A map of the Looe Estuary in south-east Cornwall is shown in Fig. 1. As it flows to the western branch of the estuary, the West Looe River passes through the Herodsfoot mining area which lies about 5 km above Sowden's Bridge. By Cornish standards, this was a productive area for lead and Dewey (1921) gives the output of Herodsfoot mine as 13470 tons of metal between 1848 and 1884 and more than 17 tons of silver between 1853 and 1884.

The bivalve mollusc Scrobicularia plana (da Costa) is very common in the estuaries of South-West England and lives in permanent burrows in intertidal sediments ranging from soft mud to fairly hard sand and sometimes gravel. Although it obtains some of its food by filtering the overlying water, Scrobicularia is mainly a deposit feeder (Hughes, 1969). In addition to its widespread distribution, Scrobicularia has several features which make it attractive as a possible indicator organism for metallic contamination. It is more tolerant of low salinities than most common estuarine bivalves, usually penetrating farther upstream than Macoma balthica, Mytilus edulis and Cerastoderma edule (Percival, 1929; Spooner & Moore, 1940; Bryan & Hummerstone, 1977). Also, it lives for over ten years, usually attaining a length of 4–5 cm, and its rate of growth can be estimated from rings on the shell (Green, 1957; Hughes, 1970).

A century or so after the cessation of almost all mining in Cornwall, certain estuaries still have extremely high sediment concentrations of toxic trace metals, particularly copper and arsenic, but also lead and zinc. These high trace metal loadings in the sediments are to a large degree bioavailable to the local infauna, especially sediment-ingesting invertebrates. Some sediment trace metal bioavailabilities are so high as to be of ecotoxicological concern, with deleterious effects on the local biota at levels of biological organization up to and including changed community structure. The estuaries of interest here are those of the Rivers Carnon (Restronguet Creek), Tamar (and Tavy), Gannel, West Looe and Hayle. These estuaries are especially attractive field sites for comparative trace metal ecophysiology and ecotoxicology research for they lack the confounding presence of other anthropogenic contaminants inevitably present in most estuaries in the developed world. The estuaries also offer a range of combinations of different trace metals and a comparative gradient of sediment bioavailabilities of these trace metals.

Analysis of the deposit-feeding bivalve Scrobicularia plana (da Costa) has been proposed as a method of assessing the biological availability of heavy metals in estuarine sediments (Bryan & Hummerstone, 1977; Bryan & Uysal, 1978). Scrobicularia has a number of attributes which are useful in this type of indicator: (i) it is common in many British estuaries, particularly in the south, and often penetrates much farther upstream than other common bivalves such as Mytilus edulis; (ii) it is a convenient size for analysis and, during its life span of perhaps 10 years, reaches a shell length of 40-50 mm (Green, 1957; Hughes, 1970); (iii) it is a good accumulator of metals and appears to reflect changes in their biological availability.

Concentrations of lead in the soft tissues of the deposit-feeding bivalve Scrobicularia plana have been compared with the physicochemical characteristics of sediments in 20 estuaries in southern and western England and one in north-west France. The results indicate that the biological availability of lead in the sediment is controlled mainly by the concentration of iron, and that the concentration of lead in the bivalve may be predicted from the Pb/Fe ratio in 1 N hydrochloric acid extracts of surface sediments.

In the first paper of this series (Bryan & Hummerstone, 1971) it was shown that in the estuarine polychaete Nereis diversicolor O. F. Müller the concentration of copper is roughly proportional to that of the surrounding sediment. On the other hand, concentrations of zinc appeared to be relatively independent of those in the sediment and it was suggested that the level of zinc in the worm is regulated.

The concentration of copper in Nereis diversicolor O. F. Miiller is roughly related to the total concentration in the sediment and particularly high concentrations are found where mining pollution occurs. In contrast, the concentration ofzinc in Nereis remains remarkably constant despite wide variations in the environment and appears to be accurately regulated.

Arsenic concentrations in estuarine sediments from England and Wales range over three orders of magnitude. The highest concentrations, up to 2500 μg/g, occur in the sediments of estuaries in south west England associated with past or present metalliferous mining activity. Strong correlations exist between arsenic and iron in 1 N-HC1 extracts of different estuarine sediments. The As/Fe ratio in those estuaries not contaminated with mine wastes is 11 × 10−4, increasing to 190 × 10—4 in metalliferous sediments.