Biomass and standing stock on sublittoral hard substrates in the Oosterschelde estuary (SW Netherlands)

Abstract

From 1979 to 1991 the species composition of communities living on hard substrata (hardsub) in the Oosterschelde has been studied — in both the littoral and sublittoral zones. From 1984 onwards, biomass was also measured. This paper deals mainly with the distribution and the development of biomass on sublittoral hardsub in the Oosterschelde. Analysis has shown that the most important abiotic factors regulating the flora and fauna are: quantity and nature of the substrate; sedimentation; exposure to water movement (mainly currents); and light. The construction of the storm-surge barrier has influenced those factors. The main consequences for the flora and fauna on sublittoral hard substrata have been through the increased amount of available hard substratum by about 10% until 1984 and a further 20% from 1984 to 1987, the main barrier construction period). Within the same period (until 1987) the biomass per square metre also increased. This caused a net increase of hardsub biomass — in the sublittoral — of about 35%.

After the barrier was completed sedimentation increased; in some parts of the basin hardsub organisms were covered by sediment and have not recovered; the total quantity of available hard substratum decreased by an amount yet to be established. For the purpose of this paper it is tentatively estimated at 20%, but the process is still going on.

Tidal current velocities are smaller in the post-barrier situation, which caused a shift from more passive suspension feeders to more actively filtering species. The relative importance of suspension feeders on hard substrata has decreased 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 nearshore 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 rather sharply, at first, but decreased very sharply in 1989. In 1990 a recovery in biomass became apparent. Developments in biomass and species composition differed per sampling location. An attempt is made to explain some of those developments, in relation to the abiotic changes brought about by the storm-surge barrier. This appeared difficult, because climatic influences obscured the effects of the barrier. The most explicit of those masking effects was brought about by a temporary, huge increase of the brittlestar (Ophiothrix fragilis). This animal covered the substratum in relatively thick layers (up to 5 cm) and more or less suffocated the other fauna. It was therefore difficult to quantify 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 equilibrium in the basin is still to be reached. Total effects in terms of species richness and of biomass will continue to be monitored, and the results used to advise the water authorities as to management and nature friendly dike building methods.