NEKTON COMMUNITIES OF AN INTERTIDAL CREEK OF A EUROPEAN ESTUARINE BRACKISH MARSH

The utilization of an estuarine brackish marsh in the Westerschelde estuary (southwest Netherlands) by fish and crustaceans visiting the intertidal creeks at rising tides was investigated over an 18 mo period. Samples were collected every month over a tidal cycle. A stow net passively fished the nekton migrating in and out of the marsh habitat. Simultaneous measurements of current speed and waterheight allowed for quantification of the catches. Multivariate clustering and ordination techniques were applied to assess temporal changes in the nekton assemblage. A total of 68 fish and crustacean species were caught but only 38 were regular visitors of the marsh creek. The marsh nekton community proved to have a very stable character. Only 40% of the observed variance could be explained. The seasonal appearance of juvenile stages of fish and crustaceans was found to differentiate between spring and summer-autumn communities. The environmental variables measured (i.e. temperature, salinity, oxygen concentration and detritus standing stock) did not correlate well with the observed changes in community structure. The mysid shrimp Neomysis integer dominated the community, both in density and biomass, during most of the year. Early postlarval flounder Pleuronectes flesus characterized the spring nekton assemblage. Early postlarval brown shrimp Crangon crangon was abundant from spring through autumn. In late summer juveniles of seabass Dicentrarchus labrax, common goby Pomatoschistus microps and shore crab Carcinus maenas were typical members of the marsh creek nekton. The hyperbenthic fauna of the marsh was comparable to the hyperbenthos found in the adjacent subtidal parts of the Westerschelde estuary while the epifauna of both areas differed substantially. It is argued that the marsh under study is a nursery ground for only a small number of estuarine inhabitants: C. crangon, P. flesus, C. maenas, P. microps, D. labrax and possibly 2 mullet species. Adult flounder, adult and juvenile common goby, juvenile seabass and the early postlarval brown shrimp all use the creeks as feeding ground, mainly preying upon the creek infauna and mysids. Given the high abundance of hyperbenthic mysid shrimp in the marsh and their presumed role in detritus-based food chains, we believe marsh studies should not refrain from considering this component of the nekton. This is the first study reporting on the nekton use of a European marsh. The need for similar research in other European marshes is stressed.