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Limnol. Oceanogr., 25(S), 1980,855-864@ 1980, by the American Society of Limnology and Oceanography, Inc.

In situ measurement of the settling velocity of organiccarbon particles and 10 species of phytoplanktonN. M. Burns and F. RosaNational Water Research Institute, Canada Centre for Inland Waters,P.O. Box 5050, Burlington, Ontario L7R 4A6

AbstractAn in situ method of simultaneously measuring the settling velocity of three size ranges ofparticulate organic carbon (POC) particles and 10 species of phytoplankton is described.The measurements took 2 h and were done at four different periods during the diurnal cycle.The POC fluxes of particles in the size ranges l-10 p, lo-64 p, and ~64 p were about equalfor each size range and showed very little diurnal variation. In contrast, the phytoplanktonshowed considerable response to the varying light intensities of a day. The flagellates mi-grated downward at sunset; a species of diatom settled more slowly during daylight than atnight; a blue-green species showed maximum buoyance at midnight and some green species

changed from downward sinking on a bright day to upward movement on a dull day.

The settling velocity of phytoplanktonwas considered as an important variablein one of the earliest quantitative phyto-plankton system models (Riley et al.1949). Since then, numerous estimates ofsettling velocities of phytoplankton andparticulate carbon have been used in dif-ferent models; however, relatively few insitu measurements have been made ofthe settling velocities of different phyto-plankton species (Eppley et al. 1968;Reynolds 1975, 1976; Happey-Wood1976; Kamykowski and Zentara 1977) orparticulate organic carbon (POC) (Burnsand Pashley 1974; Spencer et al. 1978;Linnegren 1979). In situ measurementsare essential because the settling velocityof phytoplankton can vary with season,nutrient concentrations, age of the pop-ulation, time of day, and relative bright-ness of the day. More such measurementsare needed if we are to reach an under-standing of the factors controlling the set-tling of living and detrital organic parti-cles.We here compare the POC settling ve-locities and fluxes in three size ranges:l-10 p, lo-64 p, and ~64 p, as definedby the mesh sizes of the screens used.We also compare the settling velocitiesof phytoplankton with those of POC par-ticles of similar size. Finally, we estimatethe contribution of phytoplankton carbonto the settling flux of POC from the epi-limnion. The effect of the brightness and

time of day on the settling velocity of 10different phytoplankton species is de-scribed.We thank L. Janus for counting thephytoplankton and P. J. Wade for remov-ing the zooplankton from POC samples.We also thank J. Leslie for his help.Methods

The settling experiments were carriedout at Lake St. George, a small, eutrophiclake (area = 5.8 ha, maximum depth = 15m), about 40 km north of Toronto, Ontar-io. Mooring systems were installed in themiddle of the lake so that cables ran fromsubmersible floats through pulleys at-tached to anchor weights and then towinches mounted ashore. The floatscould be submerged by winching the ca-bles in, thereby providing taut-wiremoorings. The floats were allowed to sur-face and the settling chambers retrievedby releasing the winched cables.During the experiment the epilimnionwas 4 m thick and isothermal at about20C and the thermocline extended from4 to 11 m. We sampled 10 liters of waterfrom the 3-m depth, Axed this water indarkened containers, and then poured itinto two settling chambers which wereplaced on the mooring cables. The cableswere winched in so that the floats werejust submerged and the l-m-long settlingchambers sat between 2.5- and 3.5-mdepth.

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856 Burns and Rosa

SECTION A-A

I- -REMOVABLE BAFFLES,-PLATE RELEASE

DRAIN HotES

SPRINGS

SEPARATOR PLATEOPEN POSITION

Fig. 1. Cross-sectional diagram of in situ settlingchamber.

The settling chamber used (Fig. 1) isa refinement of an earlier design (Burnsand Pashley 1974). The chambers andbaffles were made from clear acrylic plas-tic. The chambers were submerged andleft for 2 h; a messenger from the sub-merged float then released the plate fromits position beside the chamber and seg-mented the chamber into a 4.O-liter topsection and a 0.35-liter bottom section.After the chambers were retrieved fromthe mooring, water samples were drainedfrom the two sections of each and pre-pared for analysis.Duplicate 30-ml subsamples of waterfrom each sample were set aside for phy-toplankton analysis. The rest wasscreened through a 64-pm mesh screenand then through a lo-pm mesh screen;200 ml of this filtrate was then filteredthrough a Whatman GF/C glass-fiber fil-ter for POC analysis. The material re-tained on the 64-pm screen was resus-pended and preserved with Lugolssolution, the zooplankton manually re-

moved, and the samples were analyzedfor POC content. The material retainedon the lo-qn screen was washed directlyonto glass-fi her filters and analyzed forPOC content; examination of a randomselection of i,hese samples indicated veryfew zooplancton in them.Although each day was divided intofour periods (sunrise, 0600-0800; mid-day, 1100-1300; sunset, 1700-1900; mid-night, 2300-.OlOO), we could only carryout two expf:riments per day. Over the 4days, we were able to do two experi-ments for each period.The 10 dominant species were select-ed from a riumber of samples from the3.0-m depth. These species had cell orcolony sizes which would permit almostall of them to pass through the 64-pmmesh screerl, but be retained on the lo-pm mesh; they were counted in all sam-ples and formed the basis of comparisonof phytoplar kton with particulate organiccarbon in tire 10-64-p size range. Thecarbon contc:nt of the different algae wasestimated from their size (Mullin et al.1966).Each 30-ml duplicate was settled outand examined under an inverted micro-scope. The number of cells counted var-ied with the species: a minimum of 100individuals of the rarer species werecounted and 2,000 or more individualcells of the colonial species. A three-wayANOVA of the phytoplankton data wascarried out and the mean-square error ofthe complete data matrix per speciesused to calculate the variance in the de-termination of a concentration of thatspecies. This variance was then used incalculating the standard deviation in thecomputed settling velocities.The calclllation of the settling velocityand settlirrg flux of any variable isstraightforwfard if the concentration of thevariable in ,:he top and bottom sections ofthe settlirq: chamber have been deter-mined, The volume of water in the upperand lower sections is measured and thesevalues, together with data on the concen-tration difference between the upper andlower sections, make it possible to cal-culate the net settling velocity and net

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Settling velocities of organic particles 857settling flux of the particulate organic car-bon or phytoplankton.The initial concentration of POC in thesettling chamber, Ci, is given by

ci = (V,C, + V,C,)/(V, + V& (1)where Vt is volume of top section; Cl,final concentration of POC in top section;Vfi, volume of bottom section; and Cb,final concentration of POC in bottom sec-tion. The net transport from the top sec-tion of the bottle to the lower section, f,is given by

f= WCb - G> (2)d( V&/V, + VJ (3)

where d = Cn - Cl. If F is the net settlingflux, then F = f/(cross-sectional area) x(settling time). The net settling velocity,S, is given byS = FICi. (4)

The relatively short settling time ensuresthat the concentration of particles at thelevel of the separator plate (Fig. 1) re-mains close to the original concentration.The only variable in the right-hand sideof Eq. 3 is d and thus the uncertainty ofthe flux is determined by the uncertaintyin d.Validation of settling chambertechniques

The in situ settling chambers shouldnot alter the settling velocities of the phy-toplankton or POC particles if this tech-nique is to be valid. Since the settlingchambers are a new design, we hope todemonstrate that the settling velocitiesdetermined using them are similar tothose under natural conditions.Since the chambers are baffled, the de-gree of turbulence in them is less thanthat in the lake. Turbulence, however,only affects particle settling velocitieswhen the particle Reynolds numbers are~0.5 (Bloesch and Burns 1980; Hutch-inson 1967) and even these larger parti-cles are hardly affected by the degree ofturbulence prevalent in most lakes andoceans. In our study, very few of the par-ticles were large and the turbulencelevels in the small protected lake were

very low; thus the decrease of turbulencein the settling chamber would be unlike-ly to affect the settling velocity of the par-ticles. The reason for minimizing the tur-bulence in the settling chambers was toprevent the resuspension of material thathad settled onto the bottom of the cham-bers. Turbulence can be induced in un-baffled chambers by their rotation on themooring line or by jerking as the cham-bers are being set or retrieved. Also, set-tled material can be resuspended by con-vection currents caused by a temperaturedifference between the water in thechamber and the surrounding lake waterwhen the chambers are immersed.We did tests to determine whether thesystem of baffles was adequate to preventstirring of the water in the bottom seg-ments of the chambers. KMnO, crystalsput on the bottom of the chamber coloredthe water strongly for a distance of about5 mm from each crystal; the chamberswere then placed in an environment 5Ccolder, and later 5C warmer, than thewater inside. The baffling prevented con-vection currents from reaching the floorof the bottom section and did not disturbthe water colored by the crystals. Thechambers with KMnO, crystals in thebottom section were then hoisted on asimulated mooring system in a workshopand rotated on the mooring line: again,the colored water remained undisturbed.Finally, after closing the dividing plate,we simulated a violent retrieval of thesystem from the lake and found that thisdid mix the colored water throughout thebottom segment but none of this coloredwater mixed into either the box on theside of the chamber or the top segmentof the chamber. A violent retrieval wouldthus not invalidate the experiment pro-viding that the plate segmented the cham-ber before retrieval, which is the normalexperimental procedure.We have compared the settling veloc-ities determined for some species by oursettling chamber technique with veloci-ties determined by other investigators.The average settling velocity for Fragi-lariu crotonensis measured during ourstudy was 0.27 + 0.13 m * d-l (see Fig. 3d).

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858 Burns and Rosa

04 a0PHYTOPLANKTON CARBON lo-64~.

Fig. 2. Settling velocities and fluxes of phyto-plankton and detrital organic carbon in 10-64-prange.

This is as close as could be expected toa value of 0.43 5 0.22 m-d-l determinedfor the same species by Reynolds (1976)from observations of its disappearancefrom the epilimnion of a small Englishlake. In an in situ experiment, we foundAsterionellu formosu to settle with amean velocity of 0.40 + 0.28 m * d-l. Lab-oratory measured settling velocities forthis species (Smayda 1974; Titman andKilham 1976) averaged 0.31 m * d-l, withobserved values ranging from 0.09 m * d-lfor cells in growth phase to 0.63 for cellsin stationary phase. Phytoplankton set-tling velocities cannot be comparedstrictly because even for the samespecies they can be quite variable. How-ever, the above comparisons do illustratethat -the in situ settling chambers mea-

sure flux and settling velocity of the nat-ural organic particles at least as well asother techniclues now in use.Our settling chamber technique mea-sures the intrinsic settling velocity of par-ticles, not their loss rate from an epilim-nion. The loss rate can only be obtainedif both the c.ownward settling velocitiesand the upward transport of particles byvertical water movements are known.The upward transport of particles and thesuspending action of Langmuir circula-tions, however, seldom reach the fulldepth of an epilimnion (Scott et al. 1969)and thus caL se relatively little resuspen-sion of particles positioned just above thethermocline, For this reason the down-ward settling; flux, measured with settlingchambers jltst above the thermocline,will approximate the loss rate of particlesfrom the ep limnion (Bloesch and Burns1980).We attempted a direct check on the set-tling chamber technique by comparingdownward Cluxes measured by settlingchambers with those measured by a sed-iment trap. This is not an easy compari-son because the settling chambers mea-sure an almost instantaneous flux with a2-h settling period, while sediment trapsmeasure a flux averaged over an exposureperiod of a few days or more. In this test,we set two settling chambers at 8 m andtwo more at 25 m four times during a 24-hperiod and simultaneously exposed asediment tr,ip for 24 h midway betweenthe other dt:pths and within the thermo-cline at 15 n. Even when placed in tur-bulent water, sediment traps of good de-sign will measure the downward flux ofparticles in the calm water just above thebottom of the trap (Hargrave and Burns1979; Bloe:ch and Burns 1980); for this

Table 1. Average particulate organic carbon concentrations (mg C * m-), fluxes (mg C * m- * d-l), andsettling velocities (m.d-1) in three different size ranges, Lake St. Gt orge, 5-9 September 1977.L-10 /.I, 10-61/L MP

Concentration 1,107 + 66 198 + 3.0 136 & 8.8Fluxes 271 -+ 36 306 k 41 312 k 46Settling velocities 0.24 k 0.03 1.54 ?I 0.22 2.32 k 0.162 avz flux = 889 + 71

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Settling velocities of organic particles 859(a)

(d)UPWARD

BEHAVIOR 5

-SLlNklSE MID&Y SUNSET M~DF~~GHT

041it0211PWAm

(b)

It01 UPWARD(e)

NORMAL DAY

SUNRISE MlDbAVcr MloFilGHT

0.5

02 DOWNWARD

051 I---.SUNRISE MIDDAY SUdSET MIDNIGHT

a. Cryptomonas erosa AVCJ SETTLING VELOCITY q 0.31 + 032m*DAYb. Cryptomonas marsonll Avg SETTLING VELOCITY q 0.32f 032m-DAY-c. Rhodomonas mlnuta Avg SETTLING VELOCITY q 0.07 f 0.2lm*DAY-d Fragilaria crotonensis Avg SETTLING VELOCITY = 0.27f 013m*DAY-e Gomphosphaeria lacustns Avg SETTLING VELOCITY q 0.11 f 0.05m.DAY-f. Anabaena spiroides Avg UPWARD VELOCITY = 0.10 k Ollm - DAY-g. Selenastrum minutum Avg SETTLING VELOCITY = 0.15 f 013m *DAY4h. Closter ium parvulum Avg SETTLING VELOCITY = 0.18 f Ollm-DAY-i Scenedesmus acutlformls Avg SETTLING VELOCITY q 0.10 f O.O6m*DAY-1 Lagerhaemla quadnseta Avg SETTLING VEUXITY q 0.08 f O.llm-DAY-

Fig. 3. Settling velocities of different phytoplankton species as a function of time and relative bright-ness of a day. Numbers refer to dates in September 1977 when experiments were done.

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860 Burns and RosaTable 2. Light intensities (lux x 103) measured at midday at Lake: St. George, 6-8 September 1977.

0.5 1.0Depth (m)

2.0 3.0 4.0 Weather conditions

6 Sep7 Sep8 Sep5,9 Sep

3.90 2.76 1.59 1.04 0.60 Overcast, occasional dull sun2.15 1.55 0.94 0.58 0.31 T-hick cloud cover all day- 11.09 6.44 3.34 1.93 E:arly morning mist thencontinued bright sunNo midday values measured

reason, the flux measured in the calmwater inside settling chambers should besimilar to that measured in the calmwater inside sediment traps in the samewater column. The average fluxes mea-sured by the settling chambers in our ex-periment were 0.40 and 0.38 gPOC * mm2 d-l at the 8- and 25-m depths.The flux measured by the sedimentationtrap was essentially the same as that mea-sured by the settling chambers, 0.40 gPOC. rnm2. d-l.Red ts

The results of the organic particle set-tling investigation are summarized in Ta-ble 1. The values shown are averages ofall those determined except for the>64-p fraction, which does not includeany midnight values. There were largenumbers of zooplankton in the bottomsegment samples taken at midnight, andalthough nearly all were manually re-moved, the few remaining were enoughto contaminate the samples and invali-date these values.The small particles in the 1-10-p rangerepresented more than five times the car-bon of the particles in the other two sizeranges. Thus, although the small parti-cles settle slowly, the large concentrationof small particles resulted in their carbon

.flux being almost equal to the flux ob-served for the other two size ranges.The carbon contained in the differentphytoplankton cells was estimated fromthe cell volume (Mullin et al. 1966). Thetotal phytoplankton carbon concentrationand flux in .:he 10-64-p range was thenestimated by summing the values foreach param zter for each species. Thedata in Fig. 2 clearly show that the phy-toplankton carbon flux was negligiblecompared with the detrital carbon fluxand that the average settling velocity ofphytoplankton was

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Settling velocities of organic particles 861with time of day. While examining thesedata, one should keep in mind that thefigures represent the response of the totalpopulation of a species. Since senescentor recently dead cells (counted as beingalive) will show little diurnal pattern orresponse to light, they will settle down-ward at a relatively constant rate; thiswill diminish the estimated magnitude ofthe upward or downward movement of agroup of live, migrating cells.The scale of Fig. 3a and b is twice thatof Fig. 3c-j; thus, the flagellates Crypto-monas erosa and Cryptomonas marsoniican have greater settling velocities thanthe other species shown. Although C.erosa had a volume of about 2,000 p3 percell and C. marsonii about 400 t,cn, theyshowed almost identical settling veloci-ties and patterns of movement. Bothspecies remained almost neutrally buoy-ant from before midnight to after midday,but sank or migrated downward at a rel-atively high rate just before darkness.Rhodomonas minuta v. nannoplankti-ca, also a flagellate, had a pattern fairlysimilar to that of the other two crypto-monads, but showed considerable re-sponse to the relative brightness of theday. It continued its early morning move-ment upward until midday on the dullday, 7 September, but began movingdownward after sunrise of the normalday, 6 September.The only diatom, Fragilaria crotonen-sis, showed (Fig. 3d) a response differentfrom that of the flagellates, with the low-est settling velocity at sunset at the endof a bright day.The two blue-green species in thestudy reacted differently from each otherin their dirunal pattern. Gomphosphaerialacustris (Fig. 3e) did not show any dra-matic changes in settling velocity, al-though these colonial cells do appear tohave settled a little faster at the end ofthe photosynthetic period, at sunset atthe end of the brightest day, 8 Septem-ber. Anabaena spiroides (Fig. 3f) was up-wardly buoyant except in the morningwhen it was probably neutrally buoyant.Its buovancv increased from sunrise to

midday and then again from sunset tomidnight.The last species under consideration(Fig, 3g-j) are green algae, which oftenform a large part of the phytoplanktoncommunity in Lake St. George towardthe end of the stratified period. The mostinteresting feature of this group was thetremendous difference in response ofthose species to very dull conditions.Closterium parvulum and Selenastrumminutum reversed the normal downwardsinking trend to move upward during themidday period of the very dull day, whileScenedesmus acutiformis became neu-trally buoyant at the end of the dull day.Each species of green algae had a verylow average settling velocity.Discussion

The data in Fig. 2 show that virtuallyall the carbon flux out of the epilimnionin the 1-64-p size range is the result notof phytoplankton settling but of the set-tling of organic detritus. While phyto-plankton carbon made up about 25% ofthe organic carbon in that size range, thephytoplankton only accounted for about1.7% of the downward flux of carbon asa result of the low mean phytoplanktonsettling velocities (Fig. 3).The settling velocity and flux of detritalorganic carbon varied diurnally (Fig. 2),with the maximum in both variables dur-ing the sunset experiments. There was nosignificant change in the concentration ofdetritus at sunset; the increased flux wasentirely due to the increased settling ve-locity of the particles. We at first had dif-ficulty in finding a possible reason for thediurnal variation in the settling velocityof inanimate material. However, the dataon the zooplankton removed from thesamples showed that the sunset experi-ments contained higher numbers of zoo-plankton than any of the experimentsdone at other times (the water for theseexperiments was sampled 3 h beforedarkness). Enright (1977) hypothesizedand confirmed (Enright and Honneger1977) that zooplankton migrate upwardbefore darkness and begin grazing heavi-

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862 Burns and Rosaly when first arriving in the zone of pho-tosynthesis. The data in Fig. 2 wouldtend to support these ideas in that, justbefore darkness, an increased number ofactively grazing zooplankton would con-vert phytoplankton and smaller detritalparticles into fecal pellets at a higherrate. Since most fecal pellets settle fasterthan 1.7 m . d-l (Ferrante and Parker1977; Smayda 1971), the increased num-ber of fecal pellets would increase theaverage settling velocity of the organicparticles at sunset.The different species of phytoplanktonshowed very different responses to thevarying intensities of light during a day.The most striking feature about the set-tling behavior of C. erosa and C. mar-sonii is the downward migration of theseorganisms at sunset (Fig. 3a, b). The 2-hsettling period ran from 1700 to 1900hours with the onset of darkness later, atabout 1945. These organisms were prob-ably migrating downward in response tothe length of exposure to daylight ratherthan to the gradual decrease in light in-tensity before sunset. The pattern of de-scent before darkness is similar to thatobserved in test tank simulations of diur-nal migration of marine dinoflagellates,in which the organisms also migrateddownward an hour or two before the lightintensity changed abruptly when thelights were switched off (Kamykowskiand Zentara 1977; Eppley et al. 1968).Weiler and Karl (1979) found similar be-havior in a laboratory population of Ce-ratium furca and further observed thatthis organism migrated upward shortlybefore the lights were turned on. Someof the flagellates in our investigation mayalso have moved upward before sunrise;we never did an experiment at this timeof day. The response of R. minuta (Fig.3c) to high and low light intensities wassimilar to that observed by Happey-Wood (1976) who noted that R. minutamoved upward under intermittent cloudcover from before sunrise to midday andmoved downward as the skies clearedduring the afternoon and the light be-came more intense. Although all threeflagellated species (Fig. 3a, b, c) are pho-

totactic, R. minuta is obviously far moreresponsive t 3 relative light intensitiesthan the two Cryptomonas species.Fragilaria :rotonensis (Fig. 3d) appar-ently had its settling velocity diminishedby higher light intensity. There are noother published studies on this relation-ship for this organism but a maiine dia-tom, Ditylum brightwelli, has been ob-served to hnve its settling velocitiesincreased by photosynthesis (Andersonand Sweeney, 1977).The settling behavior of two other An-abaena species, Anabaena flos-aquae 2(Kanopka et ~1. 1978) and Anabaena cir-cinalis (Reyrolds 1975), has been inves-tigated. In both studies, the Anabaenaspecies were found to be buoyant at alltimes, but their buoyancy was reducedby photosynthesis. Our finding was sim-ilar in that A. spiroides was observed tobe buoyant c.t nearly all times and mostso at midniglrt.Settling velocities of the remainingfive 0rganisinsX. lacustris, S. minu-turn, C. parvullum, S. acutiformis, andLagerhaemk quadriseta (Fig. 3e, g, h, i,j)-have not been reported in the litera-ture, and we are unable to comment ontheir hehavicr as we did not carry out anyphysiologica studies.We have lrere developed informationon the rate < at which phytoplanktonchange their depth and how these ratescan change lvith time of day; it is of in-terest to examine the other factors whichchange with depth. The main variablesaffecting pjrytoplankton, which alsochange with depth, are light intensity,temperature, and nutrient concentra-tions. Light intensity (Table 2) decreasedconsiderably with depth and cloud cover;Fig. 3 shows that five of the ten speciesreacted to tl-e varying intensities on dif-ferent days and that all the species react-ed to the varying light intensities at dif-ferent times of day.We only studied the phytoplankton ata single tem:?erature during the 4-day in-vestigation and cannot comment on theeffect of temperature on settling velocity.Nutrient concentrations were mea-sured as a lunction of depth (Table 3).

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Settling velocities of organic particles 863The only soluble nutrient to show any ganic carbon flux in the eutrophic lakesignificant gradient through the thermo- studied.Cline was ammonium ion, which was As a contrast to the pattern of constantpresent in fairly high concentration in the settling of detrital carbon, almost all ofepilimnion and was thus unlikely to be the phytoplankton species investigatedlimiting. Soluble reactive phosphorus showed significant diurnal variation in(SRP) showed no variation with depth their settling velocities.and therefore would not affect the verti-cal behavior of the plankton. ReferencesWe must thus conclude that light hadthe greatest effect on the vertical distri-bution of the phytoplankton. Happey-wood (1976) came to the same conclu-sion after a diurnal study of the verticaldistribution of phytoplankton. Reynolds(1976) studied the vertical distribution ofseveral species over a summer and con-cluded that nutrient availability didcause changes in the species present,but, for long periods when there were nomajor changes in nutrient availability andhence no changes in species composi-tion, light was the controlling variable.Conclusions

We have shown that settling chamberscan be used to measure the diurnal fluxof organic carbon out of the epilimnion.The flux was almost equally divided be-tween the three size ranges (l-10 k, lo-64 p, and ~64 p). There was little varia-tion in the carbon fluxes during the 24-hdiurnal period One exception was theslight increase in both settling velocityand flux of detrital organic carbon in thk10-64-p size range shown during thesunset period (Fig. 2). This may resultfrom an increased number of fast settlingfecal pellets produced by the presunsetupward migration of zooplankton.Most phytoplankton were in the lo-64-p size range and, although phyto-plankton carbon comprised about 25% ofthe particulate carbon in this size range,phytoplankton carbon comprised only1.7% of the total downward flux becauseof the relatively low phytoplankton set-tling velocities. In the other size ranges,the phytoplankton carbon flux was amuch smaller fraction of the total carbonflux. Thus, phytoplankton carbon fluxescomprise ~1% of the total downward or-

ANDERSON, L. W., AND B. M. SWEENEY. 1977. Dielchanges in sedimentation characteristics of Di-tylum hrightwelli: Changes in cellular lipidand effects of respiratory inhibito rs and ion-transport modifiers. Limnol. Oceanogr. 22:539-552.BLOESCH, J., AND N. M. BUKNS. 1980. A criticalreview of sedimentation trap technique.Schweiz. Z. Hydrol. 41: in press.BURNS, N. M., AND A. E. PASIILEY. 1974. In situmeasurement of the settling velocity profile ofparticulate organic carbon in Lake Ontario. J.Fish. Res. Bd. Can. 3 1: 291-297.ENRIGIIT, J. T. 1977. Diurnal vertical migration:Adaptive significance and timing. Part 1. Selec-tive advantage: A metabolic model. L imnol.Oceanogr. 22 : 856-872.-, AND H.-W. HONNEGER. 1977. Diurnal ver-tical migration: Adaptive significance and tim-ing. Part 2. Test of the model: Details of timing.Limnol. Oceanogr. 22: 873-886.

EPPLEY, R. W., 0. HOLM-HANSEN, AND J. D.STRICKLAND. 1968. Some observations on thevertical migration of dinoflagellates. J. Phycol.4: 333-340.FERRANTE, J. G., AND J. I. PARKER. 1977. Trans-port of diatom frustules by copepod fecal pel-lets to the sediments of Lake Michigan. Lim-nol. Oceanogr. 22: 92-98.HAPPEY-WOOD, C. M. 1976. Vertical migrationpatterns in phytoplankton of mixed speciescomposition. Br. Phycol. J. 11: 353-369.HARGMVE, B. T., AND N. M. BURNS. 1979. As-sessment of sediment trap collection efficiency.Limnol. Oceanogr. 24: 1124-1136.HUTCIIINSON, G. E. 1967. A treatise on limnology,2. Wiley.KAMYKOWSKI, D., AND S. J. ZENTAM. 1977. Thediurnal vertical migrgtion of motile phyto-plankton through temperature gradients. Lim-nol. Oceanogr. 22: 148-151.KONOPKA, A., T. D. BHOCK, AND A. E. WALSDY.1978. Buoyancy regulation by planktonic blue-grcen algae in Lake Mendota, Wisconsin. Arch.Hydrobiol. 83 : 524-537.L;~NNEC,REN, C. 1979. Buoyance of natural popu-lations of marine phytoplankton. Mar. Biol. 54:l-10.MULIJN, M. M., P. R. GOAN, AND R. W. EPPLEY.

1966. Relationship between carbon content,cell volume and area in phytoplankton. Limnol.Oceanogr. 11: 307-3 11,REYNOLDS, C. S. 1975. Interrelations of photosyn-

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864 Burns and Rosathetic behaviour and buoyance regulation in anatural population of a blue-green alga. Fresh-water Biol. 5: 323-338.-. 1976. Sinking movements of phytoplanktonindicated by a simple trapping method. 1-AFragilnriu population. 2-Vertical activityranges in a stratified lake. Br. Phycol. J. 11:279-291,293-303.HILEY, G. A., H. STOMMEI,, AND D. F. BUMPUS.1949. Quantitative ecology of the plankton ofthe western North Atlantic. Bull. BinghamOceanogr. Collect. 12: 1-169.SCOTT, J.T.,G.E. MEYER, R. STEWART,AND E.G.WALTIIER. 1969. On the mechanism of Lang-muir circulations and their role in epilimnionmixing. Limnol. Oceanogr. 14: 493-503.SMAYDA, T. J. 1971. Normal and accelerated sink-ing of phytoplankton in the sea. Mar. Geol. 11:105-122.

-. 1974. Scme experiments on the sinkingcharacteris tic.; of two freshwater diatoms. Lim-nol. Oceanog *. 19: 628-635.SPENCER, D. W. AND OTHERS. 1978. Chemicalfluxes from a sediment trap experiment in thedeep Sargass I Sea. J. Mar. Res. 36: 493-523.TITMAN, D., ANI) P. KILHAM. 1976. Sinking infreshwater plankton: Some ecological implica-tions of cell nutrient status and physical mixingprocesses. Li nnol. Oceanogr. 2 1: 409-4 17.WEILER, C. S., AND D. M. KARL. 1979. Dielchanges in phased-dividing cultures of Ceru-tiumfurcu (Ibinophyceae): Nucleotide triphos-phates, aden {late energy charge, cell carbonand patterns of vertical migration. J. Phycol.15: 384-391.

Submitted: 20 November 1979Accepted: 8 April 1980