The impact of two applications of atrazine on the plankton communities of in situ enclosures

Abstract

Atrazine (2-choro-4-ethylamino-6-isopropylamino- s-triazine) was applied to aquatic enclosures in Lake St. George, Canada (43°57′25″ N, 79°26′30″ W) on June 1 1983 with a second application 35 days later. A nominal concentration of 0.1 mg/l was used during each application. Compared to single atrazine applications, this multiple treatment was characterized by a more gradual killing of the phytoplankton, a longer period of recovery for the green algal community, and a distinct shift in the taxonomic composition of the affected communities. Thirteen days after the first application significant declines in the populations of Elakatothrix gelatinosa, Sphaerocystis schroeteri, Tetraedron minimum, Oocystis lacustris, and Gymnodinium spp. were observed. Low numbers persisted after the second application and remained lower for 114 days. Populations of Rhodomonas minuta increased significantly on day 13, whereas those of Cryptomonas erosa were not influenced by the herbicide. On average there was a reduction from 22 to 16 prominent phytoplankton taxa after atrazine exposure. The similarity between phytoplankton communities in control and atrazine-exposed enclosures, as measured by the Morisita coefficient of association, was distinctly altered after the second atrazine application. This dissimilarity persisted for 77 days. In the spring of 1984, 323 days after the first atrazine application, the control and atrazine exposed enclosures had returned to similar phytoplankton assemblages. The rotifer community was not significantly affected. Two crustaceans, Bosmina longirostris, and Diaptomus oregonensis had significant short term reductions in numbers after the second application. These reductions were not observed after the first atrazine applications and it is suggested that their decline is a secondary effect caused by changes in other populations within the community.