The main objective of this study was to assess the impact of the major discharge, Cliff Quay Sewage Treatment works on the benthic fauna of the Orwell Estuary. One Day Grab sample was taken at each of twenty eight sites. Sub samples of the sediment were taken for particle-size analysis and organic carbon content determination. The remainder of each sample was put through a 500 micron sieve and stored for later analysis. The results obtained indicated a gradual improvement of the subtidal fauna in a seaward direction. Four species assemblages were identified correlating with the distance down stream from the Orwell Bridge, the combined percentages of silt and clay and the percentage of organic carbon content. Sites in the upper estuary had low diversity, dominated by Tubificoides pseudogaster, Tubificoides swirencoides and Tharyx sp.. A faunistically impoverished area was observed around the Woolverstone Marina (sites 7-10), whereas an area of relatively high diversity was detected down stream of Collimer Point (sites 19-28).
