Zooplankton Responses to Aluminum Sulfate Treatment of Newman Lake, Washington

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Abstract
Pelagic zooplankton were sampled prior to and following the whole-lake aluminum sulfate (alum) treatment of Newman Lake, Washington, to determine zooplankter responses. Signs of perturbation such as pronounced declines in zooplankton numbers, biomass, and species diversity were exhibited within two weeks after the treatment. These observations may be attributed to a combination of physical action of the settling alum floc, removal of and change in primary food sources, predation by planktivorous fish, and toxicity of aluminum ion. Declines were only temporary, and substantial zooplankter proliferation was evident in the following months due to the abundance of small algae and other food particles. Subsequent monitoring of the zooplankton for 21/2 years showed that the community composition changed after the treatment. Rotifers became more abundant, while biomass dominance shifted from cladocerans and copepods to copepods. Long-term compositional changes may also be attributed to a shift in zooplankter food resources. No long-term decrease in zooplankton species diversity was observed.