Biomonitoring of heavy metals in seawater

A strategy for the use of the mussel Mytilus edulis planulatus in assessing heavy metal (Cd, Cu, Pb, Zn) levels in open marine waters is suggested. Metal accumulation by mussels was linear, in laboratory experiments and the rate was directly proportional to the external concentration. Depuration was minimal, and dependent on tissue concentration. Physiological and environmental factors were observed to influence accumulation rate. Smaller sized mussels accumulated metal at greater rates, while rates were higher at low water temperatures. Mussels collected from populations in areas where metal levels were high had lower rates of accumulation than mussels from more pristine areas. Rate of accumulation was shown to be a sensitive index of external metal concentration if the influence of external factors was minimized or eliminated. When metals were presented singly, the mussel accumulated zinc and lead in direct proportion to exposure time, while this was not the case for cadmium and copper. Under combined exposure to all four metals, cadmium and lead accumulation was proportional to exposure time, while zinc and copper accumulation was not. This apparent interaction between metals during simultaneous exposure was shown to be between cadmium, copper and zinc. In general, cadmium accumulation was decreased in the presence of the other metals, while that of copper and zinc was increased. Previous exposure to either the same or another metal had no influence on the subsequent accumulation of cadmium, lead or zinc. Copper accumulation was markedly increased in mussels initially exposed for 20 days to either cadmium or zinc. The fact that these interactions occurred predominantly at high external concentrations (e.g. Cd>10 µg ℓ-1, Cu>10 µg ℓ-1, Zn>100µg ℓ-1 ) suggests that they would not seriously interfere with the monitoring strategy. In the light of experimental results, a mechanism for accumulation of each metal by M. e. planulatus is proposed. Cadmium and copper appear to share a similar carrier-assisted uptake mechanism, while zinc and lead appear to diffuse across membranes passively. The method of storage of lead within cells appears to be distinct from that of the other three metals. Cadmium, lead and zinc were found to be taken up at both the gills and digestive tissue, and were generally evenly distributed throughout the mussel body, but with high concentrations in the kidney. Zinc levels were also high in the muscle and byssus/foot tissues. Copper, on the other hand, was taken up only at the gills and stored in the foot and digestive tissue (liver). The byssal threads appear to be a major excretory system for copper, and may also be important for the other three metals as well. Field trials of the proposed monitoring techniques, involving cage suspension of organisms, revealed that the mussel provides a very sensitive and time-integrated assessment of the biologically available metals in seawater. Biomonitoring using mussels would be less costly and labour intensive than conventional techniques currently available for measuring metal levels in seawater