Contrasting effects of organic and inorganic toxicants on freshwater periphyton.

This study analyses the factors influencing the response of periphyton communities to toxicants in order to test the validity of the short-term physiological method. The effects of two model compounds, atrazine and zinc, on the photosynthesis of periphyton from different lotic systems were analyzed using short-term toxicity tests. Periphyton communities from seven sites belonging to a relatively small area were used to explore the ecological parameters affecting toxicity in flowing waters. Furthermore, time course studies of zinc toxicity on intact and suspended periphyton were carried out in order to evaluate the significance of periphyton structure (thickness) on toxicity. Atrazine toxicity varied slightly (0.42-2.42 microM) in tests with photosynthesis as an endpoint. Only environmental light was detected to lead to systematic changes in sensitivity of field collected communities. In contrast, the range of EC(10) values for zinc was very broad (7->1000 microM), and mainly related to algal biomass, while also water chemistry (alkalinity) was important. The results of time-course experiments indicated that the penetration of zinc into thick biofilms was limited. The model compounds tested in the present study may exemplify two different situations. In the case of atrazine, the effect on periphyton is easy to predict from the physiological tests used here. On the other hand, zinc toxicity assessed using photosynthesis of periphyton is much affected by many biological and chemical variables. It is concluded that toxicity tests of metals and other organic and inorganic compounds should be based on longer times of exposure.