THE USE OF BIOLOGICAL INDICATORS OF HEAVY METAL CONTAMINATION IN
ESTUARIES with special reference to an assessment of the biological
availability of metals in estuarine sediments from south-west
Report No DWI0703
The concentrations of 13 metals (Ag, As, Cd, Co, Cr, Cu, Fe, Hg, Mn, Ni, Pb, Sn, Zn) have been measured in biological indicator species and surface sediments from more than 100 sites in over 30 estuaries in south-west England and South Wales. Although the concentrations of metals in sediments reflect their input and retention by estuaries, they do not necessarily reflect the biological availability of sediment-bound metals as indicated by the concentrations in sediment-dwelling species.
Three burrowing species, the polychaete Nereis diversicolor and the deposit-feedinq bivalves Scrobicularia plana and Macoma balthica, have been evaluated as indicators of the availability of metals in estuarine sediments. Scrobicularia has been studied in most detail and is the best accumulator of metals. It exchanges metals slowly and appears to be a good indicator of changes in chronic contamination. Studies on the influence of size and season on metal levels in Scrobicularia show that sampling animals of 4 cm shell length in the autumn or early spring is the most reliable method of comparing different sites. Most of the metals lie in the digestive gland and some surveys have been conducted with the use of both this organ and the whole soft parts. The disadvantage of having to dissect the digestive gland is balanced to some extent by its ease of analysis and for most metals both tissues give comparable results. When Scrobicularia was absent, Macoma balthica (1.5 cm) was used wherever possible. The polychaete Nereis diversicolor was analysed at almost every site and proved a useful indicator for Ag, Cd, Cu and Hg although not Zn which it regulates.
Surveys with these species indicate that the biological availability of most metals differs by orders of magnitude between the sediments from uncontaminated and contaminated sites. The results for these organisms have been combined to make a preliminary classification of the estuaries for each metal. On this basis, the most contaminated are: Ag, East Looe and upper Severn; As, Restronguet Creek and Hayle; Cd, Severn and Plym; Cr, Loughor; Cu, Restronguet Creek and Hayle; Hg, Severn and Neath; Pb, Gannel; Sn, Loughor and Truro; Zn, Gannel.
Results for the burrowing indicator species are compared with those for other benthic species which have been used as indicators of the availability of metals in estuarine waters.Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.