Estuarine, Coastal and Shelf Science (1986) 23,263-276

Fluctuations in the meiofauna of the Aufwuchs community in a brackish-water lagoon

Colin Little Department of Zoology, Bristol University, Bristol BS8 1 UG, U.K.

Received 22 August 1985 and in revisedform 18 November I985

Keywords: lagoons; meiobenthos

The organization of the Aufwuchs community in a brackish-water lagoon (Swanpool, Falmouth, U.K.) is described. Changes in the population densities of encrusting bryozoans and mobile meiofauna are described for a period of 3 years.

Most meiofaunal species reached peak densities in the spring (January- March). These included tardigrades (Macrobiotus sp.), oligochaetes (N&s elinguis, Chaetogaster diuphanus), the harpacticoid copepod Schizopera clandes- tina, ostracods, the nematodes Dichromadora geophila and Theristus spp., and

possibly the nematodes Chromadorina germanica and Atrochromadora micro- laima. Other meiofaunal populations peaked in summer (July-September), and these included the chironomid Chironomus salinarius, the harpacticoid copepod Nitocra spinipes and the nematode Adoncholaimus thalassophygas. Two further species, the mite Halacarus balticus and the nematode Aphelencoides sp., showed irregular bursts in numbers.

It is concluded that the spring-peaking species increased in numbers depen- dent upon the growth of the Aufwuchs, and particularly of the surface film of diatoms, while the summer-peaking species may have been controlled more by limiting values of salinity and temperature. These conclusions are contrasted with the general view of salinity as the over-riding factor in brackish-water ecosystems.

Introduction

In a previous paper (Little, 1984) the population densities of an oligochaete, Nuis elinguis

Miiller, living in the Aufwuchs community of a brackish-water lagoon were followed over a period of 4 years, and an attempt was made to relate densities to cyclic changes in physical factors. It was concluded that although the timing of fluctuations in population density of N. eknguis usually coincided with changes in temperature and salinity, a direct causal effect of such physical factors was unlikely to account for the population fluctuations. Studies on the macrofauna of the same lagoon (Barnes et al., 1979) showed that in some species major fluctuations in density coincided with migrations into and out of the pool, while in others fluctuations were correlated with particular combinations of salinity and temperature. In the present paper, fluctuations of a number of meiofaunal species inhabiting the Aufwuchs film (the microscopic flora and associated detritus attached to

263

0272-7714/86/080263+ 14 $03.00/O 0 1986 Academic Press Inc. (London) Limited

264 C. Little

hard surfaces) are considered, and are discussed in relation to changes in temperature, salinity and physical structure of the ecosystem.

Methods

Locality Observations were made in Swanpool, a brackish-water lagoon at Falmouth, Cornwall, U.K. Previous studies have provided a general background to the conditions in this lagoon (Barnes, Dorey & Little, 1971; Dorey, Little & Barnes, 1973; Little, Barnes & Dorey, 1973; Barnes et al., 1979; Crawford et al., 1979).

Investigation of meiofaunal populations The study was intended to follow changes in a meiofaunal ecosystem over a number of years. In order that this could be achieved, it was important to be able to follow changes in the physical structure of the environment as well as changes in the fauna. The layer of sediment, algae and sessile fauna coating rocky surfaces in Swanpool was selected as ideal for this purpose because it could be observed in the laboratory without disturbance, unlike shingle and mud environments, which are inevitably disturbed during sampling. Removal of natural rock surfaces would, however, have been inconvenient for routine analysis, and artificial substrates were provided instead. These were terracotta tiles approximately 100 x 150 x 10 mm. The tiles were placed in the pool at least one year before they were used for analysis, and during this time developed a coating of sediment and algae apparently like that found on natural stones (Little, 1984).

Tiles were placed at the north end of the pool near the inflow from a storm drain (referred to as the ‘ storm-drain station ‘), and at the south end near the outlet from the pool, through which sea water flows into the pool at high water of spring tides (the ‘ outlet station ‘). Tiles were collected and preserved as described by Little (1984). Meiofaunal populations were sampled using a circular quadrat of area 284 mm’, positioned within a squared grid using random numbers. The material enclosed was shaved off with a scalpel and examined and sorted using a magnification of x 25. Efficiency of extraction was tested only for the oligochaete Nais elinguis (by adding preserved worms to cleared samples) for which it was 88% + S.D. 5.1. It was undoubtedly lower for smaller animals. Five quadrats were taken from the top, and five from the bottom, of tiles collected at intervals of 2 months.

Hydrography Measurements of surface salinity and temperature and vertical profiles were taken every 2 months with an MC5 bridge (Electronic Switchgear Ltd.). Vertical profiles were necessary in order to estimate possible short-term variations in salinity between the 2 month intervals (Dorey, Little & Barnes, 1973).

The height of the water surface in the pool to some extent reflects the volume of fresh water entering through the two stream systems. It was measured at the outlet in relation to an arbitrary ‘ pool zero ’ (see Dorey, Little & Barnes, 1973). From 1970 to 1981, 24 observations were made of the height of the water surface in the pool and that in the main stream. These two heights are strongly correlated (r = 0.92; P < 0.001). Measurements of flow rate in the stream, as opposed to stream height, were made on only 5 occasions. Flow rates of 64-73 m3 h-’ occurred at pool heights of - 7 to - 12 cm. Flow rates of

Meiojauna ojan Aujwucks community 265

Salinity ( %o)

1

52 -c EO 8

1

2

+6 LI 0

1 +12

2 r

+29 E, +22

t;

-8 K -2 K -8 IT

-8 K -11 FT

-8 F +28 “:, 1980

1981

1982

Jan. Mar. May July Sept. Nov

Figure 1. Salinity profiles in Swanpool. For each date, the number shows the height of the pool surface (in cm) with reference to ‘pool zero’.

370-390 m3 h- ’ occurred at pool heights of + 6 to + 9 cm (Dorey, Little & Barnes, 1973, and personal observations).

Results

Hydrography The data provided by Little (1984) show that over the study period surface levels of oxygen did not fall below saturation, although vertical oxygen profiles varied both over seasonal cycles and between years. Temperature showed peak values in mid-summer, while salinity tended to rise gradually over the summer to peak in September. The pattern of salinity changes may be considered in more detail by reference to Figure 1, in which vertical salinity profiles are given. In January, surface salinities were low, and the halo- cline was usually at or well below 1.5 m depth. This correlates well with the high rate of freshwater inflow (pool heights of +6 to + 12 cm). The spring tides of March/April produced a rise in the vertical position of the halocline (except in 1979), and this allowed rises in surface salinity to develop in March (1980,198 I), or May (1979,1982). During the summer months, freshwater inflow was low (pool heights - 2 to - 11 cm) and surface salinity remained high. In September, spring tides entered the pool to a greater extent than in March, presumably due to low pool heights (see Dorey, Little & Barnes, 1973 for discussion), and surface salinities reached their highest levels at this time. After

266 C. Little

Dichromadora

~dogoni!~~opyrguk~cod Diatom layer

“7 ygbya

Schizopera

Adoncholaim

Algal film- :

3

2 mm

1

0

Manganese layer

Old statoblast F

0

1

2 of Plumatella

/

/ Macrobiotus

1 \

\ Chironomus

Plumatella Zoothamnium zooid Nais

Figure 2. Diagrammatic section showing the surfaces of a tile from the storm drain with maximum development of Aufwuchs (March 1980). Stipple shows sediment.

September, increasing freshwater inflow, presumably coupled with increased wind action, reduced the height of the halocline and also reduced surface salinity. It should be noted, however, that the fairly repeatable pattern recorded in this study ignores short- term variation, such as that recorded by Crawford et al. (1979). Particularly when the halocline was near the surface, sudden short-term changes in the salinity of surface water may have occurred.

General outline of the ecosystem, and encrusting fauna The upper surfaces of the tiles showed a fairly regular alternation between growth of Aufwuchs in the spring and early summer (March-May), and a very much reduced cover in late summer and autumn (July-September), although the exact periods of growth varied over the four study years. Very little surface growth was present in early winter (November-December).

At the storm drain station, the dominant alga was the bluegreen Lyngbya Zimnetica Lemmerman which formed a mat up to 3 mm deep, in which fine sediment was entangled. Usually a thin layer of encrusting algal cells (not identified) was present covering the tiles beneath the Lyngbya. A layer of diatoms such as Nitzschia sp. and Navicula sp. often formed at the surface of the alga/sediment mat, and occasional growths of Oedogonium sp. and Microspora sp. penetrated through the mat. Figure 2 shows the organisation of this Aufwuchs layer.

At the outlet station, the upper surfaces of the tiles were often similar to those at the storm drain, but in some summers the bryozoan Victorellapavida Kent was often present and at times almost replaced the algal mat. Occasional growths of Calothrix sp. were found, and the Lyngbya was usually underlain by an encrusting algal film. Both the

Meiofauna of an Aufwuchs community 267

Enteromorpha Victorella zooid LYngbYa

Adoncholaimu 3

2 mm

1

Manganese lay

Halacarus

\ resting body

Zoothamnium Figure 3. Diagrammatic section showing the surfaces of a tile from the outlet with maximum development of Aufwuchs (May 1979). Stipple shows sediment.

bryozoan growths and the algal filaments trapped sediment particles to form a system very much like that at the storm drain. This system is shown diagrammatically in Figure 3.

The bottom surfaces of tiles at both sites were coated with a black inorganic film. The composition of this film was investigated by X-ray microanalysis. It consisted of manganese nodules with a very low iron content, and was entirely inorganic (D. W. Thompson, personal communication). High concentrations of dissolved manganese have already been reported from the waters of the pool (Crawford et al., 1979).

During the winter months the undersides of the tiles supported very little encrusting growth. In the spring, the tiles at the storm drain were dominated underneath by the bryozoan Plumatella repens (L.) (Figure 2), but during the summer this was to some extent replaced by Victorella pavida. At the outlet, V. pavida was the only common bryozoan, appearing in early summer and lasting sometimes as late as November. In the winter months, the resting bodies of I’. pavidu were often abundant, either attached directly to the manganese nodule layer, or to the encrusting algal film.

Oligochaetes The most abundant oligochaete present was Nais elinguis Miiller. This species showed a highly seasonal pattern of abundance, being found during the study years in large numbers only from January to May. Details were given by Little (1984), and the species is not considered further in the present paper.

A second species of oligochaete, Chaetogaster diaphanus (Gruithuisen) was occasionally present, but only at the storm drain, and only from March to May. Observations in April 1978 suggested that this species was in the main carnivorous. It was seen to eat individuals

268 C. Little

b : Outlet (top)

Figure 4. Density of chironomid larvae on upper surfaces of tiles at the storm drain and the outlet. Points are means (n = 5), derived from quadrats of area 2.84 cmz. Bars show SD.%.

of its own species, and its gut also contained moribund rotifers and tardigrades. Numbers were always low, and no quantitative estimates are given here.

Chironomid larvae These were most frequently recorded in the summer months (Figure 4). At the storm drain, densities were high on the tops and bottoms of the tiles (Little, 1984), but at the outlet high densities were only recorded on upper surfaces. Peak densities at the two stations often did not coincide. It was also noted that larvae were often common on the sides (edges) of the tiles, but no quantitative estimates of abundance there were made.

Because of difficulty in identifying larvae, especially early instars, a sampling pro- gramme for examining pupal exuviae was carried out between March 1981 and May 1982. A fine-mesh dip net was used to collect pupal exuviae from the accessible areas of the south, east and north shores of Swanpool. Approximately 200 exuviae were examined in each collection, or as many as were available if less than that. Identifications were made by reference to Wilson & McGill (1982), Langton (1984) and the reference collections of Dr. R. S. Wilson. Results are presented in Figure 5. For most of the year the Chironomini dominated the community. The most abundant member of this group was Chironomus salinarius Kieffer, but in May and July this was accompanied by Glyptotendipes sp., Parachironomus sp., Dicrotendipes sp. and Kiefferulus sp. The Tanytarsini (Microspectra sp., Paratanytarsus sp. and Rheotanytarsus sp.) were never common, nor were the Prodia- mesini (Prodiamesa sp.). The Orthocladiini (Smittia sp., Cricotopus sp., Eukiefferiella sp., Brillia sp., Paratrissocladius sp., Psectrocladius sp., Rheocricotopus sp. and Bryophaeno- cladius sp.) formed an insignificant fraction of the sample except in November 1981, when Bryophaenocladius sp. comprised more than 50%. The Tanypodini (Procladius sp.,

Meiofauna of an Aufwuchs community 269

80-

7 3 9 1 4 204

! 100 63 12 57 219

Mar. May July Sept. Oct. Nov. Jan. Mar. May

1981 1982 Figure 5. Percentage relative abundance of taxa of chironomid pupal exuviae. The numbers below the histograms show number of taxa (upper figure) and number of exuviae in the sample (lower figure). White circles show the Menhinick diversity index ( S/N@5, where S = total taxa, N = total number of exuviae).

-2 ~ m Tanypodini

c

i!

9 Prodiamesini

0 o Orthociadini -1 y

1: IW Chironomini L (except C.sallnarlus)

k m C.salinarius 2

-0 (=I Tanytarsini

Macropelopia sp. and Conchapelopia sp.) were present throughout the year, but formed more than 100/b of the samples only in October 1981 and March 1982.

Chironomid species diversity, as expressed by the Menhinick diversity index, was never high, and rose above 1.0 only in September and November 1981. At all other times, it remained near or below 0.5.

Mites The halacarid mite Halacarus (Halacarellus) balticus Lohmann was only occasionally present at the storm drain, but showed pronounced population peaks on the undersides of tiles at the outlet (Figure 6), in association with colonies of Victorella pa&a. The two periods of high mite density were not, however, seasonally related. The first was in May-July 1979, and the second in November-January 1981. In the 1979 period, the bryozoan colonies were actively growing, but in January 1981 the zooids had to a large extent decayed, and only resting bodies were intact. Mites were abundant within and around the decaying zooids.

Copepods Both cyclopoid and harpacticoid copepods were recorded in this study. The cyclopoid Cyclops agilis (Koch), which was also found in the plankton (see Crawford et al., 1979), was common on the upper surfaces of tiles at the storm drain in the early summer of 1979: densities up to amean of4.4 cmm2 wererecorded in July. However, this ’ bloom ‘appears to have been an isolated one, and the copepod populations usually consisted in the main of harpacticoids. The varying abundance of these is shown in Figure 7. At the storm drain, populations were highest in January-March. At the peak of these periods, the population consisted entirely of Schizopera clandestina (Klie), and it is assumed that this species was the only one common at this station. At the outlet, populations were less dense, and peaked rather later in the year. In May 1979, January 1981 and May 1981, they consisted entirely of Nitocra spinipes (Boeck), and it is assumed that the record in Figure 7 shows the densities of this one species. Both the species identified are typical of brackish water (Dr

270 C. Little

2- 2- -a:Outlet (top) -a:Outlet (top)

l- l-

o o b 7 T y++ 7 b 7 T y++ 7

8 8r

7 c 7-

6 - ” b : Outlet (bottom)

Figure 6. Density of the mite Halacarus balticus on tiles at the outlet. Points are means (IV= 5), derived from quadrats of area 2.84 cm’. Bars show S.D.‘s.

5-

a : Storm Drain (top)

1 I 1 1 1 1 IA

b : Outlet (top)

O,,,rr,,

1979 I

I I 1 I 1

I

ITTTII MMJSNJMMJSNJMMJSNJMM

1980 1981 I 1982 Figure 7. Density of harpacticoid copepods on upper surfaces of tiles at the storm drain and the outlet. At the storm drain, all identified individuals were Schizopera clundestina. At the outlet, all identified individuals were Nitocra spinipes. Conventions as in Figure 6.

J. M. Gee, personal communication), but in Swanpool S. clandestina appeared only at times of salinity minima, with a pattern very similar to that of the oligochaete Nais elinguis. The annual pattern of N. spinipes at the outlet was also similar to that of N. elinguis at that station, with later and smaller population peaks.

Ostracods These were almost exclusively confined to the upper surfaces at the storm drain, and were present only from January to May (Figure 8). The species were not identified, but all were

Meiofauna of an Aufwuchs community 271

6-

5- Storm Drain (top)

MMJSNJMMJSNJMMJSNJMM 1979 1980 1981 1982

Figure 8. Density of ostracods on upper surfaces of tiles at the storm drain. Conventions as in Figure 6. Arrows show times at which tardigrades (Macrobiotus sp.) were recorded.

smooth-shelled, indicating an essentially freshwater origin. Population peaks occurred at the same times as those of N. elinguis and S. clandestina.

Tardigrades Macrobiotus sp. (M. ambiguus or M. macronyx) were found only at the storm drain, mainly on lower surfaces, up to an average density of 1.8 cm - ‘. They occurred only from January to May (Figure 8), and thus had a pattern of occurrence similar to those of N. elinguis, S. clandestina and the ostracods.

Nematodes Densities of nematodes were always higher on upper surfaces than on lower surfaces, where they seldom rose above 10 cm - ‘. Figures for upper surfaces are presented in Figure 9. The patterns at the outlet and storm drain were different, with the storm drain showing high densities each summer, but the outlet showing no overall seasonal pattern. Densities at the outlet rose to high levels in the autumn of 1979 and remained high until spring 1980. They then fell and did not rise again until the spring and summer of 1981.

The high densities at the storm drain were comprised almost entirely of AdonchoZaimus thalassophygas (De Man), which therefore at that site appears to be a ‘ summer species ‘. Only two other taxa (Dichromadorageophila (De Man) and Theristus spp.) were abundant, and these only in January/March, so that at this station these were considered ‘ spring species ‘. Theristus spp. were commonly seen to contain many diatoms (NavicuZa sp.).

At the outlet, more species were recorded. In addition to A. thalassophygas, D. geophila and Theristus spp., three species were abundant: Chromadorina germanica (Biitschli), Atrochromadora microlaima (De Man) and Aphelencoides sp. A further two species, Eurys- tomina terricola (De Man) and Metachromadora remanei Gerlach were recorded but never formed large populations. A. thalassophygas was abundant in May, July and September (in different years), and its label as a ‘ summer ’ species derived at the storm drain was confirmed. Aphelencoides sp. formed the next largest populations at the outlet, but on only three occasions (November 1979, July 1980 and September 1981). The remainder of the species were for the most part common in January, March and May, but C. germanica was also found in significant numbers in July, September and November. While these species may therefore be termed ‘ spring ’ species in Swanpool, this description is not as clear cut as for some of the other ‘ spring ’ meiofauna.

272 C. Little

100

80

YE 60

”

:: z 40

20

0

100

80

‘;J 60 E ”

: z 40

20

0

a: Storm Drain (top)

I b: Outlet (top)

w Aphelencoides sp

se Chromadorina germanica

m Atrochromadora microlaima

n Adoncholaimus thalassophygas

o Dichromadora geophila

I Theristus spp.

Figure 9. Density of nematodes at the storm drain (a) and outlet (b). Black circles and solid line show total densities (all species), with the bars showing one S.D. Histograms show densities of individual taxa. The category ‘ The&us spp. ’ includes Theristus acer Bastian, Theristusfkvensis Stekhoven and Mesotheriws setosus (ButschZz>.

Population composition shown in the histograms was estimated by identifying a random subsample of 10-12 individuals in each quadrat, summing the totals from all 5 quadrats, and calculating percentages in this total of 5C-60 individuals.

Meiofauna of an Aujwuchs community 273

Other fauna Associated with the Aufwuchs community were large populations of rotifers and protists. Neither were preserved by the methods employed, and no identifications were made except for the colonial sessile peritrich Zoothamnium sp. This was common on the bottom surfaces of tiles at both sites at most sampling times except November.

Densities of associated macrofauna were not assessed quantitatively because the large- scale patchiness of their distribution would have involved a different scale of sampling. Qualitative notes were made of the presence of the three major species of invertebrates. At both the outiet and the storm drain, the amphipod crustacean Gammarus chevreuxi Sexton was occasionally found on the tiles, especially as juveniles. The abundance of this species has been examined by Barnes et al. (1979) who found that it showed irregular fluctuations in numbers, apparently unrelated to season. It was more active at night, and may have visited the tiles from daytime resting places in the underlying shingle.

At the outlet, the turbellarian Procerodes Zittoralis (Strom) was frequently found on the under surfaces of the tiles. It was, however, not well preserved by the fixation procedure adopted. Its occurrence was both patchy and sporadic.

At both stations, the gastropod Potamopyrgus jenkinsi (Smith) was common, especially on under surfaces. Juveniles were often extracted from the algal/sediment mat. Their distribution was extremely patchy from one tile to another.

Discussion

The taxa identified in this study fall into three groups, defined by the times of their population peaks. Several species peaked in the spring, or were only found then: Macro-

biotus sp., Nais elinguis, Chaetogaster diaphanus, Schizopera clandestina, ostracods, Pluma- tella repens, and the nematodes Dichromadora geophila and Theristus spp. The nematodes Chromadorina germanica and Atrochromadora microlaima were also commonest in the spring, but were not restricted to such a narrow period. Other species had population peaks in the summer: Chironomus salinarius, Nitocra spinipes, Adoncholaimus thalassophy- gus, Zoothamnium sp., and Victorellapavida. A third group showed less regular Auctations in population density, with no apparent seasonal pattern (Halacarus balticus) or with rather irregular bursts in numbers (Aphelencoides). These species are not considered further. Because of the fairly regular seasonal cycles of salinity and temperature in the surface waters of the pool (Little, 1984), the spring and summer groups might superfi- cially be considered a ‘ freshwater ’ group and a ‘ saline ’ group, respectively. The theme of this discussion is that although the summer-peaking species may be limited by salinity and temperature, the spring-peaking species are controlled by other factors. In more general terms, it is suggested that although salinity can be of over-riding importance in determin- ing which component species are present in brackish-water ecosystems, it may have little to do with the timing of their population growth and decline.

Spring-peaking species Taxa with population peaks in the spring increased in numbers as the alga/sediment mat of Aufivuchs increased in thickness and complexity, while salinity was low (<4!%&) and temperature was low (cu. 10 “C) but rising. Populations of N. elinguis, C. diaphanus, Macrobiotus sp., D. geophila, S. clandestina, ostracods and P. repens then declined in summer, usually by May (Figures 7, 8; Little, 1984). Populations of the nematodes Theristus spp., C. germanica and A. microlaima also usually declined, but not as rapidly. In

214 C. Little

May, the Aufwuchs was still dominant, and salinity was still low ( < 4kS), while tempera- ture had risen to about 15 “C. Of the taxa in this group, the oligochaetes, harpacticoid copepods and nematodes are characteristic of brackish waters (Brinkhurst, 1963, 1982; Lang, 1948; Warwick, 1971). The bryozoan P. repens is also found typically in brackish as well as fresh water (Prenant & Bobin, 1956). The tardigrades, alone, have not apparently been recorded from brackish waters (Morgan & King, 1976). Since the populations of all these species had either declined or disappeared before salinity reached 4%oS, increasing salinity is unlikely to have provided a direct cause of the population declines. As discussed by Little (1984), for N. elinguis, food supply, competition and predation may be more important for the whole of the spring-peaking group of species.

Sufficient information about the food requirements of the nematode species is available to allow brief discussion of the relevance of this one factor to population growth. The four common spring-peaking taxa, D. geophila, The&us spp., C. germanica and A. micro- Eaima were classified by Wieser (1953) as epigrowth feeders or non-selective deposit feeders. Generation times for these species (in culture) are of the order of 20-30 days (Gerlach, 1971). They probably fed mainly on the diatom layer present on the surface of the sediment (personal observations; Tietjen & Lee, 1977). Since the diatom layer appeared early in the spring, the population peaks of these rapidly reproducing taxa could occur early in the year.

Summer-peaking species By July-September, the time at which summer-peaking species were most abundant (Figures 4,6,7,9), the algal/sediment mat on upper surfaces had usually been reduced to a thin layer of algal filaments, salinity was high (6-8%0s), and temperature had reached 18-22 “C (Little, 1984). For several ofthe species concerned, there is already evidence that these summer values in Swanpool are near the optimum salinities and temperatures. For I/. pa&da, temperatures above 12 “C and salinities above 3%oS are optimum (Brattstrom), 1954; Carrada & Sacchi, 1964; Menon & Nair, 1972). Chironomus salinarius has been shown to be dominant only in salinities from 5-22OkS (Muus, 1967; Parma & Krebs, 1977; Krebs, 1982), while a decline of salinity from 7-190/o& down to < I%& destroyed a whole C. salinarius population (Lindegaard & Jonsson (1983). Low salinity in winter and spring would therefore have prevented the growth of large populations of this species, and low temperatures would have increased the generation times of the overwintering population (Thienemann, 1954). From the low numbers of pupal exuviae obtained in the winter months in Swanpool (Figure 5), it was also apparent that large populations of flying adults were not present until May, so that large numbers of first instar larvae would not be expected until the summer period. The harpacticoid copepod N. spinipes is common only above 2XS (Lang, 1948). For these three summer-peaking taxa, therefore, unlike the spring-peaking taxa, salinity and/or temperature are probably the most important controlling factors. The structural development of the Aufwuchs, on the other hand, is probably not so important for them, as they became common during the decline of the Aufwuchs from its maximum development in early spring.

The possible exception in the summer-peaking species is the nematode A. thaZasso- phygas. This has been recorded over a wide salinity range (Warwick, 1971), but little is known of its salinity preferences. It was classified by Wieser (1953) as a predator/ omnivore, and it has a relatively long generation time (in culture) of 55-72 days (Gerlach, 1971). It presumably did not form large populations early in the year at least partly because of this.

Meiofauna of an Aufwuchs community 275

In summary, the major meiofaunal and encrusting taxa identified in the present study can be considered in the main to fall into two groups. One group had population peaks in the spring, which increased as the Aufwuchs increased, and were probably dependent upon this for their food supply and for appropriate microhabitats. These populations were probably not greatly affected directly by changes in salinity and temperature, but died out or declined in early summer due to the effects of competition, predation and declining food supply. The second group had population peaks in the summer, increasing as salinity and temperature increased, and were not so dependent on the growth of the Aufwuchs.

It is appropriate here to draw parallels between the species examined in this paper and those of estuaries. It has been concluded above that the populations of spring-peaking species are not directly governed by factors such as temperature and salinity. This con- clusion contrasts strongly with the implicit assumption generally made for estuaries and other brackish waters, that salinity is of over-riding importance (e.g. Kinne, 1971). This assumption probably derives from early work (e.g. Remane & Schlieper, 1971) which related species numbers to salinity, and showed a minimum number of species at 5-8XS. As pointed out by Little (1984), however, physical factors such as salinity and temperature may not play direct causal roles in controlling animal populations in brackish waters. It is more probable that animals in saline lagoons and estuaries are physiologically adapted to withstand fluctuations in salinity and temperature between wide limits (see, e.g. Vernberg & Vernberg, 1972; Newell, 1979). Fluctuations in their populations would therefore to a great extent be independent of fluctuations in salinity and temperature. The evidence from the spring-peaking species in Swanpool supports this suggestion.

Acknowledgements

I would like to thank the many experts who have helped with parts of this work, particu- larly with identifications: Dr R. M. Crawford (algae), Dr D. J. Patterson (protists), Dr R. S. Wilson and Ms. E. Wells (chironomids), Dr J. M. Gee (harpacticoid copepods), and Dr C. J. Mapes (nematodes). I am grateful to Dr D. W. Thompson for examining the manganese layer and to Dr A. E. Dorey, Dr D. J. Patterson, MS P. E. Stirling and Dr C. J. Webb for help with fieldwork. Lastly, I am indebted to Dr R. S. Wilson and Dr C. J. Mapes for their comments on the paper.

References

Barnes, R. S. K., Dorey, A. E. & Little, C. 1971 An ecological study of a pool subject to varying salinity (Swanpool, Falmouth). An introductory account of the topography, fauna and flora. Journal of Animal Ecology 40,709-734.

Barnes, R. S. K., Williams, A., Little, C. & Dorey, A. E. 1979 An ecological study of the Swanpool, Falmouth. IV: Population fluctuations of some dominant macrofauna. In: Ecological Processes in Coastal Environ- ments (Jefferies, R. L. &Davy, A. J. eds). Blackwell, Oxford. pp. 177-197.

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