Freshwater community responses to mixtures of agricultural pesticides: Effects of atrazine and bifenthrin

Abstract The individual and combined effects of atrazine and bifenthrin on lake communities were investigated in a two-phased mesocosm study involving eighteen 5,500-L tanks containing natural plankton assemblages and bluegill. The factorial-designed experiment included three levels of bifenthrin (0, 39, and 287 ng L−1 in phase 1 and 0, 125 and 3,150 ng L−1 in phase two) cross-classified with three levels of atrazine (0, 15, and 153 μg L−1 in phase 1 and 0, 385, and 2, 167 μg L−1 in phase 2), with duplicate replication. In phase 1, atrazine significantly reduced chlorophyll concentrations and turbidity (day 7) but had no significant impact on primary productivity or algal cell densities. Bifenthrin significantly reduced Bosmina (days 7 and 14), cyclopoid copepodids (days 7 and 14), and copepod nauplii (day 14); however, bifenthrin increased rotifers at day 7. Bifenthrin also increased colonial green algae (day 7) and decreased turbidity (days 7 and 14). Only one fish mortality occurred during phase 1 of the study. In phase 2, atrazine resulted in significant reductions in primary productivity, chlorophyll, green algal colonies, Bosmina, and rotifers on day 7. Bifenthrin had a negative impact on Bosmina, copepod nauplii, rotifers, primary productivity, chlorophyll, and green algal colonies. A 33% bluegill mortality was observed in tanks with an average maximum concentration of 3,150 ng L−1 bifenthrin. The observed interaction effects indicated that when either compound was introduced at ecologically realistic levels, its effects were essentially masked if the other toxicant was present at high concentrations, and that the two pesticides did not act synergistically.