Agricultural pesticide runoff is a critical issue for many southeastern estuaries. The microbial food web plays an important role in nutrient cycling and transfer of nutrients to higher trophic levels. Microbial communities may serve as sensitive, early indicators of environmental stress due to pesticides. The microbial food web has not been extensively studied in tidal creeks, and toxicity data involving microorganisms and pesticides are limited. The present study examined the estuarine microbial food web under natural conditions and under pesticide stress. Artificial substrates were used to collect microbial communities from a reference tidal creek in the North Inlet National Estuarine Research Reserve. Chemical inhibitors (DCMU, antibiotics, and cycloheximide) were used to isolate individual components of the microbial food web. Three commonly used agricultural pesticides; atrazine, endosulfan, and chlorpyrifos, were tested. Productivity, biomass, and community composition were measured. Laboratory experiments, a simulated tidal creek mesocosm study and a field experiment were conducted.
The trophodynamics of the estuarine microbial food web varied with season. Phytoplankton, bacteria and protozoa were found to be interdependent in the summer, allowing nutrients to be recycled within the microbial community. In the winter, a more linear trophic system existed, permitting greater energy flow to zooplankton and benthic animals. Atrazine and its metabolite deethylatrazine were found to inhibit chlorophyll a, phototrophic carbon assimilation, dissolved oxygen, and phototrophic biovolume at 40-250 ug/L. Endosulfan concentrations of 1 and 10 ug/L inhibited bacterial abundance, but not heterotrophic bacterial productivity. Endosulfan primarily targeted the cyanobacteria. Chlorpyrifos reduced the number of heterotrophic ciliates and flagellates at 10 ug/L, and indirectly increased bacterial abundance and decreased chlorophyll a. The responses observed with chlorpyrifos were more acute than those observed with endosulfan or atrazine. The estuarine microbial food web studied went through seasonal successions in community structure and interactions. The results of this study suggest that exposure to agricultural pesticides can lead to both functional and structural changes in the estuarine microbial food web, which may potentially impact higher trophic levels.