CHANGES IN THE SUBLITTORAL HARD SUBSTRATE COMMUNITIES IN THE OOSTERSCHELDE ESTUARY (SW NETHERLANDS), CAUSED BY CHANGES IN THE ENVIRONMENTALPARAMETERS

In order to assess the effects of the execution of the Delta Project, the sessile sublittoral communities on hard substrates in the Oostersc helde estuary and the environmental parameters were quantitatively inv estigated from 1985 till 1990. During the construction period of the b arrier, three communities were sampled in the photic zone and four in the aphotic zone. The distribution of the communities in the photic zo ne seemed to be determined by the exposition to water movement and dep th, while the communities in the aphotic zone were restricted to geogr aphic areas, with differences in tidal current velocities: the mouth o f the estuary, the Hammen, the central part and the Zijpe. Two years a fter the completion of the enclosure works, the community structure ch anged rapidly, caused by decreases of tidal current velocities, increa ses of the amounts of sedimentation, especially of fine sediments, and an increase of the transparency of the water. Changes within the asso ciated vagile animals showed the same tendency as the sessile communit ies: under less exposed conditions the number of organisms remained th e same or increased, while at some locations this increase was nullifi ed by increasing amounts of sedimentation.