the release of soluble end products of...

Copepods

Fatty acid synthesis and 37-79. sho, K., and Fujita, S. !&catilis as a living feed of the Japanese Society for

, and Yone, Y. (1979). j Brachionus plicatilis and 'letin of the Japanese Society

l e intact lipids of petrel

Colattukudy), pp. 50-91.

Kilvington, C. C. (1977). In. XI. Lipids in Calanus D Marine Biological Associa-

A. E. M., and Blaxter, f marine zooplankton by no gairdneri), Marine Bio-

, J. R. (1982). Fatty acid Transactions, 10, 463-4.

i the triacylglycerol, wax caeca of rainbow trout s and wax esters. Com-

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3 The release of soluble end products of metabolism ROBERT L E BORGNE

Introduction The organic matter assimilated by a copepod undergoes a series of metabolic reactions that leads to the production of living matter (as growth, moults, storage and gametes) and energy. The latter is produced by the oxidation of organic compounds by molecular oxygen during the respiration process. End products of these reactions enter the external medium through two physiological mechanisms that are usually studied. separately: (1) respiration, stricto sensu, which is concerned with oxygen consumption and the release of carbon dioxide (CO,); (2) excretion, which deals with the liberation of the remaining end products. When catabolism is complete, the end products are inorganic (ammonium, phosphate) , whereas when it is incomplete or when the end products are further transformed before excretion (e.g. urea synthesis), the compounds released are organic. Because both excretion and respiration lead to the elimination of catabolic end products, because they have much in common in terms of the methods used for their measurement and the factors influencing them are identical, it seems logical to consider both releases in the same chapter.

Respiratory exchanges (i.e. oxygen uptake and CO, release) of copepods take place on the tegument and at the rectum. Excretion is achieved by the maxillary glands and two to four nephrocytes that are located superficially in the cephalic region. Soluble excretion, secretion and moulting losses are distinct processes that are difficult to differentiate when the products released by a marine animal are analysed. Such processes, however, give rise to compounds that differ from those of defaecation, a process concerned with the release of particulate matter than has not been assimilated (see Chapter 6).

During the last 20 years, many studies have been made of the respiration and excretion of planktonic animals, mainly because of the availability of satisfactory analytical methods and because excretion has been recognized as playing an important role in the functioning of

ORSTOM Fonds Documentaire

110 The Biological Chemistry of Marine Copepods

marine ecosystems. This latter point was stressed by Ketchum ( 1 9 6 2 ) following the studies of Harris and Riley ( 1956) and Harris ( 1959) , so giving rise to many studies of the contribution made by zooplankton excretion to the nutritional needs of phytoplankton. In addition to this kind of study, it has been found useful to combine respiration and excretion rates in order to characterize the type of substrate being oxidized by zooplankton to produce its energy (Conover and Corner, 1968) and to assess the efficiency of utilization of ingested organic matter (Butler, Corner, and Marshall 1969, 1970) .

Both aspects of the release of soluble end products will therefore be considered. The first aspect is concerned with the proportions of the different kinds of compounds released; the second deals with rate measurements and the part played by respiration and excretion in the cycling of organic matter. Most of the studies described here have been concerned with marine copepods, but occasional reference is made to related work with freshwater species and with other groups in the marine plankton.

c

Methods of measuring copepod respiration and excretion

Measurements of respiration and excretion are expressed as rates, i.e. amounts of oxygen or energy consumed, or CO,, nitrogen or phosphorus released per unit of time and body weight. Two kinds of methods have been generally used: the assay of enzymatic activity and the incubátion technique. Enzyme assays allow respiration rates to be inferred from measurements of the activity of the electron transport system (Packard 1971) and those of ammonium excretion from glutamate dehydrogenase activity (Bidigare, King, and Biggs 1 9 8 2 ) . In the incubation technique, the measurements are those of concentrations of compounds consumed or released by animals which are left in a closed, vesse! for a certain period of time. Values from enzymatic assays have to be calibrated by incubation experiments to produce the actual rate in the environment (Packard and Williams 1 9 8 1 ) . Enzy- matic assays are reviewed by Mayzaud in Chapter 5. Accordingly, here, detailed attention is given only to the incubation technique, which is still widely used and is the only one providing information about the types of compounds released by copepods.

During an incubation experiment, the respiration and excretion of one or several animals causes variations in the concentrations of oxygen or soluble end products, which can be analysed either at regular time intervals or at the beginning and the end of the experiment. The

i

The relei

main problem with in the amounts of s( as small as possible, to be so large that tl the incubation. In c

Thus, for a given tl I tion, the longer t

influencing the rate

I

1 the length of the ex ~

Short incubations a The advantages of and short incubatil hours, are that they closed systems, and larly as far as diel ever, to stress effects introduction into t crowding. Moreove animal concentrati errors. Such considc rates, and their vari tion experiments (:

I 1974, for Calanus he1 pages ppicus; Ikeda Acyocalanus gibber, C plumata).

Crowding and confine of animal concenkr that in the natura assembling numbe effect is caused by 1

by the ratio betwec flask. According tc decrease in rates di the animals are sir This may explain tl crowding effect. Fi et al. ( 1 9 8 2 a ) betwc A. gibber at concent

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Copepods

#ed by Ketchum (1962) ) and Harris ( 1959), so made by zooplankton

:ton. In addition to this mbine respiration and ype of substrate being (Conover and Corner. ln of ingested organic 370). )ducts will therefore be the proportions of the xond deals with rate ln and excretion in the s described here have masional reference is Id with other groups in

id excretion

expressed as rates, i.e. O,, nitrogen or phos- veight. Two kinds of mzymatic activity and respiration rates to be the electron transport i excretion from glut- and Biggs 1982). In : those of concentrati- ials which are left in a thes from enzymatic iments to produce the Villiams 198 1). Enzy- 7 5. Accordingly, here, n technique, which is nformatioii about the

ttion and excretion of oncentrations of oxy- ysed either at regular -the experiment. The

The release of soluble end products of metabolism 111

main pr,oblem with this technique is to measure significant differences in the amounts of soluble compounds so that analytical errors will be as small as possible, while at the same time not allowing these changes to be so large that they cause drastic alteration to the seawater during the incubation. In other words, a compromise must be found between the length of the experiment and the concentration of the copepods. Thus, for a given temperature and species, the lower the concentration, the longer the incubation, for both are important factors influencing the rates.

Short incubations and high animal concentrations The advantages of experiments that use high animal concentrations and short incubation times, lastirig from several minutes to a few hours, are that they avoid the starvation effect which often happens in closed systems, and they reflect the actual metabolic activity, particularly as far as diel rhythms are concerned. They are sensitive, however, to stress effects resulting from the capture of the animals and their introduction into the vessel, and to the effects of confinement and crowding. Moreover, physiological state is harder to control at higher animal concentrations and short incubations can lead to analytical errors. Such considerations also explain why respiration and excretion rates, and their variability, are generally higher at the start of incubation experiments (see Nival, Malara, Charra, Palazzoli, and Nival 1974, for Calanus helgolandicus, Temora sbl i fera, Acartia clausi and Centro-

pages typicus; Ikeda, Hing Fay, Hutchinson, and Boto 1982a, for Auocalanus gibber, Calanopia elliptìca , Eucalanus subcrassus and Pontellìna plumata) .

Crowding and conjnement effects. These are consequences of the increase of animal concentrations in laboratory experiments compared with that in the natural environment. The crowding effect results from assembling numbers of animals far above normal; the confinement effect is caused by the small size of the vessel and may be represented by the ratio between the volume of the animal and the volume of the flask. According to Marshall (1973), referring to Zeiss (1963), the decrease in rates due to crowding would depend upon whether or not the animals are similarly concentrated in their natural environment. This may explain the conflicting results’in the literature relating to the crowding effect. For instance, no difference was observed by Ikeda et a¿. (1982a) between the ammonium or phosphate excretion rates of A. gibber at concentrations of one individual in 4 ml and 34 individuals

I

The Biological Chemistry of Marine Copepods

in 100 ml, or for C. ell$tica at similar concentrations. By contrast, Razouls (1972) reported that T. stylifeera displays decreases in respiration rate of 22 per cent and 30 per cent when the concentration is increased from 50 to 100 and from 50 to 200/100 ml, respectively. Corresponding decreases for C. typicus respiration rates were 7 and 22

excretion rate was about three times as high for Pleuromamma spp. at

ever, quote any differences for rates of respiration and ammonium excretion. As far as the confinement effect is concerned, Menzel and Ryther (1961) found that at equal concentrations of animals, the respiration rate was lower in vessels of a volume less than 500 ml.

Stress effects. In most studies dealing with the influence of incubation time on metabolic rates, an important decrease is observed at the beginning, followed by a steady rate or a slower decrease. This is shown in Fig. 3.1 for the ammonium excretion rate of T. stylifeera. The decrease may be ascribed to abnormally high rates at the beginning, because of the stress effect. This is followed by normal values, after which starvation lowers the level of the metabolism.

Several studies have provided evidence that the high rate values at the beginning could be caused by the stress effect or the acclimatization of the animals. Skjoldal and Båmstedt (1977), for example, re-

,

per cent. Similarly, Smith and Whitledge (1977) showed that urea

1-2 animals/100 ml than at 3-4 animals/lOO ml. They did not, how-

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+

1.5-

Time (h)

Fig. 3.1. Decrease in ammonium excretion rate ( e ) of Temoru stylifeera with length of incubation expressed as pg at. NH,-N per individual per hour (from Nival et al . 1974, fig. 4).

I

The

ported that thi ;ADP)/ (ATP and recovers '

intense energy larly, the EC ( that of animal less disturbed other hand, st for the higher difference was or remained u larly, Gardner rate of Eucalan first 10 min ol animals. Anot transferred fro

80 401 O

80

O O

Fig. 3.2. Kinel N/mg ash-free Paffenhöfer 191

! Copepods

ntrations. By contrast, 7s decreases in respira- :h the concentration is 10/100 ml, respectively. ion rates were 7 and 22 377) showed that urea 3r Pleuromamrna spp. at nl. They did not, how- 'ation and ammonium oncerned, lvf enzel and itions of animals, the ne less than 500 ml.

nfluence of incubation ise is observed at the wer decrease. This is rate of T. spylifeera. The ates at the beginning,

normal values, after Aism. he high rate values at ct or the acclimatiza- U ) , for exampie, re-

7 7

f Temora stylifeera with :r individual per hour

' The release of soluble end products of metabolism 113 . .

ported that the energy charge oSzooplankton (EC), i.e. the (ATP f $ADP)/(ATP + ADP + AMP) ratio, is low just after the net haul, and recovers to a normal value 24 h later. Low EC is ascribed to intense energy utilization by the stressed animals after capture. Simi- larly, the EC of zooplankton caught during 30-min hauls is less than that of animals obtained from 3-min hauls and therefore likely to be less disturbed by the sampling conditions (Skjoldal 1981). On the other hand, starvation effects have not been found to be responsible for the higher excretion rates at the beginning of incubation, for no difference was observed by Ikeda et al . (19824 when A. gibber was fed or remained unfed before the excretion experiment ( O to 4 h). Simi- larly, Gardner and Paffenhöfer (1982) found that the higher excretion rate of Eucalanus pileatus (Fig. 3.2) was a constant feature during the first 10 min of their experiments, whatever the feeding status of the animals. Another kind of stress is thermal, caused when animals are transferred from the environmental temperature to a different experi-

Ammonium

o ; ; ; E m .- al r Y

Ammonium

40

a

c

al

c .., - U m m c

- - 5

Time after feeding (min)

Fig. 3.2. Kinetics of ammonium and amino acid excretion rates (pmole N/mg ash-free dry wt/h) of Eucalanuspileatus after feeding (from Gardner and Paffenhöfer 1982, fig. 1).


mental temperature. Thus, an ‘overshoot’ in the respiratory rate of Calanusfinmarchicus was found by Halcrow ( 1 9 6 3 ) at the beginning of his experiment, normal values being recovered 6-8 h after the temperature increase. Leaving animals in the experimental nedium before starting the actual measurements is a possible solution to the stress or acclimatization effects. But other problems then arise, namely starvation, when copepods are not fed, and chemical changes in the incubation water (oxygen, ammonium, organic compounds) when the acclimatization period is too long.

The physiological state of the copepods. This is also a crucial factor. The values for metabolic rates should be those of normal, healthy animals, and this physiological state should be sustained throughout the period of incubation. However, the higher the animal concentration in short-term experiments, the harder this is to control. In addition to the influence on rates, ratios between the excretions of nitrogen and phosphorus are lowered, for dead or injured animals release phosphate more rapidly than ammonium (Mullin, Perry, Renger, and Evans 1975; Ikeda et al. 1 9 8 2 ~ ) . The same effect can also be observed for crustaceans when moulting, phosphate release by the cladoceran Daphnia spp. then being seven times higher than normal (Scavia and McFarland 1982), and that ofEuphausia spp. even higher (Ikeda and Mitchell 1982).

The source of the excretion products. When excretion is measured under feeding conditions, soluble compounds can sometimes arise from sources other than excretion. Leakage from faecal pellets is one possibility; another is food broken up by the animals during its capture. Errors introduced by these processes are best avoided for both are concerned with unassimilated food (Butler et al. 1969, for N and P excretions by Calanus spp.; Lampert 1978, for the release ofdissolved organic carbon by Daphnia spp.). The possibility of mixing true excretion with soluble material released from unassimilated food is more critical at the beginning of the experiment, and in short incubations. The animals should therefore be left without food so that their guts are empty before the measurements start.

Long incubations and low animal concentrations Most of the problems encountered in short incubations may be elimi- nated or reduced with longer experiments: thus, there is less variability of the results, lower stress and less release from unassimilated food. Moreover, the lower concentrations used in long incubations mean

The relei

that the animals wi crowding and conf starvation effects, levels and an incrt bacterial activity, ’ types of released dependent so that mental conditions.

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1 I Starvation effects. Or

mals encounter in long-term starvatic altered to varying

Most of the stud I have not made clea

experiments are du , ment, with starva

Comparing the kin Conover ( 1 9 5 6 6 ) c( similar conclusion respiration rates o observed for the fi starvation. Howe\ recently achieved 1 1 ammonia excretior

I flow cell interface1 Fed and starved 10 min of the expel stress or acclimati: remaining higher 1 O-min period wa: the stress: for exa effect than the sim

Starvation effect if the animals arc natural levels of unnaturally conce copepods feed, foc soon occur. Anotf tured algae is that phate, even in the

opepods

le respiratory rate of 1) at the beginning of B h after the tempera- ?rital medium before lution to the stress or arise, namely starva- langes in the incuba- inds) when the accli-

a crucial factor. The nal, heal thy animals, hroughout the period ia1 concentration in ntrol. In addition to .ions of nitrogen and nimals release phos- Perry, Renger, and

can also be observed se by the cladoceran normal (Scavia and

hn higher (Ikeda and

i is measured under metimes arise from al pellets& one pos- nals during its cap- )est avoided for both t al. 1969, for N and le release of dissolved of mixing true excre- nilated food is more in short incubations. od so that their guts

X I

s

ations may be elimi- there is less variabil- i unassimilated food. g incubations mean


that the animals will be healthier and this will diminish the effects of crowding and confinement. However, other problems arise, namely starvation effects, changes in the seawater such as lowered oxygen levels and an increase in those of toxic compounds, and enhanced bacterial activity, which changes both the metabolic rates and the types of released products. All these effects are temperature- dependent so that the extent of their influence can vary with experimental conditions.

Starvation effects. Only starvation periods of several hours, which animals encounter in their natural environment, are dealt with here: long-term starvation, during which the animal's physiology can be altered to varying degrees, will be discussed in Chapter 5.

Most of the studies of the kinetics of respiration and excretion rates have not made clear whether the higher values at the beginning of the experiments are due to stress or represent normal rates in the environment, with starvation effects subsequently inducing lower rates. Comparing the kinetics of respiratory rates of fed and unfed A . clausi, Conover (19566) concluded that the decrease was due to starvation. A similar conclusion was drawn by Ikeda (19776) using the kinetics of respiration rates of Acartia tonsa over 9 h: the systematic decrease he

I 'observed for the first hour'was considered to be the consequence of 1 starvation. However, discrimination between the two effects was

rècently achieved 'by Gardner and Paffenhöfer (1982) using E. pileatus, ammonia excretion being measured every 3 min with an incubation flow cell interfaced with high-performance liquid chromatography. Fed and starved animals displayed higher rates during the first 10 min of the experiment and this may therefore be clearly ascribed to stress or acclimatization. Later, rates decreased over time (Fig. 3.2), remaining higher for fed animals than those not fed. In fact, the 10-min period was probably variable, depending on the intensity of the stress: for example, long plankton hauls would have a stronger effect than the simple addition of water into the experimental vessel.

Starvation effects can be reduced either with shorter incubations or if the animals are fed during the experiment. In the latter, using natural levels of food will lead to problems because animals are unnaturally concentrated in the experimental vessel: thus, when the copepod's feed, food levels rapidly diminish and, effects of starvation soon occur. Another problem when the food used is natural or cul- tured algae is that plants accumulate excreted ammonium and phosphate, even in the dark, which leads to underestimated rate values.

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3

!


These, however, may be corrected for algal uptake, which is a function of the nutrient concentration, by using a Michaelis-Menten relationship (see Takahashi and Ikeda 1975, for Metridia paczfica). This method has been improved more recently by Lehman (1 9 8 0 ) and used by Miller and Landry ( 1 9 8 4 ) for Calanuspanficus. Here, equal rates of uptake by phytoplankton at any cell density were ensured by inoculat- ing the seawater in the experimental and control vessels with ammonium and phosphate in excess at the start of the experiment. Finally, when copepods feed carnivorously, since animal prey are used, their excretion needs to be subtracted from that of the copepods. This was done by Corner, Head, Kilvington, and Pennycuick ( 1 976) for Calanus spp. feeding on nauplii of the barnacle Elminius modestus, although in this case no significant nitrogen excretion by the prey could be observed, even at the highest prey concentrations.

Chemical alteration to the seawater medium. During incubation, the composition of the seawater in a closed vessel is modified by the animals’ respiration and excretion. The oxygen partial pressure decreases whereas nitrogen and phosphorus concentrations increase. The influence of the latter on the physiology of experimental animals does not seem to have been studied so that conclusions can only be made about the effects of reduced oxygen levels.

Ikeda (1977a) found no significant decrease in respiration rate for oxygen saturations above 70-80 per cent. Possibly, this rather high level was due to the short incubation time (20 min to 4 h) and the small volume of the vessel (6 .8 ml) used in his experiments. Thus, in a previous review of the literature, Ikeda ( 1 9 7 0 ) quoted a limiting level of 5 0 per cent saturation for epipelagic copepods. Le Borgne (1982a) observed no significant difference between respiration or excretion rates of mixed zooplankton incubated in seawater of 80-86 per cent or 54-68 per cent oxygen saturation (animals being obtained from the same sample). The value of the oxygen saturation under which respiration decreases appears to be influenced by temperature, the activity of the animal (Ikeda 1 9 7 0 ) , and its habitat. Thus, the limiting level for the bathypelagic copepod Gaussiaprinceps, which is adapted to water of substantial desaturation, is lower than that of epipelagic copepods (Childress 1977) .

One possible way of avoiding the problem of chemical changes in the seawater medium is the ‘continuous flow technique’, which consists of circulating water through experimental and control containers and analysing its oxygen or nutrient concentrations following respiration

and excretion. tions to remai

respiratory surfaces rate is higher. Iked explanation for high that copepods captu clear why ingestion rate, unless the utili

Bacteria. Bacterial ac

duced with the ani tion of excreted or

introduced at higher I ity.

Depressed activity the effect of which w; C. finmarchicus (Mars1 results obtained with 1978) . Marshall ( 1 9 ; TVinkler method for (

biotics affect the ultra obser.vations, togethe their physiological prl respiration and excre

T h e measurements o; The incubation teci measurable increase oxygen level in the s with the measuremen will now be briefly dj

Respiration. The resp

Copepods

>take, which is a func- Lichaelis-Men ten rela- hietridia pacz fca) . This :hman (1980) and used is. Here, equal rates of re ensured by inoculat- control vessels with

art of the experiment. ;ince animal prey are n that of the copepods. .nd Pennycuick ( 1976) iacle Ejminius modestus, excretion by the prey Incentrations.

; incubation, the com- idified by the animals' al pressure decreases ns increase. The influ- ental animals does not an only be made about

in respiration rate for sibly, this rather high O min to 4 h) and the xperiments. Thus,'in a quoted a limiting level 3s. Le Borgne (1982a) spiration or excretion er of 80-86 per cent or ing obtained from the on under which respi- nperature, the activity s, the limiting level for i is adapted to water of f epipelagic copepods

if chemical changes in inique', which consists control containers and s following respiration


and excretion. This technique, which allows the experimental conditions to remain constant, has been used, among others, by Corner (1961) for the feeding of C.finmarchicus, Ikeda (19716) for the feeding and respiration of Calanus cristatus, Gerber and Gerber (1979) for the respiration and excretion of small copepods and Undinula vulgaris, and Gardner and Paffenhöfer (1982) for the excretion of E. pileatus. It seems, however, that flow intensity influences the measured rates. Feldmeth (1971) considers that under conditions of rapid flow, the respiratory surfaces become more oxygenated and so the respiration rate is higher. Ikeda (1971b), however, observed no difference. His explanation for higher ingestion rates (but not for respiration rates) is that copepods capture more food from faster-flowing systems. It is not clear why ingestion but not respiration would change with the flow rate, unless the utilization of food is spasmodic.

Bacteria. Bacterial activity is always observed in incubation experiments, even with filtered water, because microheterotrophs are introduced with' the animals. The result of such activity is the mineraliza- tion of excreted or secreted organic products, with concomitant over- estimation of the inorganic fraction on the one hand and of respiration rate on the other (Mayzaud 19736). These errors are more likely to be introduced at higher temperature becáuse of increased bacterial activity.

Depressed activity may 'be obtained by the addition of antibiotics, the effect of which was tested on Temora longicornis (Berner 1962) and C.finmarchicus (iMarshall and Orr 196 1). Other studies compared the results obtained with and without 'antibiotics (e.g. Ikeda 1970; Roger 1978). Marshall (1973) mentions that penicillin interferes with the Winkler method for oxygen and Butler et al . (1969) found that antibiotics affect the ultraviolet (UV) method for nitrogen'analysis. These observations, together with the fact that copepods need bacteria for their physiological processes, are reasons why many workers studying respiration and excretion do not use antibiotics.

The measurements of respiration and excretion rates The incubation technique discussed is a means of producing a measurable increase of catabolic end products, or a diminution of oxygenJ level in the seawater medium. The methodology concerned with the measurement of these variations is a further problem, which will now be briefly discussed.

Respiration, The respiratory end product is carbon dioxide (CO,)


which is rarely determined because the amounts released by copepods are low compared with its concentration in seawater. This leads to analytical errors. A possible solution is the use of labelling by '*C, as performed by Copping and Lorenzen (1980) in experiments with C.pacificus. This method is precise but needs a known and constant specific activity of I4CO2 in order to convert the measured, labelled amounts into total quantities of carbon dioxide (see Conover and Francis 1973).

The usual way of assessing respiration rate is to measure the diminution of oxygen in the seawater medium. The consumed oxygen is then converted into CO, using the respiration quotient (RQ) which is the ratio between the amount of released CO, and that of consumed oxygen. The RQ is equal to 1 when carbohydrates are degraded, but is less than 1 when the substrates are lipids or proteins. As yet, no measurements of R Q seem to have been made for marine copepods although the need for this was pointed out much earlier (Corner and Cowey 1968). Nevertheless, the use of a single R Q value could be misleading because of the diversity of copepod diets and therefore of the type of substrate being used during respiration.

Nowadays, oxygen concentrations may be analysed with great pre- cision by a modified Winkler method (Williams and Jenkinson 1982) or by the polarographic technique (Teal 1971), which is less precise but allows a continuous record of oxygen saturation to be made. Two other methods, discussed by Packard et al. (1984), have hardly been used for the respiration of marine copepods. These are the Cartesian diver method, described by Klekowski (1971), which measures variations in oxygen partial pressure; and microcalorimetry, which deter- mines heat production, a parameter directly proportional to respiration (Lasserre 1984; Pamatmat 1984).

Excretion. Copepods are known to excrete ammonium and phosphate, both compounds being analysed routinely in chemical oceanography. The soluble organic matter released by planktonic animals, however, is analysed less frequently. The organic molecules may be mineralized by UV irradiation in the presence of hydrogen peroxide (Armstrong and Tibbitts 1968) with a high efficiency, or by the Kjeldahl method for nitrogen (Holm-Hansen 1968) and persulphate oxidation for phosphorus (Menzel and Corwin 1965). The organic fraction is inferred from the difference between total nitrogen and inorganic nitrogen. Amino acids, amines and ammonia can be analysed by the ninhydrin

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The release o

reaction of Moore and the urease technique of Mortgan and Cundy (1

Radioactive methods excretion by C. finmarch and Lean (1973) for the and Lorenzen (1980) us used for mixed microzc Laws (1979), Glibert ( The radioactive method feed on labelled food, wf animals feeding on lab measured with this tec ments so that the anim specific activity (Conov

This brief survey of involved in the incubat solution to the problerr marine copepods. This i the use of animals or p method for the assessmc principles seem to be irr when comparing the fac the shortest time of in when using animals at h between respiration and in metabolism (e.g. the e tions (e.g. bacterial acti temperature. This last can be measured simult because, apart from the similar.

The chemical forms of The main two compou ammonium and phosph nitrogen and phosphor1 ments. The importance the biochemical process

! Copepods

ts released by copepods seawater. This leads to e of labelling by I%, as I ) in experiments with a known and constant the measured, labelled vide (see Conover and

'ate is to measure the . The consumed oxygen in quotient (RQ) which

and that of consumed rates are degraded, but or proteins. As yet, no le for marine copepods ich earlier (Corner and $e RQ value could be 3 diets and therefore of *ation. nalysed with great pre- ns and Jenkinson 1982) ), which is less precise ration to be made. Two 984) , have hardly been rhese are the Cartesian which measures varia-

lorimetry, which deter- iroportional to respira-

ionium and phosphate? hemical oceanography. onic animals, however, iles may be mineralized n peroxide (Armstrong iy the Kjeldahl method sulphate oxidation' for lrganic fraction is infer- md inorganic nitrogen. tlysed by the ninhydrin


reaction of Moore and Stein (1948). Urea can be measured using the urease technique of McCarthy (1970) or the method of Newell, Adortgan and Cundy ( 1967).

Radioactive methods have included the use of 3?p for phosphorus excretion by C.finmarchicus (Marshall and Orr 1961) and by Peters and Lean (1973) for the freshwater copepod Diaptomus spp.; Copping and Lorenzen (1980) used in studies with C. bacficus and "N was used for mixed microzooplankton by Caperon, Schell, Hirota, and Laws ( 1979) , Glibert ( 1982), and Paasche and Kristiansen ( 1982). The radioactive method is precise but is concerned with animals that feed on labelled food, which usually is phytoplankton, i.e. excretion by animals feeding on labelled detritus or microzooplankton is rarely measured with this technique. It also requires rather long experiments so that the animals' excretion products can reach a constant specific activity (Conover and Francis 1973).

This brief survey of the analytical methods and the difficulties involved in the incubation technique shows that there is no simple solution to the problem of measuring respiration and excretion by marine copepods. This is also.true of other methods that depend upon the use of animals or plants under enclosed conditions (e.g. the8'% method for the assessment of primary production). However, several principles seem to be important: (1) the use of the same methodology when comparing the factors influencing metabolic rates; (2) selecting the shortest time of incubation or the continuous flow technique? when using animals at high concentrations; (3) determining, the ratios between respiration and excretion in order to detect a possible change in metabolism (e.g. the effect ofstarvation) or in environmental conditions (e.g. bacterial activity, uptake of nutrients by plants) at a given temperature. This last point assumes that respiration and excretion can be measured simultaneously, but should pose no serious problem because, apart from the actual analysis, much of the methodology is similar.

The chemical forms of soluble compounds excreted The main two compounds excreted by copepods are known to be ammopium and phosphate, although the release of organic forms of nitrogen and phosphorus have been found during incubation experiments. The importance of this organic excretion, its composition and the biochemical processes involved, however, are not yet clear.


Nitrogen excretion Copepods are generally known as ammonotelic animals, ammonium being produced from the oxidative deamination of glutamate through GDH (glutamate dehydrogenase). The dominance of ammonium excretion has been confirmed in most of the studies in which inorganic and total nitrogen release have been simultaneously measured (Table 3.1).

However, the importance of organic nitrogen excretion was the subject of a controversy between Corner and Newell (1967), who observed a low percentage for 24 h incubations of Calanus spp., and Johannes and Webb (1965) who obtained a higher percentage of organic excretion in short incubations with mixed plankton (see Corner and Davies 1971). The first explanation for such differences was concerned with the methodology used: thus, Corner and Newell argued that organic compounds resulted from abnormally high animal concentrations in the experimental containers used for short-term incubations. Johannes and Webb, however, claimed that bacterial activity was significant in long incubations and would mineralize part of the organic nitrogen released. Neither explanation is entirely satisfactory, for very low levels of organic nitrogen excretion were found later for Calanus spp. using short incubations (4 h instead of 24 h) by Corner et al. (1976); on the other hand, a high percentage of organic excretion was found during long-term incubations (24 h) of mixed zooplankton by Le Borgne (1973, 1977).

Even if the experimental conditions and temperature can influence the amount of organic nitrogen excreted, such results indicate that the nature of the animal under study also affects the type of excretion.

Table 3.1. Ammonium excretion as a percentage of total nitrogen excretion.

Species

Calanus spp. Calanus spp. Calailus spp. feeding

on Biddulphia cells Calanus feeding as

a carnivore Acartia clausi Calanus chilenris Centropages brachiatus Eucalanus inermis

Ammonium/total N

74.7 78.3 86.0

>90.0

84.0 76.1 47.4 73.5

~~

Reference

Corner and Newell (1 967) Butler et al. (1969) Corner et al. (1972)

Corner et al. (1976)

Mayzaud (19736) Dagg et al. (1980) Dagg et al. (1980) Dagg et al. (1980)

The release

70 ~

g c 5 60- I ’ 2

50 -

Fig. 3.3. Increase in thc with that of the weight-] eastern tropical Atlantic

Figure 3.3 from Le B view. Thus, there is a ammonium excreted planktonic populatior! zooplankton other tk organic nitrogen exc results with mixed PO tion than those with c come from studies c nitrogen fraction of t amphipod Phronima st

fusiformis and Thalia a per cent for the ctenoF Roger (1982) has prl ocean: 42.5 per cent o: for sergestids, 30.0 pe: cent for amphipods ai

Results by Copping organic carbon is g released. Part of this (

pounds, such as urea i of total nitrogen excr

Copepods

:c animals, ammonium n of glutamate through inance of ammonium iies in which inorganic iltaneously measured

yen excretion was the I Newell (1967), who s of Calanus spp., and higher percentage of mixed plankton (see n for such differences s, Corner and Newell abnormally high ani- rs used for short-term :aimed that bacterial vould mineralize part ation is entirely satis- excretion were found h instead of 24 h) by )ercentage of organic ons (24 h) of mixed

:rature can influence ults indicate that the le type of excretion.

al nitrogen excretion.

eference f

orner and Newell (1 967) utler et al. (1969) xner et al. (1972)

)mer et al. (1976)

ayzaud (1 9736) tgg et al. (1 980) tgg et al. (1 980) .gg et al. (1980)

121 The release of soluble end products of metabolism

’ O i

I , I d , , , , + 1 O0

Weight 80 percentage 90 of copepods (%)

Fig. 3.3. Increase in the inorganic fraction of nitrogen excretion (at 17°C) ivith that of the weight-percentage of copepods in mixed zooplankton of the eastern tropical Atlantic Ocean (from Le Borgne 19826, fig. 2).

Figure 3.3 from Le Borgne (1982a) presents data in support of this view. Thus, there is a positive correlation between the percentage of ammonium excreted and the relative biomass of copepods in the planktonic population. In other words, the greater the proportion of zooplankton other than copepods, the greater the importance of organic nitrogen excretion. This observation would ’explain why results with mixed populations generally show a higher,organic fraction than those with copepods alone. Further confirmation of this has come from studies demonstrating the importance of the organic nitrogen fraction of taxa other than copepods: 49 per cent for the amphipod Phronima sedentaria, 30 per cent for the thaliaceans Saba

fusij5ormis and Thalia democratica (Mayzaud and Dallot 1973), and 54 per cent for the ctenophore hlnemiopsis leidyi (Kremer 1976). Similarly, Roger (1 982) has provided results for the tropical Atlantic open- ocean: 42.5 per cent of organic nitrogen for euphausiids, 30.5 per cent for sergestids, 30.0 per cent for peneids, 33 per cent for carids, 25 per cent for amphipods and 38.0 per cent for fish.

Results by Copping and Lorenzen (1980) show that the excretion of 0rganic”carbon is greater than the amounts of carbon dioxide released. Part of this organic carbon is associated with nitrogen compounds, such as urea and amino acids. Urea can represent 10 per cent of total nitrogen excreted by Calanus spp. (Corner et a l . 1976), and


could be regulated by arginase (Bidigare 1983). I t follows that urea excretion would depend on the food composition, particularly its level of arginine. On the other hand, amino acids could arise from a simple diffusion through the cellular membrane (Webb and Johannes 1967) and are released as ‘spurts) by E.pileatus (Gardner and Paffenhöfer 1982; Fig. 3.2). In the latter study, the importance of the discontinu- ous amino acid excretion compared with that of total nitrogen could not be assessed. As the ‘spurts) of amino acid excretion were not more frequent at the start of the experiment, when the animals had just been fed, amino acids are not likely to have been lost during mastica- tion or defaecation, two processes taking place during the first hour after feeding.

Phosphorus excretion The first measurements of the liberation of inorganic phosphorus by zooplankton were made by Cooper (1935) and Gardiner (1937), and those of organic phosphorus using mixed zooplankton by Pomeroy, Matthews, and Min (1963). Organic phosphorus as a fraction of total phosphorus excretion is 74.4 per cent for Oiihona similis, 68.3 and 66.5 per cent for C-II-C-IV and C-V-C-VI of A. tonsa respectively, and 66.8 per cent for Pseudocalanus minutus (Hargrave and Geen 1968). When no antibiotics were added to the experimental water, the proportions were lower, indicating a preferential organic phosphorus uptake by bacteria. Such a result conflicts with those of Peters and Lean (1973) who found that the freshwater copepod, Diaptomus spp., would release 90 per cent of 92P during short incubations and only 60 per cent during 10-h incubations, the decrease being caused by a preferential uptake of PO,-P by epibionts in general and bacteria in particular. A similar decrease in the inorganic fraction with time of incubation was observed by Le Borgne (1979) using mixed plankton. He regarded the effect as due to starvation. Temperature does not seem to influence the relative proportions of organic and inorganic phosphorus excreted (Le Borgne 1982a), but food levels have this effect with Culanus spp. (Butler et al . 1970). During a seasonal survey, these latter workers showed organic phosphorus excretion could reach 72 per cent of total phosphorus during the spring bloom, but was less at other seasons. To account for the high amounts of organic phosphorus released, Butler et al. suggested that the organic fraction represented unwanted phosphorus material that was unassimilated with other dietary constituents and was subsequently excreted unchanged. In terms of calculating food-conversion efficiency this

1 The rele

point is important;

yet been made of th sent dietary phosp

fraction although t et al . 1963).

In spite of a lack the organic compo1 high values are nc temperature and SI

the relative import;

:j Factors influencing

Respiration and ex( or excreted per tim1 hourly body loss (( those of the release body loss daily are 1

NH,-N excretion 1 vington (1972) (41

instance, on a g

Diel variations in o x vertical migrations, a (Childress 1977) or I Gerber 1979). There tions of respiration r Marshall and Orr ( Calanus hyperboreus; Be C. finmarchicus, P. mint Corner (1968) for C. longa and Pareuchaeta rí

A. clausi and C. helgola rates are concerned, tl over and Corner (19(

e Copepods

'83). It follows that urea tion, particularly its level could arise from a simple .ebb and Johannes 1967) 2ardner and Paffenhöfer rtance of the discontinu- .t of total nitrogen could excretion were not more

en the animals had just )een lost during mastica- Ice during the first hour

lorganic phosphorus by .d Gardiner (1937)) and oplankton by Pomeroy, rus as a fraction of total ma similis, 68.3 and 66.5 . tonsa respectively, and ;rave and Geen 1968)- mental water, the prop- al organic phosphorus i th those of Peters and opepod, Diaptomus spp., ncubations and only 60 ase being caused by a general and bacteria in ic fraction with time of using mixed plankton. Temperature does not organic and inorganic

t food levels have this iring a seasonal survey, horus excretion could the spring bloom, but

igh amounts of organic iat the organic fraction hat was unassimilated subsequently excreted iversion efficiency this

. The release of soluble end products of metabolism 123

point is important; for the organic phosphorus released could represent dietary phosphorus that has not been assimilated. No study has yet been made of the chemical composition of the organic phosphorus fraction although this question was raised over 20 years ago (Pomeroy et al . 1963).

In spite of a lack of information about the chemical composition of the organic compounds released by copepods, clearly the occasional high values are not simply the result of methodology. Food level, temperature and species composition may all be factors influencing the relative importance of the organic fraction.

Factors influencing copepod respiration and excretion rates Respiration and excretion rates represent amounts of matter respired or excreted per time and body weight unit. When a value for daily or hourly body loss (or uptake) is required, the weight units must be those of the released (or consumed) element. Examples of values for body loss daily are those of Ikeda and Mitchell (1982) (2.0 per cent for NH+-N excretion by Calanus propinquus) and Corner, Head, and Kil- vington (1972) (41.2 per cent for phosphorus released by C. helgolan-

' dicus). These two values are not the extremes of the range found in the literature, but they illustrate the limitations of a table that would present daily losses of body C, N, P, without taking certain key factors into account. These are now reviewed.

Variations in metabolic rates are both temporal and spatial. For instance, on a geographical scale, individuals of T. s&~lifera and C. Qpicus display higher rates in the English Channel than they do in the Mediterranean Sea (LeRuyet-Person, Razouls, and Razouls 1975). Diel variations in oxygen consumption, which are often linked with vertical migrations, are shown by the bathypelagic copepod G. princeps (Childress 1977) or U. vulgaris living in Enewetak atoll (Gerber and Gerber 1979). There have been many studies of the seasonal variations of respiration rates: Gauld and Raymont (1953) for A. clausi; Marshall and Orr (1958) for C.finmarchicus; Conover (1962) for Calanus hyperboreus; Berner ( 1962) for T. longicornis; Anraku (1964) for C. finmarchicus, P, minutus, A. clausi and Labidocera aestiva; Conover and Corner (1968) for C. &perboreus (see Fig. 3.4) , C.finmarchicus, Metridia longa and Pareuchaeta norvegica; Gaudy (1973) for C. typicus, T. stylifeera, A. clausi and C. helgolandicus. As far as seasonal variations in excretion rates are concerned, the only studies with copepods are those of Con- over and Corner (1968) for ammonium excretion and Butler et a l .


20 A

15 - -

1

1 _1 .o

B

t C I

1 O0 1 Fig. 3.4. Seasonal variations in rates of oxygen uptake (A) (in plitres 02/mg/day) and ammonium release (B) (in pgN/mg/day) and of O:N atomic ratios (C) for Calanus hyperboreus (from Conover and Corner 1968, fig. 1).

The releasc

(1970) for total nitrc C.finmarchicus and C. all studies of seaso: region and none with

The three main fa( namely temperature, considered first. 0 t h saturatian, salinity o may prevail in certai ture and food level poikilotherms and th same individual. Or account for the var stages, and between

Tem pera t u re Excretion or respirati temperature up to a I

habitat. An example and temperature (T) three rates, those of rl tion, with temperatui

The shape of the h workers (e.g. McLart erally used is

where A and B are c( the velocity of chemic is associated with thi tion faculty of poikilo, The link between M- Fig. 3.5. Three cases

(1) Q,, is steady and excretion rates do not total nitrogen excretic (2) QI,, is steady but of the rate with tempe the individual to temi between 5 and 15°C I

Copepods

1

C

i.

I F ~ M I A ~ M

uptake (A) (in @litres i/mg/day) and of O:N lover and Corner 1968,


(1970) for total nitrogen and phosphorus released by a mixture of C.finmarchicus and C. helgolandicus. It should be emphasized that so far all studies of seasonal variations have dealt with the temperate region and none with tropical areas, where seasonal upwellings occur.

The three main factors influencing respiration and excretion rates, namely temperature, amount of food and individual body weight are considered first. Other factors, such as hydrostatic pressure, oxygen saturation, salinity or light, which generally have far less effect, but may prevail in certain circumstances, are dealt with later. Tempera- ture and food level have a general effect on metabolic activity of poikilotherms and their influence may explain rate variations for the same individual. On the other hand, individual body weight can account for the variations found between various developmental stages, and between species.

Temperature Excretion or respiration rates of poikilotherms are a direct function of temperature up to a maximum depending on species, generation and habitat. An example of this relation between metabolic activity ( M ) and temperature (T) is given for A. clausi in Fig. 3.5. Variations of three rates, those of respiration, ammonium and total nitrogen excretion, with temperature (5-20°C) are represented.

The shape of the M-T relationship has been studied by numerous workers (e.g. McLaren 1963; Mayzaud 1973a) and the formula generally used is

it1 = A*B7

where A and B are coefficients derived from Arrhenius's law linking the velocity of chemical reactions with temperature. The Qlo concept is associated with this model as Qlo = BIo, and describes the adaptation faculty of poikilotherms to temperature effect over a given range. The link between M-T curves and Qlo is illustrated for A . clausi in Fig. 3.5. Three cases may be observed:

(1) Q,o is steady and close to unity, which means that respiration or excretion rates do not change with temperature. This happens for the total nitrogen excretion rate between 15 and 20°C (Fig. 3.5). (2) Ql0 is steady but greater than unity, indicating a regular increase of the rate with temperature and demonstrating a good adaptation of ,the individual to temperature. This is the case for the respiration rate between 5 and 15°C (Fig. 3 . 5 ) .


I

5 10 15 20

5 - 4 -

3 -

2- e

1- +---y--+\i 5 10 15 20

Temperature "C Temperature "C Fig. 3.5. Metabolism-temperature (M-T) curves and Qlo values for respiration or total nitrogen excretion by Acurtiu cluusi (from Mayzaud 1973~). Respiration rate in pulitres 02/mg (dry weight)/day; NH3-N excretion rate in pgNH3-N/mg (dry weight)/day; total nitrogen excretion rate in pg NT/mg (dry weight)/day.

(3) Qlo is high or less than unity, meaning that the animal does not control its metabolism, either because it is not an eurythermal species or because the temperature is too high. I t should be noted that description (3) of the Qlo value does not take into account the time to adapt to temperature changes, during which there may be a temporary and important increase of the rate, such as the overshoot found by Halcrow (1963), followed by an acclimatization period and, eventually, the appearance of genetic variants (Newel1 and Branch 1980). Thus a planktonic species may not be adapted to the sudden variations in temperature met during its vertical migrations, but will be able to cope with a more or less regular

The relea

temperature increas therefore, is concer: experimental tempe:

The QI" value of a metabolic rate is dc example, Ql0 for re (Fig. 3.5), which me excretion will be inf temperature changes by Ikeda and Hing F temperature ranging into account, the a\ ammonium excretior rate the effect of ter ingested food. Never temperature on diffei mixed plankton stuc The same observatio ture on metabolic IC (Vidal 1980).

In addition to the body weight also infl and Whitledge (1982 boreal species to ten excretion being hig€ weight compared wit erature adaptations stronger than previ changes in the lipid comparisons of rates 1982). The role of li the QI, for phosphate related to the body d is not (Ikeda et a l . l!

Qlo is also influen Thus A. tonsa, a spec Qlo (Conover 1956). Moroccan upwelling play greater Ql0 vali mamma spp. (ChamF

Copepods

-+ I I

10 15 20

-+- _L I

10 15 20 Temperature "C

:s and QI,, values for z clausi (from iMayzaud it)/day; NH,-N excretion )gen eicretion rate in ,ug

t t the animal does not Ln eurythermal species

l,,, value does not take :banges, during which Ise of the rate, such as red by an acclimatiza- : of genetic variants c species may not be .e met during its verti- I more or less regular

i

l


temperature increase during summer. Account (3) of Qltl variations, therefore, is concerned with individuals that have acclimatized to experimental temperature.

The Q,, value of a given species changes depending upon how the metabolic rate is defined, and with environmental conditions. For example, Qlo for respiration is not the same as QI0 for excretion (Fig. 3.5)) which means that the 0 : N ratio relating respiration and excretion will be influenced by temperature. This can be caused by temperature changes affecting different metabolic processes as shown by Ikeda and Hing Fay ( 1981) for Calanidae living in various regions of temperature ranging between O and 30°C. When body weight is taken into account, the average Q,, was 2.18 for respiration and 2.58 for ammonium excretion. In this case, however, it is not possible to separate the effect of temperature from that of variations in the type of ingested food. Nevertheless, it should be noted that varying effects of temperature on different metabolic processes have been observed with mixed plankton studied at two different temperatures (Table 3.2). The same observation has also been made for the influence of temperature on metabolic losses, assimilation and production of C. paczficus (Nidal 1980).

In addition to the metabolic process considered, the definition of body weight also influences Ql0 (Ra0 and Bullock 1954). Thus Vidal and Whitledge (1982) emphasize the role of lipids in the adaptation of boreal species to temperature, the Q,,, for respiration or ammonium excretion being higher when the rates are expressed per unit dry weight compared with lipid-free dry weight. 'This suggests that temperature adaptations for the metabolic rate of crustaceans may be stronger than previously reported, and indicates that neglecting changes in the lipid content of animals . . . may lead to erroneous comparisons of rates and levels of metabolism' (Vidal and Whitledge 1982). The role of lipids during thermal changes could explain why the Q,o for phosphate excretion (which is linked to lipid catabolism) is related to the body dry weight whereas that for ammonium excretion is not (Ikeda et al . 1982~).

QI, is also influenced by feeding habit if respiration is involved. Thus A. tonsa, a species more carnivorous than A . clausi, has a higher Q,,, (Gonover 1956). Similarly, species considered as carnivores in the Moroccan upwelling, such as Anomalocera Patersonì or T. styli fra, display greater Qlo values than the herbivores C. helgolandicus or Pleuro- mamma spp. (Champalbert and Gaudy 1972). The reason for such

Table 3.2. QI0 values of mesozooplankton respiration and excretion for several stations and temperature ranges.*

Temperature range ( O C )

17-27 17-27 20-28 16-23 16-22 17-22 17-22 17-23 17-28 17-28

Respiration 2.06 1.42 2.75 3.71 2.48 1.86 2.36 2.79 2.35 2.41

Excretion N l O k l l 1.80 1.96 2.39 2.64 1.92 1.37 2.62 2.39 1.91 2.58 NH4-N 2.36 1.30 2.34 3.25 2.59 2.03 3.23 3.00 2.36 2.83 P,,I,l 1.74 1.85 2.78 2.54 1.95 1.52 2.13 2.97 1.76 2.37 P0,P 1.89 1.82 1.26 3.48 1.81 1.43 2.24 2.91 2.21 1.92

' Data from Le Borgne (1982~) for eastern tropical Atlantic.

-. -

A N O 0

-I i

-7

I

I .

*


JULY 2o 1

.-.-----*

1 ° L

FEB. MARCH 1 1

Nov.

1 I

I I I , I

10 14 18 22 10 14 18 22

Fig. 3.6. Seasonal variations in respiration rate-temperature curves of Centropages typicus (from Gaudy 1973). Respiration rate in plitres 02/mg. Temperature in range 10-22°C.

differences is not clear, but might reflect the kind of substrate oxidized at various temperatures. If this is so, then differences in the O:N and 0 :P ratios would appear with temperature changes. This aspect of the problem, however, was not studied by Champalbert and Gaudy ( 1972).

Adaptation to the environmental temperature, as reflected by the Q I 0 value, undergoes changes linkedtdseasonal variations that mod- ify both the M-T curves, and hence QIo values and metabolic rates (Bullock 1955). Thus, each season presents a different relationship between temperature and respiration rate for C. ppicus (Fig. 3.6), T. splifera, A. clausi or C. helgolandicus (Gaudy 1973). Temperature would not be the sole cause of such variations, for distinct copepod generations have different abilities to adapt to temperature (Conover 1959; Bzner 1962; Gaudy 1973). Q,, values may also display geographical variations. For example, the respiratory Qlo ofA. clausi shows a steady value over a wider temperature range in the Mediterranean than in the Atlantic, which corresponds to different thermal condi-

- -


tions in the two sea areas (Mayzaud 1973a). Similarly, Gaudy (1975) reports different Qlo values for the same species living in the western Mediterranean Sea and in the Ionian Sea, areas of different temperatures. This study covered many species of copepods and took into account variations in dry body weight.

Food level The effect of food level on rates of copepod metabolism has been recognized, fed animals having been found to have higher rates than controls remaining unfed (see review by Ikeda 1976). Intermediate values are observed with animals with plenty of food and those starved for several days or weeks. Indeed, Conover and Lalli (1974) regard food level as the main cause of variations in respiration rates, although other workers found that temperature was a more important factor (Ikeda 1970; Le Borgne 1982~) . Possibly food level becomes more critical for phytophagous animals in temperate regions where marked variations occur in the phytoplankton biomass.

The relationship between rates of respiration or ammonium excretion and the amount of chlorophyll in the environmeniwas shown during a sesonal survey by Conover and Corner (1968) using C. hyperboreus (Fig. 3.4), C. finmarchicus, M. longa and P. norvegica, highest rates being found during the spring bloom. A similar relationship for total nitrogen or phosphorus excretion and levei-of plant food was also observed by Butler et a l . (1970) for a mixture of C. helgolandicus and C.finmarchicus studied throughout the year. In fact, however, such a relationship may be indirect because the amount of food available influences the rate of ingestion, which in turn influences the rate of release of metabolic end products. This has been shown for ammonium and phosphate excretion by M. paczfica feeding on algal cultures (Takahashi and Ikeda 1975) and for nitrogen excretion by C. helgolandicus feeding on an animal diet, i.e. nauplii of the barnacle Elminius modestus (Corner et al . 1976). As mentioned elsewhere, the influence of food level on metabolic rates will differ accordinp to the

The rek

tion, because their for egg production

The effect of star metabolism is to C:

of substrate oxidií minimal, or stand; is a function of boc nitrogen and phos] which also decrea2 be reduced. Reduc observe'd by many tatus; Nival et al . 1 chicus and A . clausi Ikeda (1970) for C for the species citc australis, Ikeda ( 19 tions in lipid con starvation on met; weights have alsc introduced by chs ship between bas Lalli 1974). Howe temperatures depl lipid reserves. Dar rate are very vari may be steady aft1 all; or it may be s be defined. Simil2 ammonium and indicates that a s t a parameter as w

,

I.' Body weight u

type of metabolism studied, the efficiency of food utilization being

(1984), for example, report that the rate of ammonium excretion by female C. paczficus remains constant while the ingestion rate increases with food level, thus indicating an increase in gross growth efficiency (i.e. the proportion of ingested food invested in growth, K i ) . It is not certain, however, that the same result would be obtained with excretion of organic nitrogen or respiration, instead of ammonium excre-

The inverse relatioi

confirmed (e.g. Co: Fernández 1978; V

dependent on the kind of substrate oxidized. Miller and Landry I of metabolism ( M )

where k is log k ' , k' and b its slope. Wh

7 Copepods

milarly, Gaudy ( 1975) :s living in the western is of different tempera- )pepods and took into

metabolism has been have higher rates than a 1976). Intermediate ty of food and those lover and Lalli (1974) ns in respiration rates, was a more important

lly food level becomes nperate regions where biomass. .ation or ammonium ivironment was shown . (1968) using C. hyper- noruegica , highest rates r relationship for total f plant food was also of C. helgolandicus and fact, however, such a )unt of food available influences the rate of

ias been shown for cifica feeding on algal nitrogen excretion by iauplii of the barnacle tioned elsewhere, the Wer according to the food utilization being . Miller and Landry

mmonium excretion by ingestion rate increases gross growth efficiency n growth, K , ) . I t is not be obtained with excre- .d of ammonium excre-


tion, because their experiment dealt with the utilization of the ration for egg production, a special case of food utilization.

The effect of starvation on rates of release of soluble end products of metabolism is to cause a reduction of these, depending upon the type of substrate oxidized (body proteins, lipid reserves). The basal, or minimal, or standard metabolic rate, which characterizes starvation, is a function of body weight. Thus, if the decrease in oxygen uptake or nitrogen and phosphorus release is expressed in terms of body weight, which also decreases during starvation, then the starvation effect will be reduced. Reductions in body weight during starvation have been observed by many workers (Omori 1970, and Ikeda 1971a, for C. cristatus; Nival et a l . 1974, for T. stylifeera; lMayzaud 1976, for C.f inmar- chicus and A . clausi). Losses of body proteins have been described by Ikeda (1970) for C. cristatus, Nival et al . (1974) and Mayzaud (1976) for the species cited above and Ikeda and Skjoldal (1980) for Acartia australis. Ikeda (1970) and Mayzaud (1976) have also observed reductions in lipid content. I t seems likely, therefore, that the effect of starvation on metabolic rates has been underestimated because body weights have also been reduced. In addition to the complications introduced by changes in body weight, there is no clearcut relationship between basal metabolic rate and temperature (Conover and Lalli 1974). However, it seems to be reached more quickly at higher temperatures depending upon the rate under study and the levels of lipid reserves. Data for the time required to reach the b,asal metabolic rate are very variable (see on Table 3.3). Thus, the respiration rate may be steady after 15 h, 3 days or never steady; it may not change at all; or it may be so variable that no standard rate of metabolism can be defined. Similar observations have been made in terms of rates of ammonium and phosphate excretion. Such great diversity of data indicates that a standard rate of metabolism is not so straightforward a parameter as was once thought.

Body weight The inverse relationship between the release rate of the end products of metabolism (M) and copepod body weight (w) has been generally confirmed (e.g. Conover 1960; Ikeda 1970, 1974, 1979; Gaudy 1975; Fernández 1978; Vidal 1980). The relationship is defined by

hf = k * W b

where k is log k’, k’ being the M-axis intecept of log M = k’ + b log W and b its slope. When hf is the amount of matter respired,or excreted

,


per individual and unit of time, b generally lies between 0.66 and 1 .OO, thus following the surface law of von Bertalanffy (1 95 1). According to Banse (1982) the average is close to 0.75. However, when .LI refers to dry body weight, body carbon, or body nitrogen, the slope becomes

- (k - 1). The two coefficients, k and 6, and their variations, have been well studied because of their use in modelling. For example, Ikeda and Motoda (1978), and Ikeda, Carleton, Mitchell, and Dixon (19826) used the two coefficients k and b in order to infer respiration rates from body weights in the Kuro-shio current and the Australian Great Barrier Reef, respectively.

The slope, 6 , was thought to vary with temperature and this was observed by Ikeda (1970, 1974) for the rates of respiration and ammonium excretion by many planktonic species, including copepods and other taxa. But the influence of temperature on b has not been confirmed by Vidal and Whitledge (1982), who could not observe this effect with copepods of the Southern Bering Sea and the Mid-Atlantic Bight (Fig. 3 .764 . They concluded that temperature affected k but not b , and could find no relationship between b and temperature in the data of Ikeda for crustaceans. I t should be kept in mind that an indirect temperature effect on metabolic rates could be caused by body weight, for species in cold waters are known to be larger than those of the warmer regions and to have slower rates of development. Thus, Gaudy (1975) showed that the same species collected from two

at a given temperature. He interpreted this as an effect of temperature

species undergoing no vertical migrations and avoiding the warm surface layer (e.g. Pareuchaeta gracilis, Chiridius poppe;, Arietellus setosus and Euchirella messinensis). By contrast, species with significant vertical migrations (e.g. P. abdominalis and E. marina) had no regional variations in body weight and therefore no differences in respiration rate because there was no temperature effect. Gaudy (1975), incidentally, found that temperature influences the slope b between 10 and 24°C.

Besides temperature, food level may influence the relationship be-

rates of respiration and ammonium excretion provide no evidence of this effect. Probably, as shown by Paffenhöfer and Gardner (1984) for the ammonium excretion of different developmental stages of E. pilehtus, the rate is only affected when the food available falls below a critical level. These workers also pointed out that food level may influence body weight so that metabolic rates would decrease when expressed in terms of this.

2- i

I l

I ! different areas in the Mediterranean had two distinct respiration rates

on body weight, and logically it only seems to apply to stenothermic I

J .

, tween M and W , although the data of Vidal and Whitledge (1982) for


W c E C .- c

a, W L m ö n .c

I

i n

1- 600.1L 10 100 1000 10000 /~g

1 The rek

// 1 10 100 - 1000 *10 O00 /1g

Body dry weight (W) Body dry weight (W)

Fig. 3.7. Two kinds of relationship between metabolic rates (-44) and the body dry'weight ( M I ) : Left, variable slope and 124-axis intercepts for the excretion of phosphate ( u ) and ammonium ( b ) during the warm and the cold season (Ikeda et al. 1982~). Right, with ihe same slope but variable 124-axis intercepts for respiration (c) and ammonium excretion (d ) (Vidal and Whitledge 1982). ( I ) , Subtropical species; ( 2 ) , boreal species. Ammonium excretion rate ( b ) in ng N/animal/h and in ( d ) pg N/animal/day; phosphate excretion rate ( u ) in ng P/animal/h; oxygen.consumption rate (c) in plitres O,/animal/day .

Such complex relationships between body weight, temperature and food level make it advisable to use a multivariate model for metabolic rates, instead of the A4 = k*JV* model (Conover 1978). I t should be borne in mind that k and b coefficients vary with the weight unit and the type of metabolism under study. Thus, Ikeda et al. (1982~) have given two distinct M = f(w> relationships for ammonium and phosphate excretions, with the temperature effects as discussed earlier (Fig. 3.7). Similarly, Vidal and Whitledge (1982) present two relationships with identical slopes b , but different intercepts k' on the M-axis for rates of respiration and ammonium excretion (Fig. 3.7).

Finally, the respiration rates of carnivorous species are higher than those of herbivores of the same weight (Raymont 1959; Conover 1960;

Haq 1967). This is tons and use more herbivorous specie: neutral buoyancy. ing for food. This vores was not, hov respiration rates c because he includc fore, body weight

Champalbert an tion rates of cope] small number of s i belfaviour of a par kind of problem e:

A copepod feedì ably have a lower tion rate than whe of two copepod SI carnivorous, mighi considered. This j Procházková (198 plankton: ammoni herbivores, but n rates. Another exa Calanus spp. feedir the barnacle Elmin (same temperaturi C-V and females). its daily nitrogen 1

Biddulphia and, at depending on pre cent). The assim:

ler, but the data st: solely as a herbivore could be misleading

Other factors Available oxygen. A d respiration rate. The

)pepods

I I l

100 1000 1 o o o o / ~ g

I I I

100 1000 10 000,llg

Iry weight (W)

lic rates (:M) and the xis intercepts for the he warm and the cold e but variable ill-axis :tion (d) (Vidal and I species. Ammonium nimal/day; phosphate :ion rate ( c ) in plitres

ht, temperature and model for metabolic 1978) . It should be the weight unit and i et al . ( 1 9 8 2 a ) have imonium and phos- 1s discussed earlier ) present two rela- ntercepts k' on the :cretion (Fig. 3 .7 ) . sies are highkr than 959; Conover 1960;


Haq 1 9 6 7 ) . This is possibly because the former have heavier exoskele- tons and use more energy to keep themselves up in the water, whereas herbivorous species have a higher lipid content and display positive or neutral buoyancy. Carnivores also behave more actively while hunt- ing for food. This difference between so-called herbivores and carnivores was not, however, observed by Ikeda ( 1 9 7 0 ) in his study of the respiration rates of various zooplankton species. Possibly this was because he included organisms of differing water content and, therefore, body weight (e.g. crustaceans, molluscs, jelly-like animals).

Champalbert and Gaudy ( 1 9 7 2 ) interpret their data for the respiration rates of copepods in the Moroccan upwelling as reflecting the small number of strict herbivores in their study. Defining the feeding behaviour of a particular species of copepod is, in fact, crucial in this kind of problem especially as most of the species are omnivores.

A copepod feeding on plants and ingesting less protein will probably have a lower rate of ammonium excretion and a higher respiration rate than when it feeds on animal diets. Accordingly, comparison of two copepod species, one supposedly herbivorous and the other carnivorous, might be misleading if only one metabolic end product is considered. This is obvious from the data of Blaika, Brandl, and Procházková ( 1982) comparing the excretion rates of freshwater plankton: ammonium is released more readily by carnivores than by herbivores, but no difference is observed for phosphate excretion

Calanus spp. feeding on Biddzilphia dells (phytoplankton) or nauplii of the barnacle Elminius modestus (zooplankton) under similar conditions (same temperature, 10°C; same incubation period, 4 h; same stages, C-V and females). On average, Calanus spp. released 26.6 per cent of its daily nitrogen ration per day as soluble nitrogen when feeding on BidduQhia and, at least, 27.2 per cent when feeding on the nauplii, depending on prey density (the maximal percentage was 129 per cent). The assimilation efficiencies were different for herbivorous (34.1 per cent) and carnivorous (79.5-99.9 per cent) feeding, so the actual difference in rates of nitrogen excretion were somewhat smal-

solely as a herbivore when considering its rate of ammonium excretion could be misleading.

Other factors Available oxygen. A decrease in available oxygen causes a decrease in respiration rate. The precise relationship between the two varies with

I 'i i

I II

I

t

\

t rates. Another example is provided by Corner et a l . ( 1 9 7 2 , 1976) for

J:

l ler, but the data still support the view that regarding Calanus spp.

*

n


species, being either linear or only occurring above a limiting-step (Prosser 1961). The first type has been observed by Ikeda (1977a) for A. tonsa; the other by Marshall, Nicholls, and Orr (1935) for C. f inmar- chicus, and by Alcaraz (1974), who used a Michaelis-Menten relationship between respiration rate and the partial pressure of oxygen for the shrimp Palaemonetes varians.

The response of an animal towards low levels of available oxygen depends upon its habitat and level of activity. Thus, a bathypelagic species like G. princeps can tolerate the very low oxygen concentrations ( 1 O- 13 mm Hg at 5- 10°C) characteristic of its habitat during daytime (Childress, 1977). Similarly, Eucalanus inermis and E. subtenuis can live in total anoxia during 12 h in the Peru upwelling and Calanus chilensis or Centropages brachiatus at 0.2-0.8 ml of oxygen/litre (Boyd, Smith, and Cowles 1980). In fjords of British Columbia the limiting oxygen concentration for zooplankton respiration, as measured as electron- transport system activity, is 5 pg-at. oxygen/litre or 0.06 ml/litre (Devo1 1981). But the lethal oxygen concentration is not as low for the majority of the pelagic species that do not meet such drastic anoxic conditions. Ikeda (1977a) defines the percentage of oxygen+saturation at which the respiration rate of animals decreases to 50 per cent of the rate at saturation as the P jo index. He concludes that the animals most sensitive to under-saturation are those with high respiration rates that are accustomed to oxygen-rich environments.

Hydrostatic pressure. Species with extensive vertical migrations are affected by pressure and their sensitivity is a function of their habitat. For instance, MacDonald, Gilchrist, and Teal (1972) report from a study of locomotive activity that the hyponeustonic species A. patersoni is more sensitive than the bathypelagic Negacalanus longicornis, Euaugaptilus magnus, P . gracilis and Pleuromamma robusta. The combined effects of hydrostatic pressure and temperature have been demonstrated for G. princeps by Childress (1977) who found that both factors tended to reduce the oxygen-consumption rate at the lower temperatures and higher pressures found at the greater depths inhabited by the animals during daytime.

Salinity. Marine copepods, especially in the open ocean, are adapted to a constant salinity and generally react to any accidental decrease in this by increasing their respiration rate. They thus have rather low powers of osmoregulation (Marshall 1973). However, euryhaline species living in estuaries or lagoons, such as A. clausi in the Ebrie lagoon of the Ivory Coast (Le Borgne and Dufour 1979) can stand

I The rele

very low salinities ( the metabolic rates have been studied.

Light . Artificial ligh by 31 per cent abo increase is 36 per (Mayzaud 1971). described for ammc (1977), who regard tion from intense however, no rate in

(e.g. Conover I in dim light and dr

Two conclusion copepod respiratio metabolic Ï-ates are graphical or diel v; levels of available metabolic rates shc temperature, body tors, such as oxyge be considered. Secc in terms of one foi automatically appl kind of metabolic e strate oxidized. A follows.

The ratios of oxygc phosphorus releasc

Differences betwee. or any other chemi' pared or when the any one species is efficiencies with w dietary constituent the substrate beir depends on the pre copepod. Thus, fed

The release of soluble end products of metabolism

very low salinities (down to 4 parts per thousand). The effect of this on the metabolic rates of euryhaline copepods, however, does not seem to have been studied.

Light. Artificial light causes the respiration rate ofA. clausi to increase by 31 per cent above that in darkness. For ammonium excretion the increase is 36 per cent and for total nitrogen excretion 27 per cent (Mayzaud 1971). This stimulating effect of light has also been described for ammonium and phosphate excretion rates by Fernández (1977), who regarded such increases as resulting from an escape reaction from intense light. Above a certain threshold of light energy, however, no rate increases are observed and this is why certain workers (e.g. Conover 1956) did not find any difference in metabolic rates in dim light and darkness.

Two conclusions may be drawn regarding factors influencing copepod respiration and excretion rates. First, those influencing metabolic Ï-ates are interdependent so that an interpretation of geographical or diel variations simply in terms of either temperature or levels of available food is inadequate. Any model incorporating metabolic rates should take into account three main factors, namely temperature, body weight and food level. Eventually secondary factors, such as oxygen availability, pressure and light may also have to be considered. Secondly, coeficients which are assessed or measured in terms of one form of metabolic rate, such as respiration, do not automatically apply to others, such as excretion. This is because the kind of metabolic end products released depends on the type of substrate oxidized. A more detailed consideration of this aspect now follows.

The ratios of oxygen uptake to the amounts of nitrogen and phosphorus released

Differences between the release rates of carbon, nitrogen, phosphorus or any other chemical element appear when different species are compared or when the influence of a particular environmental factor on any one species is considered. Such differences can arise because the efficiencies with which the animals use their food varies from one dietary constituent to another. A second source of difference is that of the substrate being used during catabolic processes. The latter depends on the prey being ingested and on the nutritional status of the copepod. Thus, fed animals degrade dietary carbohydrates and lipids


through the normal processes of glycolysis and P-oxidation, respectively. Proteins are brokwn down to amino acids which then undergo oxidative deamination and transamination, the resulting metabolites then entering the tricarboxylic cycle. Starved animals, however, degrade body proteins into amino acids, which then undergo oxidative deamination and the carbon skeletons are subsequently used in gluconeogenesis. The amounts of phosphorus and nitrogen released that correspond to a given amount of respired oxygen are different in the two cases and this appears in the O:NH, (ratio of oxygen uptake: ammonium excretion), O:PO, (ratio of oxygen uptake: phosphate excretion) and NH,:PO, ratios.

The first calculation of such ratios for zooplankton was made by Harris (1959), and this early study was followed by those of many others, some of whom also considered excretions of total nitrogen and phosphorus (NT and P T ) .

The O:N ratio Complete oxidation of the particulate organic matter (POM) in the sea, the average C:N:P elemental composition of which’is 106: 16: 1, needs 276 atoms of oxygen (Redfield, Ketchum, and Richards 1963). Therefore, the oxidation of one nitrogen requires 276: 16 = 17.25 oxygen atoms, a value of the O:N ratio which would apply to copepods feeding on POM of average composition. If the oxidized substrate has a different composition from the average POM in the sea, and if it consists of lipids and carbohydrates with low nitrogen content, the O:N ratio will be greater than 17.25, and will eventually tend towards infinity if there is no nitrogen. The other extreme case is a substrate consisting entirely of protein, for which the O:N ratio has a minimal value, depending upon the carbon and nitrogen composition of the protein metabolized. For example, for a protein of 16 per cent nitrogen and 45 per cent carbon, Conover and Corner (1968) calculated an 0 : N ratio of 8.2. For other percentages of carbon and nitrogen in proteins, values of O:N ratios were 6.8 (Ikeda 19776)) less than 7 (Snow and Williams 1971) and even 4, for the body proteins of C.finmarchicus (Mayzaud 1976).

As yet, the form of excreted nitrogen in the O:N ratio has not been dealt with. When the nitrogenous end product is ammonium, O:N is the O:NH, ratio. When part of the nitrogen released is organic, however, the O:N ratio is less than O:NH, because N , is greater than NH,. The problem of the origin of the organic nitrogen release is therefore a major one: is it excretion of an end product of metabolism or a secretion? (See p. 119).

I The re

Because the t€ composition of tl significance and (

uncertain, it is carbohydrates an

As far as marin ratio were those (

was 13.5, less thai of Redfield et al . ( tion and respirati and Corner (1968

’ratio for C. Ig’perbc before the spring after, when anim; following summe (1968) for this SF will be now used O:N ratio (Table

The oxidized subsiral rich in carbohydrz ing this period, thl quent decrease o (1968) as the res1 Omnivorous and are not so depenc display lower and species. A similar I

as deduced from mouth parts, has ;

The use of a pro to the starvation e physiological reaci variable. The 0:N: tion at 13°C when (Nival et al . 1974). Paracalanus parvus, plumchrus than for 1 study on the varia content, found tha sporadically, protl According to May:

Zopepods

d ß-oxidation, respec- Is which then undergo : resulting metabolites :d animals, however, i then undergo oxida- subsequently used in

and nitrogen released Ixygen are different in atio of oxygen uptake: :n uptake: phosphate

lankton was made by red by those of many s of total nitrogen and

matter (POM) in the of which is 106:16:1, , and Richards 1963) . uires 276:16 = I 7 2 5 iich would apply to ition. If the oxidized average POM in the tes with low nitrogen 5, and will eventually other extreme case is ich the 0 : N ratio has nd nitrogen composi- ir a protein of 16 per r and Corner ( 1 9 6 8 ) ntages of carbon and ..8 (Ikeda 1 9 7 7 ~ ) ) less r the body proteins of

:N ratio has not been s ammonium, 0 : N is ased is organic, how- e N , is greater than c nitrogen release is -oduct of metabolism


Because the theoretical minimal 0 : N ratio as a function of the composition of the proteins degraded is dificult to assess and the significance and origin of the organic fraction of excreted nitrogen is uncertain, it is hard to 'know the relative proportions of lipids, carbohydrates and proteins used during catabolic processes.

As far as marine copepods are concerned, the first values of the 0 : N ratio were those of Corner, Cowey, and Marshall ( 1 9 6 5 ) . The mean was 13.5, less than the theoretical value of 17 calculated from the data of Redfield et al . ( 1 9 6 3 ) . Later, seasonal variations of nitrogen excretion and respiration and the O:NH, ratio were studied by Conover and Corner ( 1 9 6 8 ) using temperate and boreal species. The minimum ratio for C. hyperboreus (Fig. 3 . 4 ) , a phytophagous species, is observed before the spring bloom, and reaches its maximal value immediately after, when animals are rich in lipid. Afterwards, it decreases till the following summer. The main conclusions of Conover and Corner ( 1 9 6 8 ) for this species and C.finmarchicus, M. longa, and P. norvegica will be now used to describe the principal causes of variations in the 0 : N ratio (Table 3 . 4 ) .

The oxidized substrate. During the spring bloom the food of herbivores is rich in carbohydrate and their O:NH, lies between 20 and 30. Follow- ing this period, the use of lipid leads to the highest ratios. The subsequent decrease of O:NH, is interpreted by Conover and Corner ( 1 9 6 8 ) as the result of the utilization of dietary or body proteins. Omnivorous and carnivorous species (e.g. hí, longa and P. norvegica) are not so dependent on the phytoplankton bloom and, therefore, display lower and less variable O:NH, ratios than do the herbivorous species. A similar relationship between O:NH, and feeding behaviour, as deduced from the amy1ase:trypsin ratio or the anatomy of the mouth parts, has also been found by Gaudy and Boucher ( 1983) .

The use of a protein substrate at the end of winter can be compared to the starvation effect that has been studied by many workers. The physiological reactions of copepods towards starvation appear to be variable. The O:NH, ratio of T. stylifeera decreases after a 4-day starvation at 13°C when the animal has to use its own proteins to survive (Nival et al . 1 9 7 4 ) . A similar conclusion is drawn by Ikeda ( 1977c) for Paracalanus parvus, although the ratio is higher for starved Calanus plumchrzks than for fed individuals. Finally, Mayzaud ( 1 9 7 6 ) , from his study on the variations of O:NH, and that of the lipid and protein content, found that C. finmarchicus uses lipids during starvation and, sporadically, proteins, whereas A . clausi primarily uses proteins. According to Mayzaud ( 1 9 7 3 6 ) such differences among species indi-

I

Table 3.4. O:NH.,, O:PO,, and NH,,:PO,+ atomic ratios For copepods. (Mean values given, with range in parentheses).

spccics O:NI-14 O:P04 NH.,:PO., Arca / season Reference

Pleuroniatnina .w)fiias Calntius

j i i ~ i ~ i m G/U'CII.I. (feind e ) Calaiu~~s

Jinmarchicus Calanus hjperboreus Melridia

Pai euchaela tioruegica Cetrf ropages /&icus Euchirella roslrata Pleuromanima robusla Rliiticalaiius 11asuks Calanus

fintnarcliictu and Calanus fiels olaiidicus mixed small copepods Calanus spp.

loriga

214 14.5 Bermuda Napora (1963) in Kremer ( I 975)

13.5 (9.8-15.6) Clyde Sca area, spring Corner el al. (1965)

11-54

10-150

Conover and Corner Gulf of Maine, seasonal variations ( 1968)

11-32

13-54

8.2

29.3

17.3

27.3

12.2" Clyde sea area, (10.0-14.0) spring to autumn

Butler el al. ( 1969)

11.5" Clyde sea area, autumn

11.1" Clyde sea area, (10.4-1 1.8) annual cycle

Butler el al. (1970)

- ,

Cnlaiius 16.5'' fielgolandicus (13.2-20.7) C111nnus cris falus 5.7 (3.9-9.9) 110 (76-127) 19 (12-24)

Calaiius /)1uiiicliriis 6.8 (5.8-9.2) 89 (76-124) 13 (12-20) Acartia clausi 1.61 Teinora splgera 15 Calatuus 58.9

finmarchicus Pnramlmiii r sn o En

Corner el al. (1972)

Taguchi and Ishii (1 972)

Northcrn North Pacific

Villefranche (starved animals) Mayzaud (19736) Upwelling off Morocco Nival el al. (1974) Nova Scotia (I-day starvation) Mayzaud (1976)

/ U U l l J l U

Rhincalanus nasutus Calanus

jumarchicus and Calanus helgolaridicus mixed small copepods Calanus spp.

Calanus helgolandicus Calanus cristatus

Calanus plumchrus Acartia clausi Temora splifera Calanus

jìmnarchicus Paracalanus parvus Calanus plumchrus Acartia clausi

L

Calanus helgolandicus and C. finmarchicus Temora spl fera Centropages gpicus Pleuroinamina gracilis Anomalocera patersoni Clausocalanus arcuicoriiis Calanus robustior Gaussia princeps Calanus propinguuJ

27.3

2.2* 10.0-

1.5”

5.7 (3.9-9.9)

6.8 (5.8-9.2) 1.61

15 58.9

30

20

16.8-2 1.3

23.3-30.4

18.2-28.8

18.6-19.5

27.1-35.8

39.7-44.7

15.0-22.5

13.1- 18.4 21.7 10.8 (7.5-13.9)

4.0)

11.1” (10.4-11.8)

--

Clyde sea area, spring to autumn

Clyde sea area, autumn

Clyde sea area, annual cycle

250

450

Butler et al. (1969)

16.5* (13.2-20.7)

110 (76-127) 19 (12-24) Northern North Pacific

89 (76-124) 13 (12-20) Villefranche (starved animals) Upwelling off Morocco Nova Scotia (1-day starvation)

4 Saanich Inlet (start of experiment on starvation)

10

120-294 6.6-14.0

138-191 7.37-10.6

329 9.52

Butler et al. (1970)

Barcelona (study of temperature effect)

6.73-33.1 129-2 17 Midwater species Antarctic Ocean

Corner et a l . (1972)

Taguchi and Ishii (1 972)

klayzaud (1973b) Nival et al. (1974) Mayzaud (1976)

Ikeda (19776)

Fernández (1978)

Quetin et al. (1980) Ikeda and Hing Fay (1981)

Table 3.4. Cont.

Reference Spccics O:NH, OTO, NHjPO, Area/season

Calanus aculiis Calatius /iro/)inquiis Melridia gerlacliei Eucalanus sii bcrassus Calanus pauper h o c a l a n u s gibber Temora turbinala Centro/)ages

Jicrcatus Calano/)ia ellif)lica Labidacera spp. Acartia paczjica Tortanus gracilis Corycaeus spp. Eucalanus allenualus Calauus robustior Euchirella spp. Euchaetn spinosa

119.0 (69.1-190.2) 13.9 138

12.8 168

4..4

3.66

8.64 17.2

29.6

57.1 84.4

10.0

13.1

6.35

8.8 8.6 7.6 6.6

6.7

7.6 3.9 5.5 5.7

(4.35-8.48)

9

8. I

5.9 9.2

Euchaela marina 12.8 Eucliaela acuta 28.6

intermedin Undeucliaela 10.6 inajor Chirundina 26.0 s l r e e t s i Pleurornnniina 2.7

Undeuchaeta 8.7

64.3 5.4 296.9 10.4 59.6 6.9

60.3 5.7

59.0 2.3

36.4 17 4

Ikcda and Hing Fay (1981) Ikeda and Mitchell ( 1982)

Great Barrier Reef (warm and cool season)

Ikeda el al. ( I 982a)

Indian Ocean equatorial area

Gaudy and Boucher (1 983)

Labidocera spp. Acartia pacifica Tortanus gracilis Corycaeus spp. Eucalanus 4.4 attenuatus Calanus 3.66 robustior Euchirella spp. 8.64 Euchaetn spinosa 17.2

Euchaeta marina Euchaeta acuta Undeuchaeta L

intermedia Undeuchaeta major Chirundina streetsi Pleuromamma x$hias Pleuromanima abdominalis Arietellus plumger Pontella spp. Pontella

12.8 28.6

8.7

10.6

26.0

2.7

12.6

11.1

5.41 11.17

7.6 3.9 5.5 5.7

29.6 8.1

57.1 5.9 84.4 9.2

Indian Ocean equatorial area (1 983)

Gaudy and Boucher

64.3 5.4 296.9 10.4 59.6 6.9

60.3 5.7

59.0 2.3

36.4 13.4

223 17.8

410 37

51.0 76.5

7.9 6.9

atlantica Pontella fera 6.0 1 73.28 12.35 Oncaea venusta 5.89 261.89 44.46 Candacia 6.59 36.16 6.32 paclpdactyla Scolecithrix bradyi Undirrula darwini Temora discaudata

9.37 110.30 11.87

11.29 12.4 1 6.97 13.96 75.07 4.99

* Denotes N:P ratio between total nitrogen and phosphorus excretion.


cate variable adaptations to the effects of starvation and, as a consequence, variable sensitivities to the low winter food levels in temperate and boreal regions.

Physiological state. The second cause of variation of the O:NH4 ratio is stage of development. Thus gravid females of C. hj9erboreus have lower O:NH, ratios than non-gravid ones, probably because of the 'greater reliance on protein metabolism in the absence of any appreciable lipid reserve' (Conover and Corner 1968). Similarly, these authors found a positive correlation between O:NH, and the individual size, which they link with the existence of lipid reserves. However, such a relationship was not found for other species by Ikeda and Mitchell (1982) or by Vidal and Whitledge (1982).

Temperature. Temperature seems to have no influence on the O:NH, ratio for C. hjlperboereus between 2 and 8"C, although Conover and Corner (1968) found a high variability in their data. The same conclusion concerning the lack of a temperature effect is drawn by Gaudy and Boucher (1983) and appears in the results in Fernández (1978) with several copepod species. However, a thermal effect on O:NH, may be deduced from Q,,, values for T. stylifeera, C. hdgolandicus, A . clausi and C. typicus found by Nival et al . (1974): between 13 and 23"C, the Q,, for respiration was higher than for ammonium excretion, thus- implying increasing O:NH, ratios with temperature. Possibly, these conflicting results originate from species differences, adaptation to temperature depending upon the degree of use of stored lipids (Conover and Corner 1968).

The 0 : P ratio For particulate organic matter in the sea, an average of 276 atoms of oxygen are needed to oxidize one atom of phosphorus (Redfield et al . 1963). However, other values for the 0 : P ratio will be obtained depending upon the composition of the substrate catabolized. For instance, Ikeda (19774 calculated theoretical 0 : P ratios from the mean phosphorus and carbon compositions of carbohydrates, lipids and proteins as oc;, 263 and 383 respectively. Apart from Ikeda's results for C. plumchrus and several others (Table 3.4), however, most of the values for O:PO, fall below the 276 theoretical ratio, as pointed out by Corner and Cowey (1968). Two hypotheses may explain this.

(1) There may be an incomplete oxidation through catabolic processes so that part of the phosphorus is being released as organic

The relea i molecules, the oxid environment. When the total phosphoru 143, an 0 : P ratio cl (seeLeBorgne 1973) sed earlier, reaches i 1970). (2) Part of the phc animal (Corner an( would then reflect t overestimated. . Accordingly, the

rehult from the kin utilization. Few stuc for the O:N ratio, tl behaviour and the study by Ikeda (197 that of starved anin arises as to whether more important for tein (poorer in pho animals is greater. 1 release in his experi

I There does not a] .body weight (Ikeda temperature on O: (1982a) for mixed zc (e.g. Fernández 197 ture, which is causc compared with that methodological arti effect of a natural pr animals. According teria1 activity is re during experiments and total phosphorL effect could be imF peratures, animals c starved. Indeed, su clear from the resull starved between 6 al

I I

i

Copepods

/ation and, as a conse- òod levels in temperate

n of the O:NH, ratio is '. hyperboreus have lower because of the 'greater if any appreciable lipid , these authors found a individual size, which However, such a rela- j a and Mitchell ( 1982)

ifluence on the O:NH, ilthough Conover and r data. The same con- rect is drawn by Gaudy :s in Fernández (1978) :rmal effect on O:NH, vlifera , C. helgolandicus, 974): between 13 and for ammonium excre-

os with temperature. )m species differences, the degree of use of

vrerage of 276 atoms of phorus (Redfield et al. atio will be obtained trate catabolized. For

0 : P ratios from the ' carbohydrates, lipids . Apart from Ikeda's le 3.4)) however, most etica1 ratio, as pointed :ses may explain this.

]ugh catabolic proces- ; released as organic


molecules, the oxidation of which will require more oxygen in the environment. When this organic fraction accounts for about one-half the total phosphorus released and the O:PO, ratio is approximately 143, an 0 : P ratio close to the theoretical value (276) can be obtained (seeLeBorgne 1973). Thecontributionoftheorganicfraction, as discussed earlier, reaches as much as 72 per cent for Culanus spp. (Butleret al. 1970). (2) Part of the phosphorus released may not be assimilated by the animal (Corner and Cowey 1968) and the low observed 0 : P ratio would then reflect the fact that true phosphorus excretion had been overestimated.

Accordingly, the variations in the 0 : P ratio shown in Table 3.4 result from the kind of substrate oxidized and the efficiency of its utilization. Few studies, however, have dealt with such variations. As for the 0 : N ratio, the composition of the diet and, therefore, feeding behaviour and the amount of food are likely to influence 0:P. The study by Ikeda (19774 shows that O:PO, of fed C. plumchrus is les than that of starved animals, but this is not so for P. parvus. The question arises as to whether the excretion of organic phosphorus is relatively more important for fed individuals, or whether the proportion of protein (poorer in phosphorus than lipids) being degraded by starved animals is greater. Ikeda (1977c), however, did not measure organic release in his experiments.

There does not appear to be any relation between O:PO, and dry .body weight (Ikeda and Mitchell 1982). However, the influence of temperature on O:PO, and O:P, has been shown by Le Borgne (1982a) for mixed zooplankton (Table 3.2) and by the results of others (e.g. Fernández 1978, for copepods). The 0 :P increase with temperature, which is caused by a slower increase in phosphorus excretion compared with that of respiration, could be the consequence of some methodological artifact (bacterial activity or starvation) or to the effect of a natural process specific to the metabolism of poikilothermic animals. According to Le Borgne (1982a)) it does not seem that bacterial activity is responsible because the observed increase occurs during experiments using antibiotics and because both the inorganic and total phosphorus excretions are affected. However, the starvation effect çould be important since it is more marked at higher temperatures, animals degrading a substrate poorer in phosphorus when starved. Indeed, such an increase in O:PO, for starved animals is clear from the results of Ikeda and Skjoldal (1980) for Acartia australis starved between 6 and 49 h. Consequently, it is hard to know whether

I * ,


it is due to the effect of starvation during incubation experiments or if it actually happens in the natural environment, particularly when animals are facing temperature variations during their diel migrations.

The N:P ratio Provided the chemical composition of the sÜbstrate oxidized during catabolic processes is known, a theoretical N:P ratio may be calculated. According to Ikeda (1977c), it would be 0.63 for lipid and 56 for protein. However, utilization efficiencies of ingested nitrogen and phosphorus have to be taken into account because they are likely to be different (Harris and Riley 1956). For zooplankton, the atomic ratio N:P is 24.6, greater than the value for phytoplankton (1 5.7). Accor- dingly, for herbivorous feeding, the N:P ratio of excretion products should be less than 15.7. Such a value was found by Harris (1959): he quotes an excretion NH,:PO, atomic ratio of 7.0 (9.8 according to Butler et a l , 1969, who recalculated the mean). Table 3.4 shows that this holds for most NH,:PO, values, in spite of the different types of food used by copepods. Carnivorous copepods represent an interes- ting case because, if they have a body N:P ratio close to that of herbivores, the N:P ratio for their excretion products should be greater than the body N:P ratio of herbivores. This, however, is not shown by the data in Table 3.4 where Euchaetidae, strict carnivores, do not have NH,:PO, excretion ratios higher than those of other copepods. Organic excretion may account for this, but was not measured in most instances. Thus, the ratio N T : PT for excretion could be different from NH,:PO,.

The N:P excretion ratio is fairly constant compared with nitrogen and phosphorus excretion rates. Thus, during an annual cycle, the atomic ratio N:P for total N and P excreted by Calanus spp. is minimal in winter (9.8), when plant food is virtually absent, and maximal during the spring bloom (13.0). This gives a ratio of only 0.75 between minimum and maximum. Moreover, organic phosphorus excretion, which is high during spring, may account for part of the seasonal variation in N:P ratio, which stresses the importance of measuring both inorganic and organic excretions. Besides the feeding status of Calanus spp., which appears to be a major factor, Butler et al. (1970) found no differences in N:P ratios for CV, females and males as far as total excretion was concerned. Also, their table 6 shows a lower ratio for starved animals in five cases out of six, an observation similar to that concerning the low winter value referred to earlier. There does

I l ,

The re1

not appear to be a on NH,:PO, (Gau found an increase increasing less th; et al. (1982~) foun and the cool (24.5 Great Barrier Ree

The use of N:P rat The observed diffe ton and their excre ‘led Ketchum (196 cent for zooplanktc subsequently ex ter argument, detailec (1978), is as follou

If A N and A , are assimilated, T, ani l/v, and W , the ql relations can be WI

Assimilated quantit (i.e. the ration R ) , h D, and D,:

I f a , is the N:P ra excretion (Tx:Tp), L eficiency ratio (ON

(1) and (2) +

1 Thui

As the net growth

Copepods

mion experiments or if ent, particularly when luring their diel mig-

istrate oxidized during \$:P ratio may be cal- be 0.63 for lipid and 56 f ingested nitrogen and use they are likely to be ikton, the atomic ratio dankton (15.7). Accor- ) of excretion products id by Harris (1959): he f 7.0 (9.8 according to 1 . Table 3.4 shows that if the different types of s represent an interes- ratio close to that of

n products should be . This, however, is not tidae, strict carnivores, :r than those of other for this, but was not r: P-r for excretion could

smpared with nitrogen g an annual cycle, the Calanus spp. is minimal absent, and maximal

:io of only 0.75 between phosphorus excretion, )r part of the seasonal portance of measuring :s the feeding status of :or, Butler et al . (1970) rles and males as far as e 6 shows a lower ratio observation similar to to .earlier. There does


not appear to be any thermal effect on N T : P T (Le Borgne 1982u), nor on NH,:PO, (Gaudy and Boucher 1983), although Le Borgne ( 198%) found an increase in the latter ratio with temperature, PO, excretion increasing less than NH, excretion. This would explain why Ikeda et al . (19824 found variations in NH,:PO, between the warm (28.6) and the cool (24.5) season for species of copepod from the Australian Great Barrier Reef.

The use of N:P ratios for the assessment of net growth efficiency The observed differences between N:P ratios for particles, zooplankton and their excretion products (Harris and Riley 1956; Harris 1959) led Ketchum (1962) to calculate a net growth efficiency of c. 50 per cent for zooplankton in Long Island Sound. Ketchum’s approach was subsequently extended by Butler et al . (1969) for Calanus spp. The argument, detailed by Corner and Davis (1971) and Le Borgne (1978), is as follows:

If A N and A , are the quantities of dietary nitrogen and phosphorus assimilated, T , and T p the quantities lost through metabolism, and W, and W, the quantities laid down as production, the following relations can be written:

and

Assimilated quantities, A , and A,, are equal to the ingested quantities (i.e. the ration R ) , R , and R,, corrected for the assimilation efficiency, D , and D,:

A s = Ts + Wx A , = T P + kV,

A , = DgR, = TN f CVN

A , = Dp‘Rp = Tp + Wp (1)

(2) If a , is the N:P ratio for ingestion (R,:R,), a? that of the copepod excretion (Tx:Tp) , u 3 the body ratio (Wx:Wp) , and a4 the assimilation efficiency ratio (D ,:LI ,) , then:

(1) and (2) + ala,DpRp = u 3 W p + a2Tp = ula,6Vp + ala4Tp

Thus tb’p = Tp(~2 - U ~ U , ) (ala4 - U S ) - ’ (3)

As thelnet growth efficiency for phosphorous, K?,, is

- I then (3) and (4) + K2,, = ( a l a 4 - u?) (a3 - a,) .


j Thus the ratio between production and assimilation, K,, may be assessed from N:P ratios for the prey, the excretion, the body contents and the assimilation efficiencies. Similarly, it can be shown that K , , for nitrogen is defined by:

K?,N = a3+K,,P/a ]a4

-9 ,

l

The gross growth efficiency, K I , which is the ratio between produc- i tion and ingeshon is equal to D,*K,,, for phos- I phorus and D,.K,2, for nitrogen. Using a percentage assimilation of 62 per cent for N and 69 per cent for P (i.e. a4 = 0.90), the N:P ratio for phytoplankton, that of excretion and body contents of Calanus spp. (the latter taken as equal to the production N:P), Butler et al. (1969) calculated and as 28.3 per cent and 33.1 per cent, respectively. Net growth efficiency, K,,p is 41 per cent and K2,N 53.5 per cent. In other words, Calanus spp. uses 41 per cent of its assimilated phosphorus and 53.5 per cent of assimilated nitrogen for growth. These

approach, which supports the use of the N:P ratios method. As noted by Butler et al (1969) net growth efficiencies were here concêrned with the whole period of Calanus spp. growth, but excluding egg production.

Since that time, this kind of calculation has been used only once for copepods, namely by Le Borgne and Dufour (1979) for the quasi- monospecific plankton of the Ebrie Lagoon in the Ivory Coast, with C-IV, C-V and adults ofA. clausi. Using a4 = 1, they found average values of K,,p of 37.2 per cent and I Y , , ~ of 61.3 per cent for warm (26-30°C) and rich waters (more than 4 mg chlorophyll a/m3).

As literature values for body N:P ratios of planktonic carnivores (chaetognaths, polychaetes, siphonophores, hydromedusae and ctenophores) are greater than those of herbivorous animals (copepods, some euphausiids, mysids, tunicates), Le Borgne (1978) suggested that the N:P ratio method could be applied to the higher trophic level. However, N:P ratios for the excretion and body contents of carnivorous copepods do not seem to have been determined. Differ- ent C:N ratios have also been observed for planktonic prey and pre- dators, so the method could also be used with C:N ratios, provided the complete release of carbon compounds is known (i.e. respiration and organic excretion).

The advantages of the N:P ratios method, as discussed by Le Borgne (19824 and Mann et al. (1984)) are as follows:

(1) It enables the production rate, a difficult parameter, to be obtained from K , and the excretion rate.

results agree with those of Corner et al. (1967), from a separate I

The relea

(2) I t is rapid and (3) It applies to mi: of a given species or treated as a single b phytoplankton, may are:

(1) The choice ofa in the case of Calanu certain size-class (e. ton, detritus, microz (2) The ratio betwc difficult to determint lets; such measurem easily break during (

(3) Finally, the exter excreted is a major ,

represents the end p when secretions or L with the excreted prc

The importance of tl excreted by copepoc In addition to knowir processes, the study c the importance of CO

ecosystem. This is USI

of which consists o copepods alone is to 1 should be determine obtaining their contri seldom been done, (Table 3.5) together i ton samples. As they most-cases, a fair ide; tion can be obtained.

The importance of copepo duced by respiration marine environment. organisms may have

Zopepods

irnilation, K 2 , may be ion, the body contents an be shown that

ratio between produc- 3,.K,,, for phos- rcentage assimilation = 0.90), the N:P ratio intents of Calanus spp. '), Butler et a¿. (1 969) 33.1 per cent, respec- tnd K,,N 53.5 per cent. Fits assimilated phos- en for growth. These 57), from a separate 50s method. As noted e here concerned with uding egg production. :en used only once for (1979) for the quasi- he Ivory Coast, with , they found average 3 per cent for warm ilorophyll a/m3). danktonic carnivores hydromedusae and ierbivorous animals j ) , Le Borgne (1978) ipplied to the higher on and body contents n determined. Differ- ktonic prey and pre-

ratios, provided the (i.e. respiration and

as discussed by Le 3llows:

.t parameter, to be

The release of soluble end products of metabolism

(2) It is rapid and allows a multiplicity of measurements. (3) It applies to mixed populations, which means that various stages of a given species or several species of the same trophic status may be treated as a single biomass and that all particulate diets, not simply phytoplankton, may be used as representing the diet. The drawbacks are:

(1) The choice of a , as the N:P ratio of the prey in assessing K?; thus, in the case of Calanus spp., is it all the particulate material or only a certain size-class (e.g. 5p-50pm) or a certain fraction (phytoplankton, detritus, microzooplankton) that is used as a food? (2) The ratio between the assimilation eficiencies of N and P is difficult to determine and is based on measurements with faecal pellets; such measurements are subject to error because the pellets can easily break during collection and release their contents, (3) Finally, the extents to which organic nitrogen and phosphorus are excreted is a major problem for when the' measured excretion truly represents the end products of metabolism, a 2 is equal to N,P,, but when secretions or unassimilated compounds are analysed together with the excreted products, this no longer holds.

149

The importance of the amounts of the products respired and excreted by copepods in the marine environment

In addition to knowing the kind of substrate oxidized during catabolic processes, the study of respiration and excretion can be used to assess the importance of copepods in the transfer of organic matter in the ecosystem. This is usually done for mixed zooplankton, only a fraction of which consists of copepods. However, if the part played by copepods alone is to be assessed, their excretion and respiration rates should be determined and then multiplied by their biomass, thus obtaining their contribution to the zooplankton as a whole. As this has seldom been done, data for mixed zooplankton are presented (Table 3.5) together with the contributions of copepods to the plankton samples. As they account for at least one-half of the biomass in . most cases, a fair idea of their role in nutrient and oxygen consumption cambe obtained.

The importance of copepods in oxygen consumption. The carbon dioxide produced by respiration is not rate limiting for photosynthesis in the marine environment. However, oxygen consumption by heterotrophic organisms may have a significant effect on natural levels of oxygen.

Table 3.5. Pcrcentage contributions of zooplankton excretion to phytoplankton nutrient requirement. (Copepods as a proportion of the total zooplankton is shown when stated).

Copepods as proportion of the total Excretion/phytoplankton Reference

Sea area and season zooplankton (per cent) requirement (per cent) -- ~

Aminoiiiuin excretion Long Island Sound (Oct.-June) Narragansett Bay:

spring autumn

Villefranche Colombia river mouth Ocean off Oregon North Pacific gyre Atlantic equator:

March July

N.W. Africa upwelling

Beaufort estuary (Sept.-April) Kuro-Shio (summer-win ter)

71.4,-100 of ind. 77

2.5 181.7

90 36 40-50*

80 Acartia clausi 23-43

64-83 (weight) 57 95 of animals 17

* 44

1 O0 6”

56-65 of animals 1 1-44

Harris (1959)

Martin (1 968)

Mayzaud (1971) Jawed (1973)

Eppley et a¿. (1 973) Le Borgne (1977)

Smith and Whitledge

Smith (1978)

Ikeda and Motoda (1 978)

(1977)

Narragansett Bay (annual cycle) 4.4 Peru upwelling 20 (weight); adults 5 N.W. Africa upwelling 1.5-5 1 Australian Great Barrier Reef 52-59.5 of animals 9.0-29.2

1n Inn* Urea excretion

Vargo (1979)

Dagg et a¿. (1980) Fernández (1981) Ikeda et al. (19826)

ivorrn raciiic gyre Atlantic equator:

'ku--Ju "pp'cy C l U l . ( 1 J I . I )

Le Borgne (1977) March 64-83 (weight) 57

N. W. Africa upwelling 44 July 95 of animals 17

Beaufort estuary (Sep t.-April) Kuro-shio (summer-win ter)

1 O0 6*

56-65 of animals 1 1-44

Smith and Whitledge (1977) Smith (1978)

Ikeda ancl Motoda (1978)

Narragansett Bay (annual cycle) Peru'upwelling N.W. Africa upwelling Australian Great Barrier Reef

Urea excretion North Pacific gyre Atlantic equator Beaufort estuary (Sept-April)

Phosphate excretion Narragansett Bay;

spring autumn

North Pacific gyre N.W. Mediterranean; spring Atlantic equator

March July

N. W. Africa upwelling Australian Great Barrier Reef

4.4

1.5-5 1 20 (weight); adults 5

52-59.5 of animals 9.0-29.2

40- 100* 2.2 8*

16.9 200 110-140*

2

52.0 26.7

6.6-25.6 2-74

Vargo (1979)

Dagg e! al. (1980) Fernández (1981) Ikeda et al. (19826)

Eppley et al . (1973) Le B,orgne (1977) Smith (1978)

Martin (1968)

Eppley et al. (1973) Nival el al . (1975) Le Borgne (1977)

Fernández (1981) Ikeda e! al. (19826)

Phytoplankton nutrient uptake.

152 The' Biological Chemistry of Marine Copepods

This is particularly true for the deep oxygen minimum; although the main heterotrophs are bacteria. In the upper layers of the oceans, however, which are generally oxygen-saturated through photosynthe- tic processes and exchanges with the atmosphere, 'respiration of net zooplankton appears to be 1.5 to 2.5 orders ofmagnitude smaller than the O, production implied by I4C productivity measurements' (Shulenberger and Reid 1981, for the 0-100 m layer of the mid- latitude Pacific). For the photic zone, net zooplankton appears to consume as much oxygen as microzooplankton (<200 fim) according to King, Devol, and Packard (1978) and Ikeda (1979). According to King et a l . (1 978), the ratio between zooplankton respiration, as determined by its electron-transport system activity, and that of all organisms collected on a fibre-glass filter (Gelman, type A) is equal to 62.4 per cent. This percentage decreases at deeper levels, falling to only 6.1 per cent between 200 and 600 m, a result which shows the minor role of zooplankton in the oxygen budget. Although such values are likely to change from one oceanic area to another, it is probably correct to conclude that oxygen consumption by copepods, which comprise only part of the total zooplankton, is negligible compared with the oxygen production by phytoplankton in the photic layer and is low compared with the oxygen uptake by other heterotrophic organisms at greater depths.

The importance of copepods in nutrient recycling Excreted ammonium and phosphate may be taken up quickly by phytoplankton (see review by Corner and Davies 1971) and phosphate by bacteria. Organic molecules are utilized by bacteria and, in some instances, by phytoplankton. This is so for urea (McCarthy 1971), amino acids (Stephens and North 1971) and phosphorus compounds (Corner and Davies 1971). The part played by zooplankton organic excretion in bacterial assimilation seems not to have been studied, mainly because the chemical forms of this material are not well known, the organic fraction usually being measured simply as the difference between total and inorganic excretions.

However, numerous studies have compared excreted ammonium and phosphate with phytoplankton needs (Table 3.5). The extent of primary production accounted for by zooplankton excretion lies between 2 and 200 per cent. There are several reasons for this wide range:

(1) Young ecosystems, such as those in upwellings, are characterized by new production from nitrate, whereas production from recycled

The rele

nutrients is more c For example, the p spring bloom in tc northwest Africa (: the Atlantic equatc 1977). (2) The assessmen is often based on I'(

into nitrogen and p some instances the ments with isotope: Smith and Whitled! differ depending up (NH,-N, N03-N, ur uptake is measurec bacteria, thus unde plankton P needs. (3) Amounts excre being studied. Thu: result from using la ture small animals tions of copepods ir determine. They are and are overestima body weights of COI

mesozooplankton (: estimated when the animals such as cop The percentages shc depending upon the

Because of these v of zooplankton excr shown in Table 3.5,

Conclusions

Copepods generally and can survive hai explain why their me that, in spite of me knowledge of respirí sources of variation,

- ,ope pods

inimum, although the layers of the oceans, through photosynthe- :re, respiration of net agiill.l !h. smaller than tivity ieasurements' m la- :r of the mid-

or ' acton appears to 10 ,um) according 79). According to

3n respiration, as ity, and that of all

.îi, type A) is equal to :eper levels, falling to :sult which shows the Although such values

mother, it is probably by copepods, which

s negligible compared n the photic layer and 'r heterotrophic organ-

ing taken up quickly by

vies 1971) and phos- :d by bacteria and, in for urea (McCarthy

and phosphorus com- layed by zooplankton ms not to have been ' this material are not ieasured simply as the ns. excreted ammonium

)le 3.5). The extent of inkton excretion lies ,eral reasons for this

ngs, are characterized iuction from recycled


nutrients is more characteristic of older and equ'ilibrated ecosystems. For example, the percentages shown in Table 3.5 are low during the spring bloom in temperate areas (Martin 1968), in upwellings off northwest Äfrica (Smith and Whitledge 1977; Fernández 1981) and the Atlantic equatorial upwelling, which occurs in July (Le Borgne 1977). (2) The assessment of phytoplankton nutritional needs is variable. I t is often based on "C uptake values which are subsequently converted into nitrogen and phosphorus needs by using C:N and C:P ratios. In some instances the calculation uses direct N and P uptake measurements with isotopes (e.g. Eppley, Renger, Venrick, and Mullin 1973; Smith and Whitlkdge 1977; Smith 1978; Table 3.5) and these uptakes differ depending upon whether only one compound or all compounds (NH,-N, NO,-N, urea-N) are considered. Moreover, when phosphate uptake is measured, it is concerned with both phytoplankton and bacteria, thus underestimating ratios between excretion and phytoplankton P needs. ( 3 ) Amounts excreted are a function of the zooplankton size-class being studied. Thus an underestimate of zooplankton excretion may result from using large-meshed nets (e.g. >500 ,um), that do not capture small animals with high excretion rates. Moreover, the proportions of copepods in zooplankton populations are rather difficult to determine. They are usually based on numbers of animals (Table 3.5) and are overestimates in terms of biomass because the individual body weights of copepods are often low compared with those of the mesozooplankton (>200 ,um). However, excretion may not be overestimated when the percentages of individuals are used because small animals such as copepods have higher excretion or respiration rates. The percentages shown in Table 3.5 lie between 50 and 100 per cent, depending upon the sea area concerned and the mesh-size of the nets.

Because of these various complications, estimates of the importance of zooplankton excretion in supplying nutrients to phytoplankton, shown in Table 3.5, should be regarded as tentative.

Conclusions

Copepods generally make up the bulk of marine zooplankton biomass and can survive harsh experimental conditions. These two reasons explain why their metabolism has been the subject of extensive studies that, in spite of methodological uncertainties, have provided a fair knowledge of respiration and excretion. The influences of the main sources of variation, i.e. temperature, food level and body weight, are


known and can be jointly used in modelling to give an estimation of the role played by copepods in marine food-webs.

One of the main developments in physiological studies on copepods has been the’combining of measurements of the different end products of catabolism. Because their respective contributions depend upon the kind of substrate being oxidized, O:N, O:P, N:P ratios have given useful information about methodological questions, the interpretation of rates and about seasonal variations and the efficiency of food utilization. However, it seems that in terms of methodology the ‘state-of- the-art’ needs further development, particularly as far as organic nitrogen and phosphorus excretions are concerned. Moreover, further advances could well exploit the use of O:N or 0 : P ratios in the calculation of growth efficiency, as has been done with N:P.

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