No 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)(1986)(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 397 398 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 sev...

Developments of intertidal hard-substrate communities in the Oosterschelde estuary were examined in perpendicular transects between high-water line and low-water line in the period 1982-1992 . Prior to the beginning of the Oosterschelde estuary works a typology of communities was established and an overall survey of the estuary was carried out . The communities contain flora (algae) as well as fauna . Due to asphalting of dikes in 1986, much of the surface of several communities has been destroyed . The originally well developed communities with large species-richness have not returned . The small reduction in tidal amplitude due to the construction of the storm-surge barrier had a minor effect on the zonation of communities . In the upper part of the intertidal zone the boundaries of the communities moved 0.5-1 .0 m downward in the transects along the dike-slopes . At an average inclination of 18° this means a vertical shift of about 15-30 cm . This reflects the reduction of the tidal amplitude : the high-water line shifted ca 22 cm downward . In a number of places sedimentation has caused a reduction in the number of smaller seaweed species in the lower eulittoral zone . At monitoring locations presence of the original communities is rather unchanged . Rare species like Pelvetia canaliculata, Actinia equina and Gelidium pusillum have been able to maintain quite successfully . 281

In 1970 the Biological Working Group of the Dutch Underwatersports Association started an investigation of the hardbottom biocoenoses in the Grevelingen. Since 1979 we extended the investigation and since 1980 a monitoring investigation of the fish fauna in Lake Grevelingen has also been carried out. The aims of the investigations are to gain insight into the presence and coverage of the hard bottom biocoenoses and their reactions to changes in the aquatic environment, as well as to gain a continuous up-to-date insight into the presence of the various fish species, so that (management) measures can be advised. In this paper, the results obtained in the period 1970 to 1983 are given. The authors recommend opening the sluice in the Brouwersdam for much longer periods and carrying out an active management using this sluice and the siphon sluice in the Grevelingendam, which can be put into use at the end of 1983.

Some aspects of the flora and fauna of the Dutch artificial rocky shores are described, together with abiotic factors determining their distribution. This leads to recommendations for future management. It is shown that there are many ways for a multifunctional use of the dikes and its flora and fauna. Certain types of flora and fauna might be used to strengthen a seawall. Examples are given of the possible application of hard substrate flora and fauna for water purification purposes and for monitoring of water and substrata quality.