ANALYSING BRACKISH BENTHIC COMMUNITIES OF THE WESER ESTUARY:SPATIAL DISTRIBUTION, VARIABILITY AND SENSITIVITY OF ESTUARINE INVERTEBRATES.

Abstract

This study analyses the benthic invertebrate fauna of the brackish-water zone of the Weser estuary from km 45 to km 115, based on personal sampling and observation in the years 1994 to 2002 and additional data from the region since 1980. In order to provide the biological information for an evaluation and the conservation management of the Weser estuary, the spatial distribution of benthic invertebrates and their sensitivity to human interference was investigated in three case studies. Additional external data of benthic invertebrates were added to provide a comprehensive database that allows a transfer of recognized principles from the case studies to a benthic characterisation of the entire estuary.By including the supratidal areas in the analysis, 10 brackish subzones were identified and compared by area and benthic assemblages. In contrast to earlier characterisation by dominant or frequent species, a biotope characterisation by brackish-water species is suggested, which allows the identification of small-structured biotopes. The strong gradients from land to water and river to ocean result in a variety of special biotopes demanding extreme adaptive skills from benthic inhabitants. A low point diversity, which reflects the strong species selection of such an environment, combined with an increase of species richness on larger scales is found to be characteristic for a transitory environment as represented by an estuary.Sensitivity to impacts depended on certain biological criteria both at the species and the community level. An efficient conservation and management strategy for estuarine features as recently claimed by the EU directives needs to take into account the observed different spatial scales of individually structured biotopes and the specific sensitivity of estuarine benthic communities to impacts to ensure the continuance of the estuarine environment as an entity.