Effects of the agricultural pesticides atrazine, deethylatrazine, endosulfan, and chlorpyrifos on an estuarine microbial food web

Agricultural pesticide runoff is a critical issue for many southeastern est uaries. The estuarine microbial food web plays an important role in nutrien t cycling and transfer of nutrients to higher trophic levels. The present s tudy examined the effects of agricultural pesticides on the estuarine micro bial food web. Polyurethane foam substrates were used to collect microbial communities from a reference tidal creek in the North Inlet National Estuar ine Research Reserve (Georgetown, SC, USA). Colonized substrates were broug ht into the laboratory and exposed to the following commonly used agricultu ral pesticides: atrazine (and a metabolite, deethylatrazine), endosulfan, a nd chlorpyrifos. Chlorophyll a, phototrophic carbon assimilation, dissolved oxygen, and phototrophic biovolume were significantly reduced at concentra tions of 50 and 250 mu g/L atrazine and deethylatrazine. Generally, inhibit ion of the phytoplankton resulted in increased bacterial abundance and prod uctivity, whereas heterotrophic ciliate and flagellate abundances were not affected. Total bacterial abundance, but not heterotrophic bacterial produc tivity, was significantly reduced with endosulfan treatments of 1 and 10 mu g/L. Endosulfan was primarily found to target the cyanobacteria. Changes i n biomass reflected compositional shifts in the phototrophs. The abundance of heterotrophic ciliates and flagellates was significantly reduced at 10 m u g/L chlorpyrifos. Bacterial abundance and productivity increased, whereas phototrophic variables decreased. Agricultural pesticides were found to al ter both functional and structural aspects of the estuarine microbial food web.