Enclosures of three sizes (large, 1000 m³; medium, 125 m³; small, 20 m³) designed and constructed specifically to facilitate the evaluation of the impact of pesticides in freshwater ecosystems are described. Uniform mixing of permethrin in a corral was possible within an hour using a pump system that had no adverse effects on entrapped organisms. Methoxychlor applied to surface waters with a back-pack spray system penetrated the column of water within 24 h. Both application methods produced replicable concentrations close to the desired nominal concentration. As a result of the impact of permethrin (0.5 and 5.0 μg L⁻¹) and methoxychlor (5 and 50 μg L⁻¹), densities of macrozooplankton populations decreased and rotifer populations increased in numbers. As the macrozooplankton populations recovered, rotifer numbers decreased.

The large corrals possessed a distinctive edge zone that extended 1.0 m from the corral wall and was usually characterized by greater densities of macrozooplankton than the center zone. No such edge zone existed in the small or medium corrals. We recommend that the spatial distribution of zooplankton in each size of corral be defined before establishing a sampling regime; thus more precise population estimates may be attained.

The impact of methoxychlor on enclosed zooplankton populations differed in the three sizes of limnocorrals. The toxicity of 20 μg methoxychlor L⁻¹ to zooplankton increased with increasing size of corral, primarily because of the decreasing ratio of surface area of limnocorral walls to water volume. Methoxychlor adsorbs readily to polyvinyl chloride; thus the concentration in the water of the larger enclosures was higher for a longer period of time than that in the smaller corrals.