It has been proposed that the
herbicide atrazine may increase rates of parasitic
trematode infection in amphibians. This effect may occur indirectly as a result of increased biomass of periphyton and augmented populations of aquatic snails, which are the trematode's primary larval host. Evidence has also shown that nutrients alone may induce the same indirect responses. Since both
atrazine and nutrients commonly enter surface waters from agricultural run-off, their spatial and temporal co-occurrence are highly probable. In light of recent wide-spread declines in amphibian populations, a better understanding of the role of
atrazine in the proposed ecological mechanism is necessary. A microcosm study was conducted to quantify biomass of phytoplankton and periphyton over a range of
atrazine and
phosphorus concentrations (from 0 to 200 μg L(-1) each) using a central composite rotatable design. Over 10 weeks, biomass and water chemistry were monitored using standard methods. Regression and canonical analyses of the response surfaces for each parameter were conducted. We found significant effects of
atrazine and
phosphorus on dissolved
oxygen, pH, and conductivity throughout the study. Additions of
phosphorus mitigated the apparent inhibition of these photosynthetic indicators caused by
atrazine. Despite these changes, no consistent treatment-related differences in algal biomass were observed. These results indicate that the indirect impacts of
atrazine on total growth of periphyton and likely, subsequent effects on aquatic snails, are not expected to be ecologically significant at the concentrations of
atrazine tested (up to 200 μg L(-1)) and over a range of nutrient conditions commonly occurring in agroecosystems.