Toxicity of sediment pore water from 13 sites in the lower Fox River/Green Bay watershed was assessed using a number of test species. Sediment pore water from the 10 lower Fox River sites exhibited acute toxicity to fathead minnows (Pimephales promelas) and Ceriodaphnia dubia, and pore water samples from all 13 sites were chronically toxic to C. dubia. Sediment pore water from seven of the sampling sites was toxic to Selenastrum capricornutum, but none of the samples were toxic to Photobacterium phosphoreum. Toxicity characterization, identification and confirmation procedures indicated that a significant amount of the acute toxicity of the pore water to fathead minnows and C. dubia was due to ammonia. The identification of ammonia, a naturally occurring compound in sediments, as a potentially important sediment‐associated toxicant has implications for sediment toxicity assessment and control, not only in the Fox River and Green Bay, but in other freshwater and marine systems as well.

Toxicity of sediment pore water from 13 sites in the lower Fox River/Green Bay watershed was assessed using a number of test species. Sediment pore water from the 10 lower Fox River sites exhibited acute toxicity to fathead minnows (Pimephales promelas) and Ceriodaphnia dubia, and pore water samples from all 13 sites were chronically toxic to C. dubia. Sediment pore water from seven of the sampling sites was toxic to Selenastrum capricornutum, but none of the samples were toxic to Photobacterium phosphoreum. Toxicity characterization, identification and confirmation procedures indicated that a significant amount of the acute toxicity of the pore water to fathead minnows and C. dubia was due to ammonia. The identification of ammonia, a naturally occurring compound in sediments, as a potentially important sediment‐associated toxicant has implications for sediment toxicity assessment and control, not only in the Fox River and Green Bay, but in other freshwater and marine systems as well.

Comparisons were made among Leffler microcosms developed from four different natural communities and exposed to 0, 20, 100, 200, 500, 1,000, and 5,000 micrograms/L atrazine, a commonly used herbicide. Atrazine reduced net primary productivity, pH, and net productivity/respiration ratios in all four microcosm communities. In three of the four communities, the lowest observed (P less than 0.05) effect concentration (LOEL) was 100 micrograms/L. In the fourth community the LOEL was 200 micrograms/L atrazine. The sensitivity and accuracy of bioassays with four different microcosm communities were evaluated by comparing results with values reported for acute and chronic single species bioassays, other types of microcosms, and experimental ponds exposed to similar concentrations of atrazine. The ranges of sensitivity noted in these experiments were less than the range reported for single species bioassays using common test organisms and similar to those reported for other microcosms. The similarity between Leffler microcosm results and the responses reported for the experimental ponds suggests that the Leffler microcosms accurately reflected concentrations causing ecosystem level changes in the experimental ponds.

As part of a study to evaluate laboratory toxicity screening tests that include single species bioassays and microcosms, we examined community level responses in Taub microcosms exposed to atrazine (60, 100, 200, 500, 1000, and 5000 µg/L), a commonly used herbicide. Measurements of community metabolism included primary productivity, community respiration, primary production efficiency, and productivity/respiration (Pn/Rni) ratios. These community measurements varied in their sensitivity to atrazine. Primary production efficiency (primary productivity per unit chlorophyll) appeared to be the most sensitive measurement with greatly reduced efficiencies occurring throughout the experiment at atrazine exposure of 60 µg/L. The other community measures appeared to be more sensitive to atrazine during the interval when) ratios. These community measurements varied in their sensitivity to atrazine. Primary production efficiency (primary productivity per unit chlorophyll) appeared to be the most sensitive measurement with greatly reduced efficiencies occurring throughout the experiment at atrazine exposure of 60 µg/L. The other community measures appeared to be more sensitive to atrazine during the interval when Daphnia magna populations were highest, suggesting increased pressure by D. magna on primary producers increased the sensitivity of this test system to atrazine. All community metabolism measurements of the microcosms exposed to higher atrazine concentrations of 500, 1000, and 5000 µg/L differed from controls throughout the experiment.

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