Test of Direct and Indirect Effects of Agrochemicals on the Survival of Fecal Indicator Bacteria

Abstract

ABSTRACTWater bodies often receive agrochemicals and animal waste carrying fecal indicator bacteria (FIB) and zoonotic pathogens, but we know little about the effects of agrochemicals on these microbes. We assessed the direct effects of the pesticides atrazine, malathion, and chlorothalonil and inorganic fertilizer onEscherichia coliand enterococcal survival in simplified microcosms held in the dark.E. colistrain composition in sediments and water column were positively correlated, but none of the agrochemicals had significant direct effects onE. colistrain composition or on densities of culturable FIBs. In a companion study, microcosms with nondisinfected pond water and sediments were exposed to or shielded from sunlight to examine the potential indirect effects of atrazine and inorganic fertilizer onE. coli. The herbicide atrazine had no effect onE. coliin dark-exposed microcosms containing natural microbial and algal communities. However, in light-exposed microcosms, atrazine significantly loweredE. colidensities in the water column and significantly increased densities in the sediment compared to controls. This effect appears to be mediated by the effects of atrazine on algae, given that atrazine significantly reduced phytoplankton, which was a positive and negative predictor ofE. colidensities in the water column and sediment, respectively. These data suggest that atrazine does not directly affect the survival of FIB, rather that it indirectly alters the distribution and abundance ofE. coliby altering phytoplankton and periphyton communities. These results improve our understanding of the influence of agricultural practices on FIB densities in water bodies impacted by agricultural runoff.