Response of Macrozoobenthos to a Temporal Lowering of Chlorinity: A Large-Scale Field Experiment in the Dutch Delta Area1

Abstract

During the winter 1979/1980 the chlorinity in the Keeten-Krammer-Volkerak-area was considerably lowered, due to an experimental doubling of the freshwater discharge at the Volkerak sluices. To assess the possible impact, species composition, abundance and biomass of the intertidal macrozoobenthos was studied prior to (1978, 1979) and after (1980, 1981) the experiment. In order to prevent damage to the local commercial mussel-stocks, the condition of Mytilusedulis and water-quality-parameters were studied during the experiment. The species that were frequently found before the experiment were still abundant afterwards. Nevertheless, species number directly after the experiment was lower than before. Some of the relatively rare species that were not found immediately after the experiment, were present again later in 1980. The Shannon-Weaver diversity index did not show a significant change. Sample-classification using the Bray-Curtis dissimilarity-index, indicated that most pre-experimental samples were similar to post-experimental samples. Dissimilarity was found between March and October samples. Although locally high values of ammonia and E. colibacteria occurred, the water quality in general remained relatively good. The condition of the mussels was not demonstrably influenced by the experiment. Moreover, young mussels transferred to a control area in the Oosterschelde showed good growth, indicating also a recovery from the exposure to lower chlorinities.