A phased approach was taken for applying previously developed numerical models to address quantitatively whether application of 4 pesticides (i.e., methoprene, malathion, resmethrin and sumithrin) to combat mosquitoes carrying West Nile virus could alone have caused the massive die-off of lobster observed in western Long Island Sound during 1999. Model results show that even with an overly conservative model input assumption (i.e., that the entire mass of pesticides applied in the watershed reached the open waters of Long Island Sound without any attenuation or decay in either the watershed or the Sound) the calculated 24-h average ambient levels of methoprene in the Sound were less than 0.0005 μg/L and well below the lowest reported ecologic endpoint of lobster stress (i.e., 2.8 μg/L stage 2 larvae LC50). Under the assumed conservative model loadings, results for malathion were highest in the East River (maximum 24-h average = 10.3 μg/L) and were much lower in the western Sound area of the lobster die-off. These levels are below the lowest reported ecologic endpoint of lobster stress (i.e., 4.1 μg/L larvae LC50). Model calculations for resmethrin and sumithrin were compared with ecologic endpoints for lobster stress measured for resmethrin. The lowest reported ecologic endpoints reported for resmethrin were 0.01 μg/L and 0.095 μg/L for reduction in adult phagocytosis after weekly exposure and larval 96-h LC50, respectively. Calculated resmethrin levels on a 24-h average basis were as high a 0.225 μg/L, but were significantly lower in western Long Island Sound. For sumithrin, the maximum calculated 24-h average concentration, 0.151, occurred in Eastchester Bay in near surface waters. The calculated levels of neither resmethrin nor sumithrin reached the LC50 value for adult lobsters of >1 μg/L. Malathion, resmethrin and sumithrin were also modeled with a less conservative and more realistic set of assumptions that included decay of the pesticide within the receiving water. Based on a 24-h average malathion concentrations calculated by the model were <1 μg/L in near bottom waters throughout Long Island Sound and probably did not represent a stress to the lobsters. Calculated resmethrin levels throughout near bottom waters of Long Island Sound were <0.005 μg/L and, therefore, did not represent a stress to lobsters. Calculated sumithrin concentrations in near bottom waters were as high as 0.08 μg/L in portions of western Long Island Sound. Assuming that resmethrin endpoints are applicable to sumithrin, it is unlikely that sumithrin could have caused mortality in adult lobsters, however, we cannot fully rule out the possibility that sumithrin may have been a stressor at sublethal levels.
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