Effects of atrazine on periphyton under grazing pressure

An experiment was carried out using indoor experimental channels to assess the long-term effect (18 days) of herbivores (Physella acuta, Gastropoda) o n periphyton communities exposed to low levels of atrazine (14 mug l(-1)). We hypothesized that herbivorism modifies the response of periphyton to atr azine. Carbon incorporation, chlorophyll-a content, biovolume and algal tax onomic composition in the channels that contained atrazine were not signifi cantly different from the control channels (not receiving atrazine). In cha nnels with grazers and atrazine, there was a significant reduction of carbo n incorporation and algal density. In this treatment, physiognomic forms an d algal composition were significantly different from the others. The bioma ss of grazers (measured as change in dry mass) was not significantly affect ed by the addition of atrazine. Grazers maintained low levels of periphyton biomass, enhancing algal cell exposition to toxicant and inhibiting any ad aptation of the algae to the toxic exposure. The increase in atrazine toxic ity with grazing not only affected the metabolism, but also the structure o f the algal community, which suggests that effects were not transient but p ermanent. (C) 2001 Elsevier Science B.V. All rights reserved.