Studies on heavy metal toxicity and accumulation in the catchment area of the Derwent Reservoir

Abstract

A three year long study has been carried out of the distribution of zinc, lead and cadmium within the water, sediments and submerged plants of the Derwent Reservoir and its catchment These studies have been extended to include detailed field investigations of the accumulation of heavy metals by Lemanea fluviatilis, an alga common in the R. Derwent Further field and laboratory investigations have been carried out of the tolerance to zinc of Stxgeoclonxum tenue. Elevated concentrations of zinc, lead and cadmium were shown to be present, in the water and sediments of the R. Derwent at its point of entry to the Derwent Reservoir. These metals were derived mainly from Bolts Burn, a polluted tributary. This latter stream was found to receive inputs of heavy metals from clearly defined sources within an active fluorspar mine. Although old mine workings were present within the catchment, these had a relatively small effect on the composition of water in Bolts Burn and the R. Derwent. Detailed investigations of the composition of stream and river water enabled various aspects of the behaviour of different fractions of metals to be described and compared in polluted and non-polluted reaches. The concentrations of zinc, lead and cadmium within the water, sediments and submerged plants of the Derwent Reservoir were found to be high when compared with data from other published studies. The pollution of the reservoir with these metals may therefore be regarded as serious. Surveys of the distribution of heavy metals in the water column were carried out at different stages in the filling cycle of the reservoir. These, together with surveys of the composition of sediments and submerged plants, enabled several of the major factors influencing the distribution of metals within the reservoir to be determined. Studies of the accumulation of heavy metals by 47 populations of Lemanea fluviatilis indicated that the alga is a potentially useful 'monitor' of pollution by zinc, lead and cadmium in flowing waters. Although the enrichment ratios for these metals were not constant over a range of concentrations in the water, clear linear relationships were apparent between the concentrations of each metal in the water and in filaments of Lemanea. Intensive sampling from a single population in the R. Derwent demonstrated that a proportion of the zinc content of filaments was sensitive to short term fluctuations in the zinc content of the surrounding water. The results of a series of transplant experiments are also reported. Stigeoclonium tenue was found to be abundant in several streams carrying relatively high concentrations of zinc in the water A study of populations isolated from 35 reaches demonstrated that material growing in higher concentrations of zinc in the field had an enhanced tolerance to zinc. This tolerance was stable during long term culturing, and appeared to have a genetic basis. Assays of populations from harder waters suggested that high concentrations of calcium acted to reduce the toxicity of zinc in the field. Further studies performed in the laboratory demonstrated that increases in pH and the concentrations of magnesium, calcium and phosphate all acted to reduce the toxicity of zinc to Stigeoclonium tenue. However, the effects of these factors on toxicity were found to differ between a zinc sensitive population and a zinc tolerant population