COMBINED EFFECTS OF ALACHLOR AND ATRAZINE ON BENTHIC ALGAL COMMUNITIES IN ARTIFICIAL STREAMS

A duplicated factorial approach was used to assess the combined effect s of three environmentally realistic levels of alachlor (0.0, 5.0, and 90.0 mu g/L), cross-classified with three environmentally realistic l evels of atrazine (0.0, 12.0, and 150.0 mu g/L), on indigenous epipeli c algae (mud-dwelling species, characteristic of agricultural streams) . The Experiment was conducted in 18 recirculating laboratory streams over a 4-week period. Biovolume of viable algal cells and relative abu ndance of the six dominant algal species were utilized as experimental endpoints. Community biovolume was impacted throughout the experiment in streams treated with 12.0 and 150.0 mu g/L atrazine and in streams containing 90.0 mu g/L alachlor, but only in samples collected at 4 w eeks. The effects of atrazine and alachlor together on algal community biovolume appeared to be additive rather than synergistic. The relati ve abundance of two dominant algal taxa were impacted in streams treat ed with alachlor, but only one species remained affected over time. Th e lack of significant interactions is most likely attributable to the different modes of action of atrazine and alachlor. The additive natur e of these two herbicides should be an important consideration in the development of water quality criteria.