The mobile epifauna of the soft bottoms in the subtidal Oosterschelde estuary: structure, function and impact of the storm-surge barrier

Abstract

Data on the mobile epifauna of the Oosterschelde estuary, collected by beam trawl, were compiled from several studies. Multivariate statistical techniques brought out the fact that the Oosterschelde, when compared with neighbouring areas, has a characteristic epibenthic fauna. Diversity as measured by Hill's diversity numbers N through N + 00, is higher for the Oosterschelde (N 1= 4.5) than for the Voordelta (N 1= 3.5) and the Westerschelde (N 1 = 2.2). Four epifaunal communities can be distinguished within the Oosterschelde, the two most seaward communities 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 accounted for by only six species: starfish Asterias rubens, plaice Pleuronectes 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 compartmentalization dams on the epibenthic fauna. The increase in flatfish in the Hammen area is probably linked to the decrease in current velocities in that area. On the other hand the increase in the gadoids bib and whiting Merlangius merlangus is predominantly due to the stronger year classes in the post-barrier time period. Lower nutrient inputs through the Northern branch, in combination with the increase of the gadoids, may have caused the decline of the brown shrimp in the Oosterschelde. A decrease has also been observed in the sandeel Ammodytes tobianus and the hooknose Agonus cataphra-ctus.