Chemical and biological processes controlling the response of a freshwater ecosystem to copper stress: A field study of the CuS04 treatment of Mill Pond Reservoir, Burlington, Massachusetts1

The response of natural phytoplankton populations to copper stress and its dependence on the chemical speciation of copper was studied during CuSO4 treatment of Mill Pond Reservoir to control a nuisance bloom of Ceratium hirundinella. Potentiometric and ultrafiltration experiments showed that the chemistry of the added copper was controlled by complexation by humic substances. The Ceratium population decreased by 90% 1 day after treatment and was replaced as the dominant phytoplankter by two green algae, Nannochloris sp. and Ourococcus sp. Primary productivity decreased initially but returned to pretreatment levels in 10 days with the growth of the green algae; toxicity experiments in the laboratory with algae isolated from the reservoir showed that Nannochloris and Ourococcus can tolerate much higher cupric ion activities than C. hirundinella. The field and laboratory data together show that the concentration of the free copper ion determined the toxic response of the phytoplankton in the reservoir.