Benthic responses to organic enrichment and climatic events in the western North Sea

Abstract

Macrofaunal assemblages inhabiting stable offshore muddy sand substrata responded predictably to the effects of mild organic enrichment arising from sewage-sludge disposal off the north-east coast of England (western North Sea). At the disposal site, densities were elevated up to two-fold, but classical ‘indicator’ species were only marginally enhanced and there was no evidence of a significant waste-induced change in assemblage structure. The response following cessation of disposal was equally predictable, with a decline in densities to ‘reference’ levels some three years later. However, physical manifestations of disposal, including tomato pips and non-biodegradable artefacts, were still evident after this time.Changes in diversity at selected monitoring stations tended to track each other over time and the employment of treatment/reference ratios and limit values for acceptable change provided a useful model for the simplified expression of trends. Predictions concerning the limited scale and intensity of the effects of sewage-sludge disposal appear to have been met, indicating that the management option of sea disposal was, at the time, an environmentally acceptable one.Temporal trends in the benthic fauna were also correlated with winter values of the North Atlantic Oscillation Index for the preceding year. The densities and variety of species tended to be lower in response to warmer winters characterized by westerly airflows which were commonly encountered in the 1990s. Finally, the increasing importance of extended time-series data for the investigation of man-made impacts on the marine environment is highlighted, along with the accompanying requirement for continuity and quality assurance of sampling programmes.