Effects of methoxychlor on zooplankton in freshwater enclosures: Influence of enclosure size and number of applications

Methoxychlor was applied to 20, 100 and 1,000 m3 enclosures at a nominal concentration of 20 μg L−1. Total dissipation was more rapid than expected in the 20 m3 enclosures (slope = −0.0288) while rates in the 100 and 1,000 m3 enclosures were similar (slope = −0.0207 and −0.0187, respectively). Effects on zooplankton were less rapid and less severe and recovery more rapid in the 20 m3 than in the larger enclosures, but little difference was observed between the 100 and 1,000 m3 enclosures. Comparison of a single application of 20 μg L−1 in 100 m3 enclosures with two applications of 10 μg L−1, 35 days apart, also in 100 m3 enclosures, showed a more rapid dissipation after the first application than after the second (slope = −0.0236 vs. −0.0161). Acute toxic effects were not observed after the second treatment, and little effect on time to recovery was noted. Selection of certain species by the second application of methoxychlor was not observed.