The photosynthetic responses of marine phytoplankton, periphyton and epipsammon to the herbicides paraquat and simazine

Short-term toxicity tests using photosynthesis (incorporation of C-14) as a test parameter were performed in order to compare the sensitivities of three marine microalgal communities (phytoplankton, periphyton and epipsammon) to two herbicides, paraquat and simazine. Thirty minutes of pre-exposure to simazine were sufficient to obtain the full effect in all communities, while for paraquat 4 h was required. The bioavailability of paraquat and simazine was not limited by adsorption to sediment in the epipsammon samples. Simazine was more toxic than paraquat for the three communities at similar concentrations. Phytoplankton was slightly more sensitive for both herbicides (EC50 ranges of 9-23 mu M for paraquat and 0.37-0.99 mu M for simazine) than periphyton and epipsammon. These attached communities exhibited different results for each toxicant, periphyton being more sensitive to paraquat (EC50 range 9-21 mu M) and epipsammon to simazine (EC50 range 0.44-1.17 mu M). The three communities presented EC ranges comparable to those found in single species tests, suggesting that different levels of biological organization can exhibit a similar sensibility to toxicants, thus indicating that natural communities are suitable for use in these kinds of toxicity tests.