Two limnocorral (in situ enclosure) experiments were conducted to assess the effects of a commercial 2,3,4,6-tetrachlorophenol formulation (DIATOX®) on natural zooplankton communities. In Experiment 1, no significant reductions were noted in mean abundance of any major zooplankton taxa following a morning application with 0.75 mg a.i./l DIATOX®. Variability among zooplankton populations in replicate enclosures, especially controls, was high and was attributed to variable predation by uncontrolled populations of planktivorous fish (juvenile Perca flavescens). Lack of significant impact on zooplankton populations was attributed to rapid photodegradation of the surface applied chlorophenols (CPs), resulting in sublethal water column concentrations within 24 h of application. High variability among replicate enclosures made assessment of subtle impacts difficult.

In Experiment 2, the effects of 0.75 and 1.50 mg a.i./l DIATOX® were assessed following evening applications. Replication among zooplankton populations in control enclosures was significantly improved by removal of fish. The evening pesticide application delayed photodegradation and, at the 0.75 mg/l treatment, resulted in 1.6 times higher CP concentrations in the water 24 h after application than resulted from the same treatment in Experiment 1. Both Experiment 2 DIATOX® treatments resulted in significant reductions in all major zooplankton taxa. Rotifers were 1.5 times more sensitive than cladocerans and copepods, which responded similarly to the two treatments. All zooplankton populations had generally recovered by Days 14 to 28 and by Days 28 to 42 posttreatment at the low and high treatment concentration, respectively.

258Direct effects on phytoplankton populations were minimal. There were no significant reductions in the abundances of major phyla (Cyanophyta, Chlorophyta, Chrysophyta), but 14 to 21% and 16 to 26% decreases were observed in dissolved oxygen concentrations at the low and high treatment, respectively. A 36-fold increase in total phytoplankton abundance was observed on Days 21 to 28 at the 1.50 mg/l DIATOX® treatment, largely accounted for by two filamentous Cyanophyta species (Oscillatoria amphigranulata and O. lauterbornii). This secondary effect most likely resulted from the reduction in total zooplankton abundance which reached a maximum of 93% on Day 7. Phytoplankton populations decreased to control levels by Day 42 as zooplankton populations recovered. Analysis of water chemistry data contributed no additional information on treatment effects.