AbstractAquatic enclosures, or limnocorrals (5 × 5 × 5 m deep), installed in a 10‐ha mesotrophic lake in southern Ontario were used to evaluate the impact of four concentrations of methoxychlor (300 and 3 μg/L in 1981, and 50 and 5 μg/L in 1982) on plankton communities. Methoxychlor was acutely toxic at 300 μg/L to both the macrozooplankton (Cladocera and Copepoda) and microzooplankton (Rotifera). No recovery in macrozooplankton numbers was observed by the end of the experiment (119 d); however, densities of microzooplankton had recovered by 52 d to levels similar to those in control corrals. Changes in species composition of the microzooplankton community were such that the replicate corrals no longer contained similar rotifer populations.Methoxychlor at 50 μg/L was toxic to all Cladocera, copepod nauplii and copepodites, and the Chlorophyta. Even at this concentration, numbers of Cladocera did not recover to levels similar to those in the control corrals by the end of the experiment (145 d); however, nauplii and copepodites recovered by 55 and 100 d, respectively. Numbers of Chlorophyta were significantly lowered by 50 μg methoxychlor/L (p le; 0.05). The observed numerical recovery at 41 d was due primarily to an increase in the density ofKurchneriella lunaris.The biomass of this group remained low in these treated corrals because, despite its numerical increase,K. lunariscontributed little to chlorophyte biomass.Although 5 μg methoxychlor/L was toxic only to Cladocera and copepodites, recovery occurred within 20 d. Methoxychlor at 3 μg/L had no observable effects on density of plankton or on community structure.The responses to methoxychlor of the various groups of plankton generally appear to be exposure‐dependent, with Cladocera and Chlorophyta exhibiting the greatest
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