Probabilistic risk assessment of cotton pyrethroids: II. Aquatic mesocosm and field studies

Results of mesocosm and field studies with cypermethrin and esfenvalerate w ere analyzed and interpreted to support an ecological risk assessment of co tton pyrethroids in aquatic ecosystems. A core group of seven mesocosm stud ies conducted on two continents over the course of a decade were examined, and additional observations from mesocosm and field studies with these and other cotton pyrethroids were also brought to bear. The results for cyperme thrin and esfenvalerate were remarkably consistent. They revealed a trend i n sensitivity from amphipods, isopods, midges, mayflies, copepods, and clad ocerans (most sensitive) to fish, snails, oligochaetes, and rotifers (least sensitive). With few exceptions, populations affected by pyrethroids in th e mesocosms recovered to normal levels before the end of the year of exposu re; most populations recovered within weeks. Factors presumed responsible f or population recovery included internal refuges (areas of low exposure), r esistant life stages, rapid generation times, and egg deposition by adults from outside the treated systems. Indirect effects on fish (which have been hypothesized to occur when invertebrate food sources are reduced) were not observed. The lowest-observed-adverse-effect concentrations for the overal l ecosystems for cypermethrin and esfenvalerate corresponded to the 54th an d 41st centiles of acute toxicity endpoints (LC50s) for arthropods measured in laboratory studies with these compounds, implying that a risk character ization based on 10th centiles would be highly conservative.