1.Objectives

The DOE seeks to improve the scope of environmental monitoring programmes using marine animals by including an assessment of the impact of pesticides on marine water quality. Mussels have been used successfully to assess the bioaccumulation and toxic effects of organic chemicals of urban and industrial origin, but the effects of pesticides on the physiological parameters measured in these surveys are unknown. It is essential to establish whether mussels are as sensitive to pesticides as other estuarine and marine species that the Department may wish to protect through the establishment of appropriate environmental quality standards. The programme objectives were as follows:

1. To quantify the sublethal physiological responses of mussels to bioaccumulation of environmentally important pesticides.
   
2. To explain these responses in terms of anticipated modes of toxic action.
   
3. To apply quantitative structure-activity relationship (QSAR) modelling to facilitate extrapolation of conclusions to untested compounds of similar type.
   
4. To test whether there are interactions between the toxicity of representative pesticides and common environmental contaminants such as hydrocarbons.
   
5. To use the data to interpret results of field studies using mussels.

2.Methods

The work was carried out in the laboratory by exposing mussels to solutions of pesticides in seawater for periods of up to 7 days, and measuring the effect on feeding rate, respiration rate and absorption efficiency. These parameters are required for the calculation of the energy balance of the mussels (termed Scope for Growth (SFG)). SFG is used to compare the health of mussels in field populations. To facilitate comparison with such field studies, the levels of pesticides accumulated by the mussels in our laboratory experiments were measured.

3.Results and Conclusions

1. All the compounds tested had an effect on feeding rate, which is quantitatively the most important variable determining SFG. Respiration rate was only raised by those compounds (the chlorophenols and tributyltin) known to have specific effects on respiration at the biochemical level. Elevation of whole animal respiration may have potential as a marker of impact by these classes of compounds. Absorption efficiency was too insensitive under laboratory conditions to demonstrate any significant changes with the toxicants studied.
   
2. The physiological effects of the chlorophenols were clearly related to the structural characteristics of the molecules, so QSARs of predictive value can be established.
   
3. Although all the neurotic pesticides tested (which included organochlorines, pyrethroids, a carbamate and an organophosphate) had some effect on feeding rate, most appeared to be acting only as non-specific narcotics during the timecourse of our experiments. They were no more toxic to the mussels than typical urban contaminants like hydrocarbons. Since some pesticides are highly toxic to crustaceans and fish, physiological energetics studies with mussels cannot be used to assess their impact on these species. This limitation also applies to water quality assays using larval molluscs. There is an urgent need to develop in situ and laboratory based bioassays using neurotoxin sensitive species (eg. Mysid crustaceans). Mussels, however, were sensitive to the organophosphate pesticide tested, dichlorvos. Further research to exploit the potential of this observation is recommended.
   
4. The combined toxicity of dichlorvos and the narcotic hydrocarbon naphthalene is less than additive, so adverse effects are less than might be predicted. However, the benefit of environmental clean-up of only one constituent of such a mixture would be less than expected.
   
5. The present toxicological data base cannot explain all of the adverse effects on mussel health observed in field studies. There is a need to extend the range of chemicals analysed in monitoring studies and to increase the toxicological data base.