Interlaboratory testing of CuSO4 toxicity in the “Standardized Aquatic Microcosm” protocol consisting of multiple phytoplankton and animals in a chemically defined medium.

Four different laboratories conducted a total of ten experiments of the “Standardized Aquatic Microcosm” to test the reproducibility of results to control, low, medium, and high concentrations of CuSO4. In nine experiments, treatments consisted of six replicates of 0, 500, 1000, and 2000 ppb Cu++. One experiment, ME74, used 0, 127, 255, 509 ppb. The purpose was to test a chemically defined medium (thus negating differences due to local water supplies) and the same 10 species of phytoplankton and 5 animals, including Daphnia. Microbes were undefined. The protocol included the weekly re-introduction of small numbers of each species to allow potential recovery from toxicity. Control microcosms had a “spring algal bloom” terminated by zooplankton grazing and multiple competitive interactions. The copper inhibited some phytoplankton more than others and killed many grazers, especially Daphnia. The data set presented several interesting statistical properties that would yield new insights. (a) The results were very similar, but the timing varied— the higher the concentration of copper, the longer the inhibition and mortality of organisms, so those at 500 ppb recovered earlier, the 1000 ppb recovered later, and at 2000 ppb most never recovered. But if compared on each sampling day, e.g., 10, 14, … to 64, results appear highly variable. (b) In at least one experiment, the toxicity of copper was challenging to demonstrate statistically because high variability in the timing of recovery of the intermediate concentration increased pooled variances. (c) The elimination of highly-sensitive dominant organisms allowed less-sensitive organisms to increase in abundance. Within natural environments, the observation that some species increase in the presence of toxic substances has been used to discredit toxicity testing without considering the relative sensitivities of competing or predatory species. (d) The competitive interactions among organisms, e.g., cyanobacteria and green alga