Molluscs in biological monitoring of water quality.

Molluscs living in Lake Balaton accumulate persistent toxic substances, namely heavy metals, to a greater extent, than other organisms, and can serve as excellent passive biomonitors. Especially gills are good accumulators. Regular sampling showed that the level of Cd and Hg concentrations increased, while Pb contamination decreased during the past 20 years in mussels, corresponding probably to changes in pollution of the Lake. In functional, active monitoring various behavioral patterns of molluscs were employed. In mussels the periodicity of activity and rest (pumping activity vs. valve closure) is a sensitive indicator of unfavorable conditions and so of toxic substances. Low concentrations of inorganic and organic toxicants (heavy metals, PCBs, PAH compounds) cause reduction of the active and increase of the rest periods in a concentration dependent manner in a few hours. A second, suitable test for evaluating toxicity of chemicals is the measurement of the water flow through the exhalant siphon. Under the effect of toxicants the siphon activity, both the strength and duration of water flow changes characteristically within a few minutes. For both behavioral tests special techniques have been developed suitable for long duration recording, supported by mechano-electrical transduction and computerized data evaluation. In case of the pond snail (Lymnaea stagnalis L.) the behavior (positive/negative geotaxis and orientation) is disturbed in the presence of the above mentioned toxic chemicals. The execution and evaluation of the changes in the movement of the snail is based on video-recording and measurement of the direction and distance the animal performs in uncontaminated water and in the presence of the pollutants during the same period of time (0.5-4 h).