Abstract-The effects of 4-nonylphenol (NP) on freshwater zooplankton were evaluated in 18 littoral enclosure mesocosms in northeastern Minnesota. The 18 enclosures were allocated to three blocks of six units with each block including two untreated control enclosures and one enclosure for each of four NP treatments (3, 30, 100, and 300 g/L). Treated enclosures received 11 applications of NP over a 20-d period between July 8 and 28, 1993. Maximum NP concentrations measured in the water column 2 h after each application averaged (ϮSD) 5 Ϯ 4, 23 Ϯ 11, 76 Ϯ 21, and 243 Ϯ 41 g/L over the 11 applications. Nonylphenol dissipated rapidly from the water column but was more persistent in sediments and in/on macrophytes. All cladoceran and copepod taxa were significantly reduced in abundance at 243 Ϯ 41 g/L; some sensitive taxa were also affected at 76 Ϯ 21 and 23 Ϯ 11 g/L. While many rotifer taxa were unaffected at any of the test concentrations, several were affected at Ն76 Ϯ 21 g/L. Ostracods were only affected at 243 Ϯ 41 g/L. No zooplankton taxon was affected at 5 Ϯ 4 g/L. The period of maximum impact usually occurred within 1 to 7 d of the last NP application, and recovery to control abundance levels generally occurred within 7 to 28 d of the last NP application. Two sensitive taxa, Acroperus and Calanoida, did not recover at Ն76 Ϯ 21 g/L by the end of the study. The maximum acceptable toxicant concentration for protection of all zooplankton taxa was estimated at ϳ10 g/L, although overall community diversity was unaffected at 23 Ϯ 11. The water was the most probable route of NP exposure, but the greater persistence of NP residues in/on macropyhtes may have contributed to the lack of recovery of some macrophyte-associated taxa.

The effects of esfenvalerate [(S)-oc-cyano-3-phenoxybenzyl(S)-2-(4-chlorophenyl)-3-methylbutyrate] on plant and animal communities were observed in 12 littoral enclosures (5 x 10 m) in a 2-ha pond near Duluth, Minnesota, at six concentrations (0, 0.01,0.08,0.2, 1, and 5 pg/L, nominal) for eight weeks. Esfenvalerate exposures of 1 and 5 pg/L resulted in drastic reductions or elimination of most crustaceans, chironomids, juvenile bluegills (Lepomis rnacrochirus), and larval cyprinids. Abundance of some copepod and insect genera declined at esfenvalerate concentrations of 0.08 to 0.2 pg/L, and these effects were apparent up to 53 d. Some invertebrate communities were able to recover by day 25 in enclosures containing concentrations of 50.2 pg/L esfenvalerate.

The effects of 4‐nonylphenol (NP) on freshwater zooplankton were evaluated in 18 littoral enclosure mesocosms in northeastern Minnesota. The 18 enclosures were allocated to three blocks of six units with each block including two untreated control enclosures and one enclosure for each of four NP treatments (3, 30, 100, and 300 μg/L). Treated enclosures received 11 applications of NP over a 20‐d period between July 8 and 28, 1993. Maximum NP concentrations measured in the water column 2 h after each application averaged (±SD) 5 ± 4, 23 ± 11, 76 ± 21, and 243 ± 41 μg/L over the 11 applications. Nonylphenol dissipated rapidly from the water column but was more persistent in sediments and in/on macrophytes. All cladoceran and copepod taxa were significantly reduced in abundance at 243 ± 41 μg/L; some sensitive taxa were also affected at 76 ± 21 and 23 ± 11 μg/L. While many rotifer taxa were unaffected at any of the test concentrations, several were affected at ≥76 ± 21 μg/L. Ostracods were only affected at 243 ± 41 μg/L. No zooplankton taxon was affected at 5 ± 4 μg/L. The period of maximum impact usually occurred within 1 to 7 d of the last NP application, and recovery to control abundance levels generally occurred within 7 to 28 d of the last NP application. Two sensitive taxa, Acroperus and Calanoida, did not recover at ≥76 ± 21 μg/L by the end of the study. The maximum acceptable toxicant concentration for protection of all zooplankton taxa was estimated at ˜10 μg/L, although overall community diversity was unaffected at 23 ± 11. The water was the most probable route of NP exposure, but the greater persistence of NP residues in/on macropyhtes may have contributed to the lack of recovery of some macrophyte‐associated taxa.

Two applications of the insect growth regulator diflubenzuron were made to replicate littoral enclosures at nominal concentrations of 0.7, 2.5, 7.0, and 30 &mgr;g/L. Assessment of the effects of this insecticide on benthic macroinvertebrate community structure was accomplished by measuring changes in abundance and taxonomic richness. Chironomidae and Ephemeroptera were the most sensitive groups sampled, with no observed effect concentrations of 2.5 and 0.7 &mgr;g/L, respectively. No adverse effects were observed on Mollusca or Oligochaeta at any of the test concentrations. Taxonomic richness was noticably reduced at 7.0 and 30 &mgr;g/L on all post-application sampling dates, producing changes in community structure that persisted for >/=57 days.

Two applications of the insect growth regulator diflubenzuron were made to replicate littoral enclosures at nominal concentrations of 0.7, 2.5, 7.0, and 30 &mgr;g/L. Assessment of the effects of this insecticide on benthic macroinvertebrate community structure was accomplished by measuring changes in abundance and taxonomic richness. Chironomidae and Ephemeroptera were the most sensitive groups sampled, with no observed effect concentrations of 2.5 and 0.7 &mgr;g/L, respectively. No adverse effects were observed on Mollusca or Oligochaeta at any of the test concentrations. Taxonomic richness was noticably reduced at 7.0 and 30 &mgr;g/L on all post-application sampling dates, producing changes in community structure that persisted for >/=57 days.