BIOMASS AND STANDING STOCK ON SUBLITTORAL HARD SUBSTRATES IN THE OOSTERSCHELDE ESTUARY (SW NETHERLANDS)

From 1979 to 1991 the species composition of communities living on har d substrata (hardsub) in the Oosterschelde has been studied - in both the littoral and sublittoral zones. From 1984 onwards, biomass was als o measured. This paper deals mainly with the distribution and the deve lopment of biomass on sublittoral hardsub in the Oosterschelde. Analys is has shown that the most important abiotic factors regulating the fl ora and fauna are: quantity and nature of the substrate; sedimentation ; exposure to water movement (mainly currents); and light. The constru ction of the storm-surge barrier has influenced those factors. The mai n consequences for the flora and fauna on sublittoral hard substrata h ave been through the increased amount of available hard substratum by about 10% until 1984 and a further 20% from 1984 to 1987, the main bar rier construction period). Within the same period (until 1987) the bio mass per square metre also increased. This caused a net increase of ha rdsub biomass - in the sublittoral - of about 35%. After the barrier w as completed sedimentation increased; in some parts of the basin hards ub organisms were covered by sediment and have not recovered; the tota l quantity of available hard substratum decreased by an amount yet to be established. For the purpose of this paper it is tentatively estima ted at 20%, but the process is still going on. Tidal current velocitie s are smaller in the post-barrier situation, which caused a shift from more passive suspension feeders to more actively filtering species. T he relative importance of suspension feeders on hard substrata has dec reased by about 20% after the building of the storm-surge barrier. In 1990 and 1991 it increased again. Overall water transparency increased , but the lower limit of macroalgal growth has not gone deeper, as nea rshore turbulence and turbidity did not change significantly. Effects on hardsub were small in the beginning. During the construction period (1985-1987) no clear effects were registered. After the completion of the barrier total species diversity increased at first, followed by a decrease from the second half of 1988 onwards. Biomass increased rath er sharply, at first, but decreased very sharply in 1989. In 1990 a re covery in biomass became apparent. Developments in biomass and species composition differed per sampling location. An attempt is made to exp lain some of those developments, in relation to the abiotic changes br ought about by the storm-surge barrier. This appeared difficult, becau se climatic influences obscured the effects of the barrier. The most e xplicit of those masking effects was brought about by a temporary, hug e increase of the brittlestar (Ophiothrix fragilis). This animal cover ed the substratum in relatively thick layers (up to 5 cm) and more or less suffocated the other fauna. It was therefore difficult to quantif y the effect of increased sedimentation on the fauna. The increase of Ophiothrix is probably not caused by the storm-surge barrier, but by a succession of several mild winters. It is clear that a new equilibriu m in the basin is still to be reached. Total effects in terms of speci es richness and of biomass will continue to be monitored, and the resu lts used to advise the water authorities as to management and nature f riendly dike building methods.