Ecological and sedimentological studies on china clay waste deposits in Mevagissey Bay, Cornwall
Prior to 1973, the St Austell china clay industry discharged fine grade waste into local rivers which carried the waste into Mevagissey Bay, Cornwall. The clay waste, a mica and quartz rich silt buried a marine shell gravel substrate by up to 2 metres. The china clay waste and natural sediments on the surface and subsurface of Mevagissey Bay have been studied and the distribution of live and dead benthic faunas analysed. The benthic faunas presently living in the china clay waste and surrounding shell gravel have been compared to faunas from a similar survey taken during the period of peak discharge (1968) and during the reduction of discharges (1970-1973). Within the bay are four different benthic communities whose distribution reflects the amount and nature of china clay waste present. The only notable changes since the the cessation of dumping being the colonisation by a Tellina tenuis community of a previously azoic muddy substrate near the point of waste discharge into the bay. This research suggests that benthic communities in areas subjected toinert solid pollution will change to communities typical of the grain size of the waste. Each community can be recognised by a distinctive death assemblage although these show little resemblance to the composition of the original living community. Different taphonomic processes control the preservation within each community. Radiographs and microscopic examination of impregnated box core samples reveal the important effect of bioturbation on the orientation and position of shells and shell fragments within the substrate. Since the cessation of china clay waste dumping in 1973, shell debris formed by local benthos has began to transform the soft, fine grade substrate into a shell gravel. It is predicted that it will take within the order of 10,000 years for the substrate to resemble the original shell gravel now lying below the china clay waste.