Light history influences the sensitivity to atrazine in periphytic algae

This study examines the relationship between light history (i.e. light conditions during colonization) and sensitivity to atrazine for periphyton communities grown on etched glass substrates (colonized for 2-3 weeks) at seven stream and river sites differing in water chemistry and light regime. For each site, taxonomic composition of the community (by microscopic examination and cell counts), pigment composition, and photosynthetic parameters were measured and related to sensitivity to the herbicide. Photosynthetic parameters were estimated from photosynthesis-irradiance (P-I) curves; the responses of the algae to atrazine were analyzed using short-term concentration-response tests. Periphyton communities colonizing shaded sites were diatom dominated, had higher percentages of accessory pigments, and were more susceptible to photoinhibition at high light intensities compared to periphyton communities colonizing open sites; communities in the more open sites were dominated by green algae or cyanobacteria. Periphyton communities colonizing open sites were more sensitive to atrazine; the concentration of atrazine needed to inhibit photosynthesis by 50% in the short-term tests (EC50 values) was correlated with light conditions during colonization. The interaction between treatment factors (light conditions during colonization, irradiance of incubation, and atrazine concentration) was analyzed by three-way ANOVA. The two-way interactions were significant, indicating that the response of the periphyton to irradiance during incubation and to atrazine concentration both depend on light history. However, the magnitude of the interaction between light history and atrazine toxicity was attenuated by changes in irradiance during incubation (three-way ANOVA). This investigation presents evidence that the apparent toxicity of atrazine to lotic periphyton in natural communities is greater for algae that are adapted to high-light conditions than for those that are adapted to low-light conditions.