Changes in atrazine toxicity throughout succession of stream periphyton communities

A study was made to describe atrazine toxicity and its changes throughout succession of periphyton communities of an undisturbed Mediterranean stream. Toxicity was assessed by short-term physiological tests (concentration-effect curves of photosynthesis to atrazine) in the laboratory using artificial substrates colonized in one stream site during winter, and two stream sites (one open and the other shaded) during summer. In the winter experiment, when environmental conditions were relatively steady and chlorophyll content was low, toxicity increased according to the increases in cell density and chlorophyll content throughout colonization. EC50 (concentration inhibiting photosynthesis by 50%) was above 0.8 mu M atrazine until day 16 and below 0.4 mu M atrazine after three weeks. In the summer experiment, under more variable environmental conditions, the differences between the EC50 at the beginning and the end of the colonization experiments were not significant (one factor ANOVA) at the two sites. EC50 was on average 0.89 mu M atrazine in the shaded site and 0.29 mu M atrazine in the open site. A significant negative correlation between irradiance and EC50 was observed when all the experiments were considered together (r = 0.464, n = 20, p<0.05), suggesting that light history may have an important role in the response to atrazine. This investigation reveals that the response of stream periphyton to atrazine is likely to be influenced by colonization time and the corresponding changes in algal density and community composition as well as by environmental conditions (e.g. light regime) throughout succession.