Composition,distribution and biomass of meiobenthos in the Oosterschelde estuary (SW Netherlands)

Meiofauna composition, abundance, biomass, distribution and diversity were investigated for 31 stations in summer. The sampling covered the whole Oosterschelde and comparisons between the subtidal — intertidal and between the western-central — eastern compartment were made.Meiofauna had a community density ranging between 200 and 17 500 ind 10 cm^–2, corresponding to a dry weight of 0.2 and 8.4 gm^–2. Abundance ranged between 130 and 17 200 ind 10 cm^–2 for nematodes and between 10 and 1600 ind 10 cm^–2 for copepods. Dry weight biomass of these taxa was between 0.5–7.0 gm^–2 and 0.008–0.3 gm^–2 for nematodes and copepods respectively.The meiofauna was strongly dominated by the nematodes (36–99%), who's abundance, biomass and diversity were significantly higher intertidally than subtidally and significantly higher in the eastern part than in the western part. High numbers were positively correlated with the percentage silt and negatively with the median grain size of the sand fraction. The abundance and diversity of the copepods were highest in the subtidal, but their biomass showed an inverse trend being highest on the tidal flats.The taxa diversity of the meiofauna community and species diversity of both the nematodes and the copepods were higher in subtidal stations than on tidal flats. In the subtidal, the meiofauna and copepod diversity decreased from west to east, whereas nematode diversity increased.The vertical profile clearly reflected the sediment characteristics and could be explained by local hydrodynamic conditions.Seasonal variation was pronounced for the different taxa with peak abundance in spring, summer or autumn and minimum abundance in winter.Changes in tidal amplitude and current velocity enhanced by the storm-surge barrier will alter the meiofauna community structure. As a result meiofauna will become more important in terms of density and biomass, mainly due to increasing numbers of nematodes, increasing bioturbation, nutrient mineralisation and sustaining bacterial growth. A general decrease in meiofauna diversity is predicted. The number of copepods is expected to decrease and interstitial species will be replaced by epibenthic species, the latter being more important in terms of biomass and as food for the epibenthic macrofauna and fishes.     

Meiobenthos in mangrove areas in eastern Africa with emphasis on assemblage structure of free-living marine nematodes

A survey was conducted to examine spatial variations in the population density of major meiofaunal taxa and the assemblage structure of free-living marine nematodes within 5 mangrove areas on the west and east coast of Zanzibar. Meiofauna densities in surface sediments (0–5 cm) ranged from 205 to 5263 ind. 10 cm², being on average 1493 ind. 10 cm². Of the 17 major taxa recorded, nematodes dominated (64–99%) in all samples while harpacticoid copepods were usually second most abundant. Within all areas the numbers of meiofauna were very variable and significant differences among areas were only detected for oligochaetes and turbellarians. Densities of nematodes, harpacticoids, polychaetes and turbellarians were, however, significantly (P<0.001) higher at low water stations compared with mid and high water stations. Harpacticoids were negatively correlated with the numbers of fiddler crab (Uca spp.) burrows. Other correlations between environmental factors (grain size, temperature, salinity, oxygen tension, prop root density, fiddler crab burrows) and major meiofaunal taxa were non-significant. A total of 94 nematode genera were recorded from four mangrove areas. The most abundant and frequent genera were Microlaimus and Spirinia, followed by Desmodora and Metachromadora. Representatives of the genera most common in current study are found all over the globe. There was a high variation in nematode assemblage structure within and between sampling areas indicating the absence of a well defined nematode assemblage confined to mangrove areas. In a hypersaline area diversity was much reduced and where salinity was over 100%. the fauna was restricted to 3 nematode genera, Microlaimus, Theristus and Bathylaimus. Multidimensional scaling ordination (MDS) of the nematode genera separated samples taken from low water stations from other stations, the assemblage structure being significantly different at the low water stations. Numbers of selective deposit feeders were negatively correlated with average grain size and positively correlated with silt content.     

Composition and distribution of meiofauna,including nematode genera,in two contrasting Arctic beaches

The meiofauna of two tidal beaches, one exposed and one more sheltered, on Bjornoya (Bear Island) was investigated in summer 2000. Both meiofaunal densities and composition seem to be controlled by physical properties of the sediment, which in turn are controlled by exposure. The moderately and poorly sorted sediments in the sheltered beach were more abundant in terms of meiofaunal densities than the well sorted sediments in the exposed beach (254–481 individuals in 10 cm² vs 7–269 individuals in 10 cm², respectively). In total, seven higher meiofaunal taxa were found. Turbellaria were the numerically dominant taxon in the exposed beach. In the sheltered beach, Turbellaria also dominated, followed by Nematoda and Harpacticoida. The vertical distribution of the meiofauna was in accordance with what has been reported from other intertidal beaches. Nematoda were studied in detail and their densities ranged over 0.7–7.7 individuals in 10 cm² in the exposed beach and 2.7–186.0 individuals in 10 cm² in the sheltered beach. Nematodes were identified to genus level and a total of eight nematode genera were found. Sediment community respiration, measured as oxygen consumption, ranged between 2.3 cm³ O₂ m^–2 h^–1 in the exposed beach and 7.3 cm³ O₂ m^–2 h^–1 in the sheltered beach (respectively, the equivalent of 24 mg and 75 mg of organic carbon metabolised per day). Values from the sheltered site are within the range of results registered in much warmer localities.     

Benthic meiofauna responses to five forest harvest methods

Benthic meiofauna were collected from the pools of minute (0 order) streams in the Ouachita National Forest, Arkansas during March 21–23, 1996 to see if benthic communities responded to forest harvest methods in a similar manner as plankton communities collected two years prior. The study streams and their watersheds (2–6 ha) were located in 14–16 ha forest stands that were selected for comparability of stands. Five treatment stands were paired with adjacent undisturbed reference stands (10 total). Treatment stands were subjected to one of five harvest methods listed in order of decreasing severity of harvest disturbance to the stands: (1) clearcut; (2) pine seed-tree; (3) pine shelterwood; (4) pine-hardwood group selection; and (5) pine single-tree selection. The mean number of taxa per site was 14 with a range of 9–20 taxa including rotifers, copepods, nematodes, dipterans, ostracods and `other' meiofauna. Densities of total meiofauna (mean=2449 No. l ^–1) were significantly higher (p= 0.002) in treated sites. Highest densities occurred in single-tree and clearcut treatments. Rotifers were significantly more numerous at the single-tree treatments (p=0.03) and nematodes were significantly greater at the clearcut treatments (p=0.03). We conclude that benthic meiofauna in these headwater streams are sensitive to silviculture practices and that the impact of forest harvest persists for at least 2.5 years.     

Intertidal meiofauna of a high-latitude glacial Arctic fjord (Kongsfjorden, Svalbard) with emphasis on the structure of free-living nematode communities

Meiofauna communities of four intertidal sites, two sheltered and two more exposed, in Kongsfjorden (Svalbard) were investigated in summer 2001 at two different tidal levels (i.e. the low-water line and close below the driftline, referred to as mid-water (MW) level). A total of seven meiofaunal higher taxa were recorded with nematodes, oligochaetes and turbellarians being numerically dominant. Mean total meiofaunal densities ranged between 50 ind. 10 cm⁻² and 903 ind. 10 cm⁻². Our data showed a clear decrease in total meiofaunal densities with increasing coarseness of the sediment. Total meiofaunal biomass varied from 0.2 g dwt m⁻² to 2 g dwt m⁻² and, in general, was high even at low meiofaunal densities, i.e. larger interstitial spaces in coarser sediments supported larger meiofauna, especially turbellarians. The results on the vertical distribution of meiofauna contrasted sharply with typical meiobenthic depth profiles on other beaches, probably in response to ice-scouring and concomitant salinity fluctuations. Oligochaetes were the most abundant taxon, with a peak density of 641 ind. 10 cm⁻² at Breoyane Island. They were mainly comprised of juvenile Enchytraeidae, which prohibited identification to species/genus level. Nematode densities ranged between 4 ind. 10 cm⁻² and 327 ind. 10 cm⁻². Nematodes were identified up to genus level and assigned to trophic guilds. In total, 28 nematode genera were identified. Oncholaimus and Theristus were the most abundant genera. The composition of the nematode community and a dominance of predators and deposit feeders were in agreement with results from other arctic and temperate beaches. Nematode genus diversity was higher at the more sheltered beaches than at the more exposed ones. Low-water level stations also tended to harbour a more diverse nematode communities than stations at the MW level. Differences in nematode community structure between low- and MW stations of single beaches were more pronounced than community differences between different beaches and were mainly related to resources quality and availability.     

Meiofauna communities along a depth transect off Halley Bay (Weddell Sea-Antarctica)

Summary Meiofauna communities from 10 stations along a depth transect from approximately 500 to 2,000 m off the Halley Bay Station (Weddell Sea) are investigated. Representatives of about 30 smallsized taxa of higher category are found, most of them belonging to the meiofauna. Loricifera are recorded for the first time for the Southern Ocean. At one of the stations a maximum of 22 taxa occur, the mean number of taxa ranges from 7 to 16. Nematoda, Harpacticoida, Ostracoda, Polychaeta and Bivalvia are present at all sampling sites. Nematodes are always dominant representing more than 90% of the individuals per sample, followed by harpacticoids (3%) and kinorhynchs (1.2%). Important fractions of the meiofauna (an average of more than 50%) occur in strata below the top 0–1 cm layer. Maximal density is 3,800 individuals (10 cm^–2), the mean abundance per station ranges from 790 to 3,720 individuals (10 cm^–2) and the overall mean is 1,700 individuals (10 cm^–2). Multivariate analysis (TWINSPAN, Cluster analysis, DCA) discriminates between three communities which are correlated with depth and sediment characteristics: the near shelf-ice, the slope and the deep-sea communities.Data presented here were collected during the European Polarstern Study (EPOS) sponsored by the European Science Foundation     

Meiofauna associated with a Pacific coral reef in Costa Rica

The meiofauna of two coral reef habitats at Isla del Naño, Costa Rica was studied over a one year period. The dominant groups were: Foraminifera (21.2%), Copepoda (19.7%), Nematoda (19.1%), Gastropoda (16.5%), Polychaeta (7.2%) and Bivalvia (6.6%). The highest diversity was found in coarse, heterogeneous sands with the highest percentage of carbonates. The meiofauna showed a high degree of horizontal aggregation, which is a characteristic pattern for macro- and meiofauna in sediments of variable composition. No vertical variation in distribution was evident, probably due to the deep location of the Redox Potential Discontinuity layer. The total densities of organisms found in this study (99 to 575 ind/10 cm²) are low compared with densities in similar non-reefal sands (7 to 6116), and from fine sediments (80 to 17 000), but are comparable to densities found in other reef areas (39 to 609.5 ind/10 cm²). This is the first report on meiofauna from the eastern Pacific, and the first time that foraminiferans are the dominant group.     

Living conditions, abundance and biomass of under-ice fauna in the Storfjord area (western Barents Sea, Arctic) in late winter (March 2003)

The under-ice habitat and fauna were studied during a typical winter situation at three stations in the western Barents Sea. Dense pack ice (7–10/10) prevailed and ice thickness ranged over <0.1–1.6 m covered by <0.1–0.6 m of snow. Air temperatures ranged between –1.8 and –27.5°C. The ice undersides were level, white and smooth. Temperature and salinity profiles in the under-ice water (0–5 m depth) were not stratified (T=–1.9 to –2.0°C and S=34.2–34.7). Concentrations of inorganic nutrients were high and concentrations of algal pigments were very low (0.02 g chlorophyll a l^–1), indicating the state of biological winter. Contents of particulate organic carbon and nitrogen ranged over 84.2–241.3 and 5.3–16.4 g l^–1, respectively, the C/N ratio over 11.2–15.5 pointing to the dominance of detritus in the under-ice water. Abundances of amphipods at the ice underside were lower than in other seasons: 0–1.8 ind. m^–2 for Apherusa glacialis, 0–0.7 ind. m^–2 for Onisimus spp., and 0–0.8 ind. m^–2 for Gammarus wilkitzkii. A total of 22 metazoan taxa were found in the under-ice water, with copepods as the most diverse and numerous group. Total abundances ranged over 181–2,487 ind. m^–3 (biomass: 70–2,439 g C m^–3), showing lower values than in spring, summer and autumn. The dominant species was the calanoid copepod Pseudocalanus minutus (34–1,485 ind. m^–3), contributing 19–65% to total abundances, followed by copepod nauplii (85–548 ind. m^–3) and the cyclopoid copepod Oithona similis (44–262 ind. m^–3). Sympagic (ice-associated) organisms occurred only rarely in the under-ice water layer.     

The response of a harpacticoid copepod community to sediment disturbance in a semi-enclosed lagoon

The harpacticoid fauna of a sandy sediment in the Sluice Dock of Ostend, Belgium, was dominated by a single species, Canuella perplexa, that attained an average annual density of over 400 ind. 10 cm^–2 in 1976 and 1977, whereas other species had a total mean density of less than 10 ind. 10 cm^–2. In the summer of 1979 a dystrophic crisis with abundant growth of Ulva occurred and in March 1980 the lagoon was drained. After refilling of the lagoon, the sediment was immediately colonized by good swimming harpacticoids, that attained densities of 200 ind. 10 cm^–2. Three months after refilling Canuella perplexa reappeared and the system returned to its original state within one month.     

Biomass relationship to growth and phosphate uptake ofPseudomonas fluorescens,Escherichia coli andAcinetobacter radioresistens in mixed liquor medium

The ability ofPseudomonas fluorescens, Escherichia coli andAcinetobacter radioresistenns to remove phosphate during growth was related to the initial biomass as well as to growth stages and bacterial species. Phosphate was removed by these bacteria under favourable conditions as well as under unfavourable conditions of growth. Experiments showed a relationship between a high initial cell density and phosphate uptake. More phosphate was released than removed when low initial cell densities (10²–10⁵ cells ml^–1) were used. At a high initial biomass concentration (10⁸ cells ml^–1), phosphate was removed during the lag phase and during logarthmic growth byP. fluorescens. Escherichia coli. at high initial biomass concentrations (10⁷ cells ml^–1), accumulated most of the phosphate during the first hour of the lag phase and/or during logarithmic growth and in some cases removed a small quantily of phosphate during the stationary growth phase.Acinetobacter radioresistens, at high initial cell densities (10⁶, 10⁷ cells ml^–1) removed most of phosphate during the first hour of the lag phase and some phosphate during the stationary growth phase.Pseudomonas fluorescens removed phosphate more thanA. radioresistens andE. coli with specific average ranges from 3.00–28.50 mg L^–1 compared to average ranges of 4.92–17.14 mg L^–1 forA. radioresistens and to average ranges of 0.50–8.50 mg L^–1 forE. coli.     

Impact of cyprinids on zooplankton and algae in ten drainable ponds

To study the impact of cyprinids on algae, zooplankton and physical and chemical water quality, ten drainable ponds of 0.1 ha (depth 1.3 m) were each divided into two equal parts. One half of each pond was stocked with 0 + cyprinids (bream, carp and roach of 10–15 mm), the other was free of fish. The average biomass of the 0 + fish at draining of the ponds was 466 kg ha^–1, to which carp contributed about 80%.The fish and non-fish compartments showed significant differences. In the non-fish compartments the density of Daphnia hyalina was 10–30 ind. l^–1 and that of Daphnia magna 2–4 ind. l-^–1, whereas in the fish compartments densities were c. 1 ind. l^–1. Cyclopoid copepods and Bosmina longirostris, however, showed higher densities in the fish compartments. The composition of algae in the two compartments differed only slightly, but the densities were lower in the non-fish compartments. The significant difference in turbidity was probably caused by resuspension of sediment by carp. No significant difference in nutrient concentration between the compartments was found.     

Meiofauna sediment relations in leeward slope turf algae of Heron Island reef

As part of studies investigating the influence of grazers on reef meiofauna, we assessed the density, composition and richness of meiofauna (retained on a 100-μm sieve) on the leeward reef slope of Heron Reef, GBR, Australia using an airlift vacuum sampling device. Estimates of meiofauna densities ranged between 40 individuals 10 cm⁻² and 290 individuals 10 cm⁻², which is considerably lower than many estimates from carbonate sediments and hard substrates from other reefs and marine habitats. The 17 taxa of meiofauna were dominated by harpacticoid copepods (40%) and nematodes (32%). Varying sediment load within algal turfs explained 37% of variation of meiofauna density. A model is proposed in which increased shelter afforded by high living coral cover reduces meiofaunal losses from grazing and increases sediment loads, balanced by areas of low coral cover in which sedimentation rates are lower and grazing rates higher. At none of the four sites did major differences in abundance occur between November and March sampling events. Together these observations suggest that epilithic meiofaunal communities are generally spatially and temporally predictable at small scales in this reef system, indicating that their ecological services are similarly conservative. Handling editor: I. Nagelkerken     

Macrobenthic infauna of mangroves and surrounding beaches at Gazi Bay,Kenya

Twelve sites around Gazi Bay, Kenya, were examined for macrobenthic infauna. Stations differed in human disturbance, vegetation, and sediment type (sandflat, beach, denuded and virgin mangrove sites, with Sonneratia, Rhizophora, Avicennia, Ceriops or Bruguiera vegetation).Sixteen higher taxa were counted; Isopoda, Amphipoda, Polychaeta, Cumacea and Tanaidacea were determined to family level (sometimes to genus or species). Total densities of infauna ranged from 265 to 6025 ind m^–2.Gazi mangals had higher macrobenthos densities than other mangrove sediments described in literature and than the Gazi sandflats. The densities found in Gazi sandflats were comparable to similar habitats elsewhere.Virgin mangrove sediments were rich in mud and organic material, and were characterized by high densities of macro-Oligochaeta and Mollusca. Sandflats, beaches, exploited (denuded) and less dense mangroves or mangroves higher in the tidal zone (Bruguiera) were much more sandy and had a high abundance of Polychaeta and Nemertini.Structural patterns in the macrobenthos were only vaguely associated with vegetation type. Human impact (cutting) has resulted in a drastic decrease in densities of macro-infauna, possibly related to a reduction of organic material in the sediment.     

Horizontal and vertical distribution of meiofauna on sandy beaches of the North Sea (The Netherlands, Belgium, France)

Sandy intertidal zones were analysed for the presence of meiofauna. The material was collected on six macro-tidal sandy beaches along the North Sea (The Netherlands, France, Belgium), in order to analyse the vertical and horizontal meiofaunal distribution patterns. Eleven higher meiofauna taxa (one represented by larval stage—Copepoda nauplii) were recorded. The maximum total meiofauna abundance was observed on the Dutch beach (4,295±911 ind. 10 cm⁻²) in the Westerschelde estuary, while the lowest values (361±128 ind. 10 cm⁻²) were recorded in France at the Audresselles beach. Meiofauna of the different localities consisted mainly of nematodes, harpacticoids and turbellarians. Nematodes numerically dominated all sampled stations, comprising more than 45% of the total meiofauna density. Meiofauna was mainly concentrated at the sand surface with about 70% present in the uppermost 5 cm. Meiofauna occurred across the entire intertidal zone. A clear zonation pattern in the distribution of meiofauna taxa across the beaches was observed. The present work suggests that designation of exposed sandy beaches as physically controlled (McLachlan 1988) does not explain their biological variability.     

Foraminifera and meiofauna on an intertidal mudflat,Cornwall, England: Populations; respiration and secondary production; and energy budget

A rich and varied meiofauna inhabits a Cornish mudflat near the mouth of the Tamar River in southwestern England. Population densities range from 117 to 943 individuals · g^–1 (wet) sediment (1.4–11.4 × 10⁶ individuals · m^–2), with foraminifera, harpacticoid copepods and nematodes appearing in nearly equal numbers and comprising most of the meiofauna. Seasonally, meiofaunal numbers rise and fall with solar radiation and vary inversely with river discharge. Two species, the atestate allogromiid A and the calcareous Haynesina germanica (Ehrenberg), far outnumber other foraminifera; their population densities and growth rates reach maxima in spring and summer.Monthly rates of sediment respiration are locally variable, but clearly increase from winter (4.13 ml O₂ · m^–2 · h^–1 in December) to spring (38.87 ml O₂ · m^–2 · h^–1 in April). Experiments and calculations ascribe approximately 30% of this total to the meiofauna (including microfauna and microflora), 50% to bacteria and less than 20% to chemical oxidation. A tentative energy budget for the mudflat suggests that secondary production by meiofauna is small as compared with coastal environments elsewhere, and that meiofaunal production (426 Kcal · m^–2 · y^–1) is nearly twice meiofaunal respiration (252 Kcal · m^–2 · yr^–1).     

Response to cadmium of Daucus carota hairy roots dual cultures with Glomus intraradices or Gigaspora margarita

Ri T-DNA-transformed carrot roots were cultivated in two experiments either non-inoculated or inoculated with the arbuscular mycorrhizal (AM) fungi Glomus intraradices or Gigaspora margarita. The influence of two concentrations of cadmium (Cd) in the medium (2 mg l^–1, 4 mg l^–1) on both root and mycelium growth was tested. Both parameters were estimated at 10-day intervals for 70 or 100 days for G. intraradices and Gi. margarita, respectively. In the first experiment, G. intraradices showed a rapid spread of extraradical mycelium (ERM) and reached average densities per treatment of about 90 cm cm^–2 agar medium after 70 days. At the higher Cd level, the growth of ERM was delayed in comparison to the treatment without Cd addition. Root growth was inhibited by both Cd levels; the inhibition was, however, significantly lower in the treatments inoculated with G. intraradices compared to the non-inoculated control. In the second experiment, the ERM of Gi. margarita started to grow after a period of 50 days and reached average densities per treatment of only up to 27 cm cm^–2 by the end of the cultivation. The growth of Gi. margarita mycelium was not inhibited by Cd. No differences in root growth were observed between the Gi. margarita inoculated and non-inoculated treatments. The inhibitory effect of Cd on root growth differed between the non-inoculated treatments in both experiments. The study has shown that the AM fungus Glomus intraradices can alleviate Cd-induced growth inhibition to carrot hairy roots. The potential and limits of the monoxenic system in studying the interaction between AM fungi and heavy metals are discussed.     

On the distribution patterns and population dynamics of sublittoral and profundal oligochaeta fauna from Lake Banyoles (Catalonia,NE Spain)

Oligochaeta assemblages from Lake Banyoles were composed of one species of Naididae and eight species of Tubificidae, Potamothrix heuscheri (8–1350 ind. m^–2) being the most abundant and widely distributed species, followed by Limnodrilus hoffmeisteri (25–858 ind. m^–2) and Psammoryctides barbatus (12–792 ind. m^–2). The remaining species were Potamothrix hammoniensis, Potamothrix bavaricus, Limnodrilus claparedeanus, Branchiura sowerbyi, Aulodrilus pigueti and Dero digitata, all of which occurred at lower densities and with restricted distributions.Regardless of sampling depth, maximum densities of oligochaetes were found in winter (up to 5,142 ind. m^–2 in December) and secondarily in June. Immature Tubificidae with hair setae and mature P. heuscheri were present and dominant throughout the year. Cestode parasites were recorded infesting L. hoffmeisteri and P. heuscheri populations.Oligochaeta densities increased with depth, but low oxygen concentrations (less than 1 mg l^–1) during a long period (4 months) in some basins of the lake acted as a key factor in reducing the density of worms and the species richness to one species, Potamothrix heuscheri.     

Butyric acid bacteria of the genus <Emphasis Type="Italic">Clostridium</Emphasis> in the bottom sediments of inland basins of different types

The cell numbers and ecological characteristics of the distribution of certain species of butyric acid bacteria (BABs) of the genus Clostridium in the bottom sediments of inland basins of different types were studied using the optimal nutrient media. The seasonal dynamics of clostridial vegetative cells and spores in sediments with different ecological conditions were revealed. The cell numbers of the dominant BAB species were shown to depend on the redox potential of the sediments, the amount and composition of C_org, and the trophic state of the basin in general. C. pasteurianum was found to predominate in eutrophic lakes and reservoirs (5–11 × 10⁶ cells/cm³), C. butyricum and C. felsineum predominated in mesotrophic ones (2–11 × 10⁶ cells/cm³), and C. acetobutylicum was predominant in acidic chthonioeutrophic lakes and reservoirs (0.1–0.5 × 10⁶ cells/cm³). The lowest cell numbers of BABs were found in river sediments, whereas the highest numbers were recorded in the sediments of polysaprobic zones (0.1–1.0 × 10³ and 0.5–2.0 × 10⁷ cells/cm³ respectively).Translated from Mikrobiologiya, Vol. 74, No. 1, 2005, pp. 119–125.Original Russian Text Copyright © 2005 by Dzyuban.     

The spring-time abundance and feeding ofEurytemora affinis (Poppe) in Volkerak-Zoommeer,a newly-created freshwater lake system in the Rhine delta (The Netherlands)

Eurytemora affinis, a calanoid copepod, has been encountered in Volkerak-Zoommeer (Rhine delta region, S.W. Netherlands) both before this lake system was isolated in 1987 from the estuarine influence, and after. It was the main particle-feeding crustacean at all the 3 sampling stations in March–April 1990 when it reached densities of up to 215 ind.l^–1. Its decline from mid April onwards, and low densities through summer, coincided with increase in cladocerans, especiallyDaphnia spp. (D. pulex andD. galeata), a decrease in seston (<33m) and chlorophyll concentrations and in primary production rates. The clearance rates (CR) ofEurytemora measured in the spring period varied enormously (0.6–24 ml.ind^–1.d^–1) depending mainly on size (0.44–1.06 mm), food concentration (0.8–2.2 mg C.l^–1), and the water temperature which varied only narrowly (8.0–9.0°C). Mean ingestion rates of the animals measuring 0.68±0.02 mm during the study was 6.7±3.2 gC.ind^–1.d^–1; and assimilation efficiency varied between 27 and 53% (mean: 41±9%). The weight specific CR (SCR) varied between 0.96 and 6.4 litre.mg^–1 body C.d^–1. Pooled regression of SCR on the animal's body weight at the 3 study stations revealed a significant inverse relationship. Also daily ration and specific assimilation ofE. affinis varied greatly and inversely with the body weight. This calanoid contributed from about 50 to 100% to the zooplankton community grazing rates and assimilation rates, the former often exceeding the phytoplankton primary production.     

Effects of nutrient (N,P, C) enrichment,grazing and depth upon littoral periphyton of a softwater lake

Three field experiments were performed in Lake Lacawac, PA to determine the importance of potentially limiting nutrients relative to other factors (grazing, depth) in structuring shallow water algal periphyton communities. All three experiments measured periphyton growth (as chlorophyll-a, AFDM or biovolumes of the algal taxa) on artificial clay flower pot substrates which released specified nutrients to their outer surfaces.Control of standing crop by nutrient supply rate vs. grazing was examined in Expt. I. Substrates releasing excess N and P, together with one of 4 levels of C (as bicarbonate) were placed either inside or outside exclosures designed to reduce grazer densities. Chlorophyll-a rose from 1.1–25.6 µg.cm^–2, and some dominant taxa (e.g., Oedogonium, Nostoc, Anacystis) were replaced by others (e.g., Scenedesmus, Cryptomonas) as bicarbonate supply increased. Reductions in invertebrate density did not significantly affect chlorophyll-a at any of the nutrient levels.Reasons for the species shift were further evaluated in Expt. II, using a minielectrode to measure the elevation of pH within the periphyton mat through photosynthetic utilization of bicarbonate. The pH adjacent to pots diffusing N, P and large quantities of bicarbonate, and supporting high chlorophyll-a densities of 32 µg cm^–2, averaged 10.0 compared to 6.3 in the water column. Pots diffusing only N and P supported 0.7 µg chlorophyll-a cm^–2 and elevated pH to 8.2. We suspect that bicarbonate addition favored efficient bicarbonate users (e.g., Scenedesmus), while inhibiting other taxa (e.g., Oedogonium) because of the attendant high pH.Expt. III was designed to test effects of depth (0.1 m vs. 0.5 m) and N (NH₄ ⁺ vs. NO₃ ^–) upon the growth response to bicarbonate observed in Expts. I and II. Similar standing crop and species composition were noted on pots at 0.1 m vs. 0.5 m. Enrichment with NH₄ ⁺ vs. NO₃ ^– also appeared to have little effect upon the periphyton community.Shallow water periphyton communities in Lake Lacawac, when supplied with sufficient N and P, appear to show a distinctive response to increasing bicarbonate concentration and pH which is robust to moderate variation in grazer densities, distance from the water surface, and the form of N enrichment.