Large Ciliates Research Articles

Phytoplankton blooms are strongly impacted by microzooplankton grazing in coastal North Pacific waters

Phytoplankton growth and microzooplankton grazing were measured in two productive coastal regions of the North Pacific: northern Puget Sound and the coastal Gulf of Alaska. Rates of phytoplankton growth (range: 0.09–2.69 day−1) and microzooplankton grazing (range: 0.00–2.10 day−1) varied seasonally, with lowest values in late fall and winter, and highest values in spring and summer. Chlorophyll concentrations also varied widely (0.19–13.65 μg l−1). Large (>8 μm) phytoplankton cells consistently dominated phytoplankton communities under bloom conditions, contributing on average 65% of total chlorophyll biomass when chlorophyll exceeded 2 μg l−1. Microzooplankton grazing was an important loss process affecting phytoplankton, with grazing rates equivalent to nearly two-thirds (64%) of growth rates on average. Both small and large phytoplankton cells were consumed, with the ratio of grazing to growth (g:μ) for the two size classes averaging 0.80 and 0.42, respectively. Perhaps surprisingly, the coupling between microzooplankton grazing and phytoplankton growth was tighter during phytoplankton blooms than during low biomass periods, with g:μ averaging 0.78 during blooms and 0.49 at other times. This tight coupling may be a result of the high potential growth and ingestion rates of protist grazers, some of which feed on bloom-forming diatoms and other large phytoplankton. Large ciliates and Gyrodinium-like dinoflagellates contributed substantially to microzooplankton biomass at diatom bloom stations in the Gulf of Alaska, and microzooplankton biomass overall was strongly correlated with >8 μm chlorophyll concentrations. Because grazing tended to be proportionally greater when phytoplankton biomass was high, the absolute amount of chlorophyll consumed by microzooplankton was often substantial. In nearly two-thirds of the experiments (14/23), more chlorophyll was ingested by microzooplankton than was available for all other biological and physical loss processes combined. Microzooplankton were important intermediaries in the transfer of primary production to higher trophic levels in these coastal marine food webs.

Dynamics and growth estimates of planktonic protists during early spring in Lake Pavin, France

The in situ growth rates of the dominant planktonic protists were estimated in Lake Pavin during early spring, from unfiltered (UF, i.e. raw) and fractionated (<50 and <10 µm) water samples incubated in diffusion chambers. The early spring phytoplankton bloom was largely domi- nated by the plastidic dinoflagellate Gymnodinium uberrimum. Because they were not efficiently removed by filtration through the 10 µm pore-size mesh, small ciliates (10 to 25 µm in length: Urotricha spp., Balanion planctonicum and Strobilidium/Halteria) affected the growth rates of nanoflagellates in the <10 µm samples. Low or negative growth rates were also found for oligotrichs and tintinnids, which appeared to be very sensitive to manipulation and incubation. Except for the latter 2 taxa, species-specific theoretical growth rates (derived from cell volume and temperature) were close to those calculated from changes in natural abundances. Our results suggest that the small ciliate Urotricha may act as an essential trophic link between autotrophic nanoflagellates (ANF) and metazooplankton, and that large ciliates may be able to efficiently transfer energy between hetero- trophic nanoflagellate (HNF) prey and metazooplankton grazers during early spring in Lake Pavin. Our experiments highlight the importance of taking into account each individual protist species, and of not considering flagellates and ciliates as being a uniform group in pelagic systems. Because of the great diversity of shape, size and flexibility of these cells, it is essential during fractionation experi- ments to confirm the effectiveness of the experimental protocols so as to identify possible artefacts and to obtain a more reliable estimate of the flows of material and energy in aquatic ecosystems.

Zooplankton feeding ecology: grazing on phytoplankton and predation on protozoans by copepod and barnacle nauplii in Disko Bay, West Greenland

Grazing experiments were conducted with Calanus spp. and Balanus cf. crenatus nau- plii incubated with natural plankton from Disko Bay, West Greenland during the post-spring-bloom period. Both copepod and barnacle nauplii were feeding on different types of protists although at dif- ferent rates. Calanus spp. nauplii preferred large ciliates and dinoflagellates whereas flagellates ~5 µm in diameter and Myrionecta rubra were hardly ingested at all. B. cf. crenatus nauplii preferred diatoms and also consumed the small flagellates at relatively high rates. Compared to Calanus spp. nauplii, B. cf. crenatus nauplii ingested ciliates and dinoflagellates at low rates suggesting a more herbivorous feeding mode than the more predaceous copepod nauplii. The daily grazing impact of the nauplii community in Disko Bay was estimated using the weight-specific mean clearances from the grazing experiments and field biomass values of different categories of prey and nauplii. These calculations showed that the grazing impact by the nauplii community on all prey categories was generally modest (1.3 to 9.2%). However, the dominant part of the total food intake by Calanus spp. nauplii in the surface water was composed of ciliates and dinoflagellates, most of which were phagotrophic

The effects of the ciliate Paramecium cf. caudatum Ehrenberg on toxin producing Cylindrospermopsis isolated from the Fitzroy River, Australia

The large ciliate Paramecium cf. caudatum Ehrenberg was found to be a successful grazer of toxin producing Cylindrospermopsis in the laboratory. The feeding rate increased with increasing cell concentration to 1367 cell animal hr-1 at 4.1 x 10(5) cells mL-1 but declined slightly at cell concentrations greater than this. Preliminary studies on the effects of this grazing on toxin concentrations in cultures of both straight and coiled forms of Cylindrospermopsis resulted in the production of different amounts of the toxin cylindrospermopsin in the different isolates. Differences in toxin production were also found between cultured and field populations from the lower Fitzroy River indicating that toxin production may be influenced by a suite of genetic and environmental factors. The proven ability of this ciliate to graze toxic Cylindrospermopsis provides some insight into interactions that may be able to control some toxic blooms in semiarid Australian conditions.

Factors regulating summer phytoplankton in a highly eutrophic Antarctic lake

Lakes from Maritime Antarctica are regarded as systems generally inhabited by metazoan plankton capable of imposing a top-down control on the phytoplankton during short periods, while lakes from Continental Antarctica lacking these communities would be typically controlled by scarcity of nutrients, following a bottom-up model. Otero Lake is a highly eutrophic small lake located on the NW of the Antarctic Peninsula, which has no metazoan plankton. During summer 1996, we studied the density, composition and vertical distribution of the phytoplankton community of this lake with respect to various abiotic variables, yet our results demonstrated neither light nor nutrient limitation of the phytoplankton biomass. Densities of heterotrophic nanoflagellates (HNAN) and ciliates from three different size categories were also studied. Extremely low densities of HNAN (0–155 ind. ml−1) could be due to feeding competition by bacterivore nanociliates and/or predation by large ciliates. A summer bloom of the phytoflagellate Chlamydomonas aff. celerrima Pascher reached densities tenfold those of previous years (158.103 ind. ml−1), though apparently curtailed by a strong peak of large ciliates (107 ind. ml−1) which would heavily graze on PNAN (phototrophic nanoflagellates). Top-down control can thus occur in this lake during short periods of long hydrologic residence time.

Atrazine effects on the microbial food web in tidal creek mesocosms

Atrazine effects on the microbial food web were assessed in mesocosms designed to simulate a southeastern tidal creek ecosystem. The test chambers simulate the major features unique to estuarine systems, such as tidal flow and salinity changes. Microbial communities were colonized on artificial substrates and analyzed for productivity, biomass, and community composition. Atrazine was found to reduce chlorophyll a, phototrophic carbon assimilation and phototrophic biovolume in the 40 and 160 μg l −1 treatments. Significant reductions in phototrophic variables occurred within 24 h of pesticide exposure and no recovery was detected after 9 days. Dissolved oxygen content in the mesocosms was not significantly affected by atrazine. Atrazine induced changes in composition of the algal community. Most taxa were reduced in abundance by atrazine exposure; however, cyanobacterial taxa generally became more abundant. Bacterial abundance was significantly elevated in the highest dose after 24 h, but decreased in a dose–response manner after 48 h. There was no significant effect on small ciliates; however, large ciliates and small flagellates increased significantly in number after 48 h and remained elevated after 9 days. Atrazine at 160 μg l −1 significantly reduced large flagellate density throughout the experiment. These results suggest that exposure of the estuarine microbial food web to agricultural pesticides can lead to both functional and structural changes, which may potentially impact higher trophic levels.

Temporal changes of microbial assemblages in the ice and snow cover of a high mountain lake

It has recently been shown that a rich community of microorganisms inhabits the slush layers of the winter cover of high mountain lakes. In this study, temporal changes in species assemblages and environmental conditions in the ice and snow cover of Lake Redó in the Pyrenees (Spain) are presented. The winter cover was a highly dynamic environment, in which major changes occurred in physical structure and chemical characteristics. The biomass and species composition of the microbial groups present (autotrophic and heterotrophic flagellates, ciliates, and bacteria) reflected these physicochemical alterations. After an initial phase, during which the ice sheet formed and the first snows accumulated with little or no development of slush layers, there were two stages of microbial assemblages, which coincided with the two main phases in the physical change of the cover: the growth period (from January to mid‐April) and the ablation period (from mid‐April to ice‐out in June). Initially, microbial biomass originated from inputs of phytoplankton‐rich (5.9 μg chlorophyll a liter−1 ) surface lake water that flooded the cover as a result of the hydrostatic adjustment induced by the progressive accumulation of snow on top of the ice sheet. This was followed by a differential growth of mixotrophic or heterotrophic flagellated species, such as Ochromonas sp., Cryptomonas spp., Monosiga ovata and Oikomonas termo, which peaked at different times. Species depended primarily on bacterivory for their growth, since during this period, light did not reach the slush layers. By April, the transition from cover growth to ablation was marked by a reduction in biomass and in the number of species. During the ablation period, the rate of change of the cover was greater, and microbial assemblages were characterized by the vertical segregation and by the appearance of new species, some of which were typically nonplanktonic. During that time, the cover was highly influenced by the large amount of water from the melting of the snowpack within the catchment. This water likely provided chemicals and organisms, while increased light availability allowed for the growth of algae (Pteromonas sp. and other volvocales) and of the associated food web in the upper slush. In deeper layers, however, a bacteria‐based food web provided prey for large ciliates (Urosoma sp., Dileptus sp., and Lacrymaria sp.), probably carried from littoral or watershed soils.

A mixotrophic ciliate as a major contributor to plankton photosynthesis in Australian lakes

A large mixotrophic ciliate (∼200 µm long) of the genus Stentor is a common constituent of the protozooplankton of Australian lakes. We investigated the photosynthetic rates of populations of this ciliate from two lakes, one in the Australian Capital Territory and the other on the New South Wales/Victorian border, in relation to photosynthesis by the whole phytoplankton community. The concentration of the ciliate varied between 192 and 4,267 cells liter−1 during the study period of May–January (the austral winter, spring, and autumn) and it contributed between 4.3 and 69.3% of total plankton photosynthesis. Individual photosynthetic rates ranged between 1.03 ± 0.8 and 3.98 ± 0.6 ng C cell−1 h−1 and individual Chl a content between 925 ± 62 to 1,461 ± 63 pg cell−1, giving assimilation numbers of 1.00–2.74. Light‐response curves indicated that the ciliate achieved its highest rates of photosynthesis at high photon fluxes, typical of the surface waters. Vertical distribution patterns of Stentor in the water column of one of the lakes supported these physiological data. Southern hemisphere lakes seem to have a protozooplankton that may contain substantial numbers of a large ciliate capable of contributing a significant portion of carbon fixation in the plankton.

Evaluation of toxicity of medical devices using Spirotox and Microtox tests: I. Toxicity of selected toxicants in various diluents

Significant effort has been directed toward developing in vitro alternatives, which can be the first step of toxicity analysis. Tissue culture assays are currently the most popular in vitro tests for evaluating acute toxicity. The possibility of applying two bioassays using microorganisms in assessing the toxicity of extracts of medical devices was investigated. The Microtox test system—a luminescent bacteria toxicity test—assesses changes in light output from a luminescent bacteria, Vibrio fischeri. Spirotox used a large ciliate protozoan: Spirostomum ambiguum. The most widely used extraction solvent, 0.9% NaCl, must be concentrated up to 2% for Microtox and diluted nine times for the Spirotox test. The organic solvents ethanol, DMSO, and polyethylene glycol 400 were not toxic in either test in concentrations of 1–2%. The toxicity of reference compounds Hg, Cd, Zn, Pb, and SDS was examined in various diluents. The sequence of toxicity of the tested compounds in Spirotox and Microtox was: Hg > Cd > Zn > Pb > SDS, and Hg > Pb = Zn > SDS > Cd, respectively. Addition of organic solvents changed the toxicity of compounds tested in 60% of Spirotox tests and only in 25% of Microtox tests. Changes were low, not exceeding 100% in almost all cases. No correlation was observed between diluent and toxicant in either bioassay. © 1997 John Wiley & Sons, Inc.

Evaluation of toxicity of medical devices using Spirotox and Microtox tests: I. Toxicity of selected toxicants in various diluents.

Significant effort has been directed toward developing in vitro alternatives, which can be the first step of toxicity analysis. Tissue culture assays are currently the most popular in vitro tests for evaluating acute toxicity. The possibility of applying two bioassays using microorganisms in assessing the toxicity of extracts of medical devices was investigated. The Microtox test system--a luminescent bacteria toxicity test--assesses changes in light output from a luminescent bacteria, Vibrio fischeri. Spirotox used a large ciliate protozoan: Spirostomum ambiguum. The most widely used extraction solvent, 0.9% NaCl, must be concentrated up to 2% for Microtox and diluted nine times for the Spirotox test. The organic solvents ethanol, DMSO, and polyethylene glycol 400 were not toxic in either test in concentrations of 1-2%. The toxicity of reference compounds Hg, Cd, Zn, Pb, and SDS was examined in various diluents. The sequence of toxicity of the tested compounds in Spirotox and Microtox was: Hg > Cd > Zn > Pb > SDS, and Hg > Pb = Zn > SDS > Cd, respectively. Addition of organic solvents changed the toxicity of compounds tested in 60% of Spirotox tests and only in 25% of Microtox tests. Changes were low, not exceeding 100% in almost all cases. No correlation was observed between diluent and toxicant in either bioassay.

Predation by a photosynthetic dinoflagellate Gyrodinium instriatum on loricated ciliates

Feeding of a naked photosynthetic dinoflagellate, Gyrodinium instriatum, on loricated cili- ates was investigated. Gyrodinium instriatum preyed on Favella azorica and Eutintinnus tubulosus by engulfment through the posterior end of the sulcus. In the case of E.tubulosus, G.instriatum preyed on this small ciliate keeping the original gymnodinioid cell shape. On the other hand, G.instriatum preyed on Favella taraikaensis by absorbing the cell contents of this large ciliate, which resulted in a balloon- like inflation of its body size. It seemed that G.instriatum can change its feeding style according to the size of prey. Thus, the present study shows, by using direct observations on the feeding of G.instriatum on loricated ciliates, a reversal of energy flow processes in the food chain in which photosynthetic organisms eat primary consumers. The growth of G.instriatum after feeding on F.taraikaensis and E.tubulosus is also described briefly.

The cryopreservation of some large ciliate entodiniomorphid protozoa taken from the rumen.

Large entodiniomorphid ciliates taken from the rumen cannot be reliably kept in vivo, and cryopreservation was investigated as an alternative approach to storage. Three species of large entodiniomorphid ciliated (Polyplastron multivesiculatum, Epidinium ecaudatum forma caudatum and Ophryoscolex caudatus forma tricoronatus) were cryopreserved by one-step freezing, with mean recoveries of 43, 51 and 80%, respectively. For Ophryoscolex caudatus forma tricoronatus two-step freezing was investigated. The mean recovery was 30%, and of tested parameters, the equilibration time and temperature of the cryoprotective additive of dimethyl sulphoxide and the length of holding phase at -40 degrees C were found to be critical for the success of the method.

The Contribution of Ciliated Protozoa to Plankton and Benthos Biomass in a European Ricefield

The densities and biomass of ciliates inhabiting the water‐sediment interface and the water column of an experimental ricefield were investigated throughout four annual cycles. Ciliate abundance and biomass were higher at the water‐sediment interface than in the water column. In both sites, large ciliates (&gt; 105μm3) contributed the higher biomass values, but the highest densities were found in the intermediate size class (104‐105μm3). The prorodontid Coleps hirtus dominated the ciliate assemblage and usually comprised &gt; 50% of total ciliate density. Blooms of C. hirtus, occurred in June in the water column and in July at the sediment surface. During the four cycles of rice cultivation, the average daily values for production of the entire ciliate community was 69.6 mgC/m2/d, and the net production efficiency (K2) was 72.0%. The estimated production values in the present study are high if compared to production measured for ciliates in other freshwater ecosystems.

An ultradian clock controls locomotor behaviour and cell division in isolated cells of Paramecium tetraurelia.

An ultradian clock operates in fast growing cells of the large ciliate, Paramecium tetraurelia. The period of around 70 minutes is well temperature-compensated over the temperature range tested, i.e. between 18 degrees C and 33 degrees C. The Q10 between 18 degrees C and 27 degrees C is 1.08; above 27 degrees C there is a slight overcompensation. The investigation of individual cells has revealed that two different cellular functions are under temporal control by this ultradian clock. First, locomotor behaviour, which is an alternation between a phase of fast swimming with only infrequent turning, and a phase of slow swimming with frequent spontaneous changes of direction. In addition, the ultradian clock is involved in the timing of cell division. Generation times are not randomly distributed, but occur in well separated clusters. At all of the six temperatures tested, the clusters are separated by around 70 minutes which corresponds well to the period of the locomotor behaviour rhythm at the respective temperatures. Whereas the interdivision times were gradually lengthened both above and below the optimum growth temperature, the underlying periodicity remained unaffected. Also cells of different clonal age had identical periods, suggesting that neither the differences in DNA content, not other changes associated with ageing in Paramecium have an effect on the clock. A constant phase relationship was observed between the rhythm in locomotor behaviour and the time window for cell division; this strongly suggests that the same ultradian clock exerts temporal control over both processes.

Comparison of ciliate communities in the anoxic hypolimnia of three lakes: general features and the influence of lake characteristics

The ciliate communities and their food sources were investigated in the anoxic hypolimnia of three lakes with differing water chemistry. Bacterial biomass and, as a result, the biomass of bactenvor- ous ciliates were correlated with lake trophy. Additionally, high sulfate and sulfide concentrations led to high bacterial biomass of sulfate reducers and anaerobic phototrophic and heterotrophic bacteria, which in turn sustained large ciliate populations. The anaerobic ciliate communities of the lakes shared many characteristics. They were comprised of the same or closely related species; this was attributed to a low diversity of food sources. Ciliate to prey biomass ratios were 1.2-3.8% which is consistent with a low theoretical growth efficiency of anaerobic metabolism. Grazing pressure on anaerobic ciliates by metazoa was insignificant. In all three lakes, ciliate populations showed distinct vertical non-random distribution patterns which were often correlated with the distribution of the corresponding food sources. It is suggested that the microbial communities in anoxic water bodies are largely influenced by few common environmental conditions and are therefore often inhabited by similarly structured ciliate communities.

The Fauna of Floating Cyanobacterial Mats in the Oligohaline Eulittoral Zone off Hiddensee (South‐west Coast of the Baltic Sea)

Abstract. The fauna — including macrofauna, meiofauna, and large ciliates — of floating cyanobacterial mats in a brackish shallow‐water area was studied by analysing six 20 cm2 pieces of mat. Although these microbial aggregations were scarcely 1 cm thick, their total meiofauna abundance was about five times as high as in the uppermost 4 cm of the adjacent sediment. The mat fauna was dominated by harpacticoids, although large ciliates, rotifers, nematodes, and oligochaetes were also markedly more abundant than in the sediment. All species occurring in the mats were members of the surrounding sediment fauna. Out of the 47 species found, only a few, among them predominantly the harpacticoid, Mesochra lilljeborgi Boeck, 1864, and the nematode Daptonema setosum (bütschli, 1874), accounted for the majority of the individual abundance in the mats. Both are regularly found in sulphidic biotopes near Hiddensee. As SEM micrographs revealed, the oligochaete Paranais litoralis (O. F. Müller, 1788) and the harpacticoid Cletocamptus confluens (Schmeil, 1894) were apparently optimal substrates for dense lawns of cyanobacteria. This indicates possible close interactions between the meiofauna and these microbiota. The frequent predation of histophagous ciliates on nematodes and harpacticoids, which were probably weakened by oxygen deficiency and/or high sulphide concentrations, were not only a sign of a generally neglected pathway in the food chain, but also impressively emphasized the huge variety of interactions taking place between meio‐ and microfauna within this specific benthic microcosm. Because of their floating character, the mats can play an important role in the dispersal of benthic fauna.

Meiobenthic distribution and nematode community structure in five European estuaries

Meiofauna from the intertidal zone of five European estuaries (Ems, Westerschelde, Somme, Gironde, Tagus) was investigated. Samples represented a cross section of various benthic habitats from near-freshwater to marine, from pure silts to fine-sandy bottoms. The meiobenthic community comprised everywhere a fauna strongly dominated by nematodes, with meiobenthic density increasing with increasing salinity. The Ems differed from the other estuaries due to the presence of a well developed community of Copepods, Gastrotrichs, large Ciliates and/or soft-shelled Foraminiferans in some sites. The Westerschelde stood out due to the near-absence of harpacticoid copepods and, as in the Tagus, the lower meiobenthic densities in the marine part of the estuary. For nematode community analysis, we also included data from the Tamar which were obtained from the literature (Warwick & Gee, 1984). This resulted in the enumeration of 220 species, belonging to 102 genera, each with a characteristic distribution along the salinity, sedimentary and latitudinal gradients. Using the multivariate technique CANOCO, a zonation along these different physicochemical determinants was observed as well although salinity and sediment characteristic (scale of hundreds of meters to kilometers) proved to be more important in explaining community structure than latitudinal differences (scale of hundreds of kilometers). Nematode diversity was nearly entirely determined on the genus level and was positively related to salinity. Deviations from this general trend in the Gironde and the Tamar were attributed to sedimentary characteristics or to low macrobenthic predation. The presence of a typical opportunistic colonizing nematode species Pareurodiplogaster pararmatus in the low-salinity region of the Gironde could indicate (organic?) pollution or disturbance of the intertidal mud-flats.

Trophic relations between cyclopoid copepods and ciliated protists: Complex interactions link the microbial and classic food webs

Two field experiments examined the effects of cyclopoid copepods on ciliates. The presence or absence of Cyclops abyssorum, Cyclops kolensis, and zooplankton &gt; 64 µm was manipulated to determine the relative importance of direct cyclopoid predation on protists vs. indirect effects mediated through cyclopoid predation on other metazooplankton. In the second experiment, presence or absence of C. abyssorum was cross‐classified with five concentrations of the metazooplankton community. Cyclopoid effects on ciliates were dependent on predator and prey species and on the abundance of alternate prey for cyclopoids. A trophic cascade was also observed, but only for two small ciliates, and only with the larger C. abyssorum. C. abyssorum had a stronger predation effect on oligotrich ciliates when metazooplankton had been removed, and this effect appeared at a lower metazooplankton concentration with a larger ciliate, compared to a smaller species of the same genus. These results suggest that for cyclopoid‐ciliate interactions, switching behavior in the predator may be at least as important as a trophic cascade.

Size structure of plankton in a temporary, saline inland lake

The study of the size structure of a planktonic community in a relatively extensive (1350 ha), very shallow (0.46 m maximum depth), temporary inland saline lake during a relatively rainy year (1987–88) showed an average biomass-size spectrum with four clear modes at nano-, micro-, meso- and macroplankton size ranges. The size-abundance distribution was dominated by medium-sized phytoplankton cells (Gymnodinium cf. excavatum, Tetraselmis apiculata, Dunaliella spp. and several diatom species), large ciliates (Fabrea salina) and a few zooplankton species adapted to changing temporary environments within a wide size range (Cletocamptus retrogressus, Moina salina, Branchinella spinosa). This polymodal size distribution is compared with results obtained in other systems, including small physically controlled lakes, large lakes, oceans and marine benthos. Consideration of the size structure within homogeneous groups of organisms shows slopes less negative than the average for the whole size range analysed, a feature which is common in small and physically controlled ecosystems. A seasonal study of the size structure revealed uneven distributions throughout the year, resulting in discontinuities in the size spectra as well as in differences of total size range. There were noticeable differences among the size spectra at each particular time, showing phases of primary producer predominance and phases with a marked predominance of consumers. The time-integrated size spectrum did not exhibit any distinct discontinuity. The influence of extreme shallowness and resuspension on the shape and dynamics of the size spectrum is examined and discussed compared to deeper aquatic systems.

Spatial Patterns of Westerschelde Meiobenthos

The meiobenthic fauna of the Westerschelde, a highly polluted and physically disturbed estuary in the south-west Netherlands, was investigated. Samples were taken in spring from six transects, including the intertidal, subtidal and channel area, and located along the salinity gradient. The samples were subdivided into slices to examine the vertical distribution of meiobenthos. Meiobenthos densities were higher in the intertidal than in most permanently submersed areas; the subtidal sites below 7 salinity were nearly devoid of meiobenthic life. Nematodes were by far the most abundant meiobenthic organisms in the intertidal, but were less dominant in the other areas. Gastrotrichs, turbellarians, copepods and large ciliates were usually more numerous in the subtidal and channels compared to the intertidal, both in relative and absolute terms. Vertical distribution of the meiobenthos was rather heterogeneous. Most intertidal stations exhibited a subsurface density peak, whereas in the subtidal and channel area, both subsurface and surface maxima were found. The nematode fauna was examined in more detail and the distributional characteristics of the most important species, with respect to salinity, grain size, water depth and sediment depth were reported. The majority of species had their centre of distribution in the intertidal, although some extended substantially into the subtidal zone. Only a few species had a predominantly subtidal distribution. Most nematode species penetrated relatively deep into the sediment and only some species were real surface dwellers. The nematode diversity per unit of surface reflected more or less the density differences and was higher in the intertidal than in the subtidal sites within a comparable salinity regime. When expressed per common number of individuals, however, there were no differences in diversity in the different areas. Canonical correspondence analysis showed sediment depth to be as important as water depth, salinity or sedimentary characteristics in the determination of community structure. Intertidal communities exhibited a well-developed community gradient with depth into the sediment, whereas the vertical structure of subtidal and channel stations was different from the intertidal zonation and in some cases showed a distorted pattern. This was probably caused by sediment disturbance due to higher current velocities and dredging activities in these regions. It is argued that, although at some subtidal sites a characteristic subtidal nematode population may persist, in many cases the sublittoral of the Westerschelde is either too dynamic an environment or food availability is too low to meet requirements for growth and reproduction of the nematodes. The populations are probably not self-sustaining but persist due to continuous replenishment from less harsh areas by means of the estuarine circulation.