THE MOBILE EPIFAUNA OF THE SOFT BOTTOMS IN THE SUBTIDAL OOSTERSCHELDEESTUARY - STRUCTURE, FUNCTION AND IMPACT OF THE STORM-SURGE BARRIER

Data on the mobile epifauna of the Oosterschelde estuary, collected by beam trawl, were compiled from several studies. Multivariate statisti cal techniques brought out the fact that the Oosterschelde, when compa red with neighbouring areas, has a characteristic epibenthic fauna. Di versity as measured by Hill's diversity numbers N through N(+ infinity ) is higher for the Oosterschelde (N1 = 4.5) than for the Voordelta (N 1 = 3.5) and the Westerschelde (N1 = 2.2). Four epifaunal communities can be distinguished within the Oosterschelde, the two most seaward co mmunities being the richest. Annual production is estimated at about 6 gADW M-2 yr-1, annual consumption is estimated at over 25 gADW M-2 yr -1. These results are highly dependent on the assumptions. Over 85% of the epibenthic production and consumption in the Oosterschelde is acc ounted for by only six species: starfish Asterias rubens, plaice Pleur onectes platessa, bib Trisopterus luscus, brown shrimp Crangon crangon , shore crab Carcinus maenas and dab Limanda limanda. In spite of its abundance, the sand goby Pomatoschistus minutus contributes little to the production. From the available data it is difficult to assess the impact of the construction of the storm-surge barrier and the compartm entalization dams on the epibenthic fauna. The increase in flatfish in the Hammen area is probably linked to the decrease in current velocit ies in that area. On the other hand the increase in the gadoids bib an d whiting Merlangius merlangus is predominantly due to the stronger ye ar classes in the post-barrier time period. Lower nutrient inputs thro ugh the Northern branch, in combination with the increase of the gadoi ds, may have caused the decline of the brown shrimp in the Oosterschel de. A decrease has also been observed in the sandeel Ammodytes tobianu s and the hooknose Agonus cataphractus.