SPATIAL AND TEMPORAL PATTERNS OF INTERTIDAL MACROBENTHIC POPULATIONS IN THE OOSTERSCHELDE - ARE THEY INFLUENCED BY THE CONSTRUCTION OF THE STORM-SURGE BARRIER

The construction of a storm-surge barrier in the mouth of the Oostersc heide caused important hydrodynamical and morphological changes that c ould influence the macrobenthic populations. This paper is one in a se ries of five all dealing with the effects of the storm-surge barrier o n macrozoobenthos and analyses the spatial and temporal distribution o f macrozoobenthos in the Oosterschelde and its relationship with some environmental parameters, based on two large scale sampling campaigns, one before and one after the completion of the barrier.The sediment o f the sampling stations was fine, well sorted sand, with an average mu d content of about 2.5%. Only in the Krabbenkreek the sediment was coa rser in 1989. The tidal elevation of the sampling sites decreased sign ificantly in 1989. The density of macrozoobenthos was significantly lo wer, the biomass higher in 1989. The density was dominated by deposit feeders, the biomass by filter feeders. The difference in biomass betw een both years was mainly due to a substantial increase of the biomass of filter feeders in 1989. The number of species per station was sign ificantly smaller in 1989 than in 1985. Between 1985 and 1989, frequen cy of occurrence decreased in 34 versus 13 which increased, density in creased in 13 species and decreased in 34 species, biomass increased i n 18 species and decreased in 29 species. Based on TWINSPAN several cl usters of stations, each with a different faunal composition, were ide ntified. These clusters did not form distinct zones on the tidal flats but were dispersed widely. The relationship between density and bioma ss of different trophic groups and the mud content of the sediment and the depth was analysed. This relationship sometimes differed clearly between years. The correlation coefficient of a multiple regression be tween density and biomass of individual species and environmental fact ors, although significant in most cases, was very low, indicating that only a small proportion of the species variability was explained. The relationship between benthos and environmental factors was further an alysed by canonical correlation analysis and multivariate discriminant analysis that gave different results for the 1985 and 1989 data. This is probably due to the broad tolerance of the species to the range of the environmental variables found in our study area. From a TWINSPAN of the density data of 1985 and 1989 together we could conclude that, although the environmental parameters in a group of stations, showing a large faunal similarity in one year, did not change, the faunal comp osition did. This indicates that faunal changes are not necessarily li nked to changes in the measured environmental parameters. In the discu ssion the different factors affecting macrobenthic populations are sit uated and it is suggested that the macrobenthic populations are probab ly more towards the nonequilibrium end of the continuum between nonequ ilibrium and equilibrium communities as defined by Wiens (1984). Altho ugh the impact of the construction of the barrier on the macrobenthic community seems at present to be rather small this does not mean that on the long-term there will be no effect.