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The importance of biodiversity for the functioning of ecosystems is still very unclear. Many hypotheses, mainly based on terrestrial studies, have been formulated, focussing on the plant diversity-productivity relationship. In this study, diversity-related and species-specific effects of bacterivorous nematodes on the decomposition rate of cordgrass detritus Spartina anglica and on the associated microbial community were investigated using laboratory microcosm experiments. Four bacterivorous nematode species (Diplolaimelloides meyli, Diplolaimelloides oschei, Diplolaimella dievengatensis and Panagrolaimus paetzoldi) were added either separately or in combinations of 2 or 3 species to the microcosms. In contrast with previous reports, no stimulation of the decomposition process was observed in the presence of nematodes. Still, clear differences in process rates were found between nematode treatments. P. paetzoldi, reaching considerably higher densities than the other nematode species, suppressed bacterial activity and diversity, probably due to overgrazing. This was, however, not translated into slower decomposition. Within treatments exclusively containing monhysterid nematode species (D. meyli, D. oschei and D. dievengatensis), differences in microbial activity and decomposition rates were found, but again no link was observed between activity and decomposition. Our data did not support any hypothesis predicting enhancement of process rates with an (initial) increase in numbers of nematode species, or redundancy among the studied species. Rather, we obtained support for an idiosyncratic diversity model, as differences in the effect of bacterivore nematode species and species combinations could not be predicted at the start of the experiment. This could be explained by the inhibitory interactions between nematode species.

Estuarine soft sediments support a diverse group of eukaryotic and prokaryotic organisms though the role of the sediment per se for the functioning of these organisms remains largely unknown. The present study aimed to test the effect of sediment grain size on the grazing activities of harpacticoid copepods. In controlled experiments, two common intertidal harpacticoid species (Paramphiascella fulvofasciata and Nitokra spinipes) were each offered a mix of two benthic diatom species (Navicula phyllepta and Seminavis robusta) in different sedimentary conditions. Several microcosms were created using a variety of sediment types, including fine silt (13C to facilitate tracing in the harpacticoids. Both copepod species were able to graze on the diatoms with highest uptake when sediment was absent. In contrast, both harpacticoid species showed no uptake in silty conditions. In general, grazing was favoured when mean sediment grain size increased. The strong negative effect of fine grains on the grazer’s efficiency can be explained by the resulting differences in the structure (and accessibility) of the diatom biofilm on the one hand and the mobility of the grazer on the other hand. In view of the subtle equilibrium between primary producers and grazers, these results might have important implications for the effect of siltation of tidal flats due to, e.g., human activities.

International audience; The composition and seasonal dynamics of biofilm-associated eukaryotic communities were analysed at the metre and kilometre scale along a salinity gradient in the Westerschelde estuary (The Netherlands), using microscopy and a genetic fingerprinting technique (PCR-DGGE). Microphytobenthic biomass, measured as chlorophyll a (chl a), varied seasonally over 2 orders of magnitude, being highest in spring. Communities were dominated by epipelic diatoms, in particular by members of the genus Navicula. In spring, a few smaller epipelic diatom species dominated during biomass peaks, while during the rest of the year, communities were more diverse and were characterised by larger species. The microphytobenthic community collapsed when grazers appeared, which happened concomitantly with a rise in temperature. Spring biomass development was associated with marked changes in porewater nutrient concentrations, especially towards the estuary mouth. In the DGGE data, diatoms, ciliates, amoebae, copepods, nematodes, annelids and platyhelminthes were detected. Ordination analysis of the species counts and DGGE data were largely congruent and indicated that on the scale of the whole estuary (i.e. km scale), taxonomic turnover in microphytobenthos composition was mainly associated with the salinity gradient. At smaller spatial scales, the position of sampling localities along the tidal exposure gradient appeared to be the main determinant of species turnover, in particular in the brackish reaches of the estuary.

During inundation of tidal marshes, fine-grained suspended matter is transported to and partly deposited on the marsh surface. In this research the complex spatial patterns of sediment transport and deposition are studied at the temporal scale of individual inundations and spatial scale of a small tidal creek basin (ca. 6 ha) within the salt Paulina marsh, Western Scheldt. Field measurements are used for the implementation and validation of 2-dimensional numerical models for tidal marsh sedimentation. Near the mouth of the creek system, the incoming and outgoing sediment mass is estimated, by way of water level, flow velocity and suspended sediment concentration (SSC) measurements. Spatial variations in SSC, at the moment of marsh inundation, are measured at about 35 locations within the creek system and above the marsh surface, using siphon samplers. Finally the sediment that is deposited on the marsh surface is sampled with sediment traps on 50 sites, both during 4 individual inundations (about 4-5 hours) and 2 spring-neap tidal cycles (15 days). First, it is investigated how the 2-dimensional pattern of SSC and sedimentation can be described by statistical models, incorporating detailed topographic information on the creek network, surface elevation and vegetation pattern. Secondly, the application of physically-based hydrodynamic models, coupled with sediment transport models, is explored and evaluated against the field data. Once validated, these models may be useful to simulate flooding and sedimentation patterns in other tidal marshes and controlled inundation areas in the Scheldt estuary.