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Benthic organisms living in shallow high-energy areas are regularly suspended into the water column, where they dwell for longer or shorter periods before they re-enter the benthos. Marine free-living nematodes, normally the most abundant metazoans in soft sediments, lack a pelagic larval stage and are exceedingly poor swimmers. Although empirical data are lacking, nematodes are regularly considered to settle as passive particles through the water column. We carried out an experiment to assess whether nematodes are able to choose settling spots when descending from the water column. In the laboratory, nematodes were extracted from sediment collected from a shallow sandy bay and allowed to settle in a water column of 1 m to the bottom, which was seeded with 4 small containers containing: nothing, sediment without organic matter, sediment with added pelagic algae and sediment with added benthic algae (BA). The nematodes showed a clear preference for the BA containers, being about 7 times as abundant there than in the other treatments (p < 0.001). The nematode composition was different among treatments with 3 abundant species more or less confined to BA. The size distribution of the nematodes shows that the smallest nematodes are also the most efficient in choosing habitat. This is in concordance with the prediction that swimming ability is coupled to nematode size and water viscosity. The results obtained show that the conception of nematodes as passive particles in the water column is not entirely valid.

ABSTRACT:The diatom Skeletonema costatum is one of the dominant phytoplankton species during spring in the northern Baltic Sea. We followed the uptake of radio-labelled S costatum by all major meiofauna species in a laboratory experiment. The uptake of labelled diatom carbon varied greatly among major meiobenthic taxa and anlong species belonging to the same class or phylum. Both total uptake and uptake per unit biomass were by far highest in the ostracod Candona neglecta accounting for 4 6 % of the total meiofauna uptake. The total uptake of I4C was significantly different among all 3 common ostracod species, Candona neglecta taking 10 and 100 times more than Paracyprideis fennica and Heterocypndeis sorbyana respectively. Nematodes accounted for over 40% of the total uptake of 14C in the microcosms, of which 84 % was taken up by large nematode species such as Paracanthonchus spp. Nematodes with similar buccal cavities and of similar size showed surprisingly large differences in the uptake of the radio-labelled material. There seems to be quite strong selection both for and against the diatom among epistrate feeders as well as anlong so-called non-selective deposit feeders Only a small portion of the total meiofauna population was found below 1 cm in the sediment; this was composed almost solely of nematodes. These nematodes assimilated as much I4C per unit biomass as the surface ones did, which contradicts the hypothesis claiming that meiobenthic animals react in 2 ways to phytoplankton sedimentation, with surface feeders directly assimilating sedimented phytoplankton. while subsurface feeders experience a more stable food supply and rely only indirectly on sedirnented phytoplankton KEY WORDS: Pelagic-benthic coupling. Food-web . Meiofauna . 14C radio label