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 perip hyton communities grown on etched glass substrates (colonized for 2-3 weeks) at seven stream and river sites differing in water chemistry an d 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 photosyn thesis-irradiance (P-I) curves; the responses of the algae to atrazine were analyzed using short-term concentration-response tests. Periphyt on communities colonizing shaded sites were diatom dominated, had high er percentages of accessory pigments, and were more susceptible to pho toinhibition at high light intensities compared to periphyton communit ies colonizing open sites; communities in the more open sites were dom inated by green algae or cyanobacteria. Periphyton communities coloniz ing open sites were more sensitive to atrazine; the concentration of a trazine needed to inhibit photosynthesis by 50% in the short-term test s (EC50 values) was correlated with light conditions during colonizati on. The interaction between treatment factors (light conditions during colonization, irradiance of incubation, and atrazine concentration) w as analyzed by three-way ANOVA. The two-way interactions were signific ant, indicating that the response of the periphyton to irradiance duri ng incubation and to atrazine concentration both depend on light histo ry. However, the magnitude of the interaction between light history an d atrazine toxicity was attenuated by changes in irradiance during inc ubation (three-way ANOVA). This investigation presents evidence that t he apparent toxicity of atrazine to lotic periphyton in natural commun ities is greater for algae that are adapted to high-light conditions t han for those that are adapted to low-light conditions.