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.

The tellinid clam Macoma balthica (L.), a key member of numerous marine temperate soft-bottom communities, was used in laboratory experiments designed to evaluate its impact on developing microalgal mats and meiobenthic assemblages. Experimental jars (100 ml, 33 cm 2 ) were filled with azoic sediment, seeded with bivalves at various densities, placed in a large outdoor water tank with a constant flow of brackish water and left for 65 d. The bivalves efficiently kept the sediment surface clean of microalgal mats. At the end of the experiment, jars without clams were on average 99% covered by algae, while jars with clams (300 to 4800 ind. m -2 ) had less than 10% cover on average. There was a highly significant positive correlation between algal cover and the number of individuals belonging to the major meiofaunal taxa, i.e. Nematoda (r 2 = 0.61, p < 0.001) and Copepoda (r 2 = 0.79, p < 0.001). In containers with high clam densities (4800 ind. m -2), numbers of nematodes were significantly lower than in containers with low clam densities (300 and 600 ind. m -2 ), but no such difference was found for the copepods. Multidimensional scaling ordination indicated 3 distinct significantly different assemblages (ANOSIM, p < 0.01) of nematodes in jars with clam density of 0, 300 and 4800 ind. m -2 , while assemblage structure of copepods was only significantly different between jars with or without clams. In a separate experiment, we tested if the size of the clam and an initial organic enrichment of the sediment would affect the colonizing meiofauna. The results indicated that small bivalves also effectively prevented algal formation and that the development of meiobenthic communities depended on initial organic matter in the sediment. We concluded that indirect effects of Macoma balthica on the assemblage structure of meiobenthos by hindering development of microalgal mats are much larger than any direct effects. KEY WORDS: Microalgal mats · Soft-bottom · Macoma balthica · Colonization · MeiofaunaResale or republication not permitted without written consent of the publisher

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