ECOTOXICOLOGIC IMPACTS OF AGRICULTURAL DRAIN WATER IN THE SALINAS RIVER, CALIFORNIA, USA

The Salinas River is the largest of the three rivers that drain into the Monterey Bay National Marine Sanctuary in central California (USA). Large areas of this watershed are cultivated year-round in row crops, and previous laboratory studies have demonstrated that acute toxicity of agricultural drain water to Ceriodaphnia dubiais caused by the organophosphate (OP) pesticides chlorpyrifos and diazinon. We investigated chemical contamination and toxicity in waters and sediments in the river downstream of an agricultural drain water input. Ecological impacts of drain water were investigated by using bioassessments of macroinvertebrate community structure. Toxicity identification evaluations were used to characterize chemicals responsible for toxicity. Salinas River water downstream of the agricultural drain was acutely toxic to the cladoceran Ceriodaphnia dubia, and toxicity to C. dubia was highly correlated with combined toxic units (TUs) of chlorpyrifos and diazinon. Laboratory tests were used to demonstrate that sediments in this system were acutely toxic to the amphipod Hyalella azteca, a resident invertebrate. Toxicity identification evaluations (TIEs) conducted on sediment pore water suggested that toxicity to amphipods was due in part to OP pesticides; concentrations of chlorpyrifos in pore water sometimes exceeded the 10-d mean lethal concentration (LC50) for H. azteca. Poten- tiation of toxicity with addition of the metabolic inhibitor piperonyl butoxide suggested that sediment toxicity also was due to other non-metabolically activated compounds. Macroinvertebrate community structure was highly impacted downstream of the agricultural drain input, and a number of macroinvertebrate community metrics were negatively correlated with combined TUs of chlorpyrifos and diazinon, as well as turbidity associated with the drain water. Some macroinvertebrate metrics were also correlated with bank vegetation cover. This study suggests that pesticide pollution is the likely cause of ecological damage in the Salinas River, and this factor may interact with other stressors associated with agricultural drain water to impact the macroinvertebrate community in the border of the Salinas Valley. Although the creek carries some natural water flow during the wettest winter months, headwater flow is underground above the study area. Flow in the lower portion of the creek is dominated by agricultural drain water. In the current study, we investigated the impacts of this drain water in the Salinas River over an 18-month period. Salinas River water and sediment toxicity were characterized by using the cladoceran Ceriodaphnia dubiaand the amphipod Hyalella azteca, respectively. The results of these tests were compared to physical and water-quality analyses, as well as selected pesticide measures in both water and sediment ma- trices. Ecological impacts were assessed by characterizing macroinvertebrate community structure upstream and down- stream of the drain-water input. Possible causes of toxicity and impacts on macroinvertebrate community structure were investigated by using a combination of toxicity identification evaluations (TIEs) and chemical analyses, dose-response in- formation from the literature, and habitat and physical factor assessments. The results were combined in a weight-of-evi- dence evaluation of the impacts of agricultural drain water on the river ecosystem, and were used to investigate chemicals responsible for toxicity and ecological degradation.