Contaminants in fine sediments and their consequences for biota of the Severn Estuary.

When the first MPB special issue was published 25 years ago it was suggested that high body burdens of metals and selected organic pollutants in the Severn Estuary were the result of anthropogenic loadings from a variety of sources. The objective of this synopsis is to illustrate recent trends for contaminants (metals, PAHs, PCBs) in sediments and benthic biota and to consider the evidence for improved environmental quality over the last quarter of a century. Contaminants in sediments and sediment-dwelling fauna such as Hediste(=Nereis)diversicolor are, generally, evenly distributed over the estuary - which is the consequence of extensive re-suspension and redistribution of fine sediment by strong tidal currents. Such dispersal tends to mask the influences of individual discharges and physical characteristics are considered to be the major drivers affecting biodiversity in the Severn Estuary, often overshadowing contaminant concerns. Following the closure of major industries and the introduction of stricter pollution control, many inputs have ceased or been reduced and there are indications that environmental concentrations are now lower. Bioaccumulation of most contaminants has declined accordingly (with the possible exception of Cr). Intuitively, better environmental quality should be linked to ecological improvements. However, due to the dynamic nature of the system (and a lack of biological-effects data) it is difficult to establish direct relationships between inputs, body burdens and biological/ecological consequence. Uniquely, the long-term integrated monitoring program of AstraZeneca (Avonmouth) indicates that recovery of faunal diversity and abundance has occurred in mid-sections of the estuary in recent years implying that contaminants have indeed been a forcing feature for Severn biota. In this context, we highlight contaminant issues and biogeochemical changes which may need to be addressed in connection with the development of proposals for tidal energy schemes.