Changes in the benthic community of china clay waste deposits in Mevagissey Bay following a reduction of discharges

Abstract

Macrobenthic infauna was sampled in Mevagissey Bay, off the south coast of Cornwall, England, when the bay was receiving fine-grained mineral waste from the china clay industry at a rate of about 450000 tons per annum. Quintuplicate 0·1 m2 grab samples, washed on a 0·5 mm mesh, were taken on 15 occasions over two years at a station about 1 km from the outfall and at a water depth of 13 m. The fauna had a range of density of 755–2144 individuals/0–5 m2 and in composition resembled a partially impoverished Echinocardium cordatum/Amphiurafiliformis community. Population densities of Goniada maculata (Polychaeta), Cingula semicostata (Gastropoda), Nucula turgida, Mysella bidentata, Venus striatula and Tellina fabula (Bivalvia) showed no clear trends during the two years' sampling. Magelona filiformis (Polychaeta), Abra alba and Phaxas pellucidus (Bivalvia), Acrocnida brachiata and Amphiura filiformis (Ophiuroidea) declined in abundance, whereas the densities of Nephtys hombergi (Polychaeta) and Labidoplax digitata (Holothurioidea) increased. L. digitata became the commonest species, with a peak density of 708 individuals/0·5 m2. A month after sampling began the rate of waste discharged was reduced from 700000 tons per annum, but this did not appear to improve conditions for the benthic community. Total faunal density remained relatively stable but species diversity declined. It is argued that the suspended solid concentration was unlikely to have adversely affected the predominantly deposit-feeding community, but that persistent sediment instability continued to be a source of stress. Nevertheless, the observed major changes of community structure were similar to natural fluctuations of coastal soft-bottom populations resulting from species interactions and differences of larval success.