Untersuchungen über die Auswirkung kommunaler Abwässer auf das benthische Ökosystem mariner Watten

Abstract

The influence of domestic sewage on intertidal sand and mud flat benthic communities has been studied at three minor outfalls around the North Sea island of Sylt (F. R. Germany) and at the main outfall of Vancouver, B. C. (Canada). During high tide, domestic waste water was found at the water surface due to its lower specific gravity. Consequently, O2 depletion and salinity decrease occured mainly in surface waters. Oxygen deficiency prevailed in the sediment-water layer, when settled sewage particles were re-suspended by strong tidal currents. This resulted in anaerobic conditions at the sediment surface for up to 40% of the day. Substantial pH decrease and increase in seston load of the water were observed only along the sewage channels and around the outlets when large quantities of sewage were discharged. The amount of particulate organic material increased up to 10% and more in muddy sediments due to sedimentation of sewage sludge. Considerable eutrophication of the mud flats resulted from nutrient load of the sewage and from decomposition of faecal material deposited on the sediments. The vertical distribution of microalgae in the sediment was not influenced by sewage. Primary productivity of microalgae on the sediment surface was positively correlated with the degree of pollution. In heavily and mediumly polluted zones, heterotrophic bacterial production exceeded primary production rates at the sediment surface. Compared with unpolluted intertidal flats the number of macrobenthic species was not reduced in polluted mud flats. In regard to overall species abundance, the macrofauna associations of the disposal area showed three distinct zones: (1) Heavy pollution around the outfall resulted in a degradation zone either without benthic invertebrates or with a very low density of individuals; (2) medium polluted areas were characterized by a zone of maximum density; (3) with decreasing or diminishing pollution the number of individuals returned to normal values ("background values"). The distribution of species within these zones indicated a significant difference between the response of the "channel fauna" and the "flat fauna" to pollution: while the number of species increased continously inside the channels with decreasing pollution, the species variety of the tidal flats was positively correlated with the number of individuals. The diversity of the benthos inside the channels was negatively correlated with the degree of pollution, whereas the diversity indices for the fauna on the tidal flats increased quickly from the centre of pollution, remained at a high level in the medium polluted zone, and decreased again to the area of diminishing pollution. The number of dominating species in the sewage channel increased from the outlet towards the low-tide level, with ratios of 0.9 to 0.33; on the flats the number remained constant throughout the three zones, with maximum ratios of 0.5, demonstrating that no qualitative changes for the macrobenthos of intertidal flats result from domestic sewage disposals. In polluted intertidal flats the macrobenthos is dominated by muddy-sand species whose adaptability to varying sedimentary factors, such as grain size distribution and content of organic material (coefficients of variability: 50.7 and 45.7%, respectively), is relatively high compared with mud and sand species. A special indicator community for polluted intertidal areas does not exist. Intensified waste-water treatment prior to discharge reduced the size of the degradation zone around the outfalls while it simultaneously caused a lower density of the macrobenthos in the medium polluted zone. © 1979 Biologischen Anstalt Helgoland.