Preliminary observations of the effects of rainwater addition on Lake Michigan phytoplankton primary productivity

The rate of phytoplankton primary production may be increased by phosphate additions from rain or reduced by toxic trace elements in the rain. Hypothetically then, the co-occurrence of a phytoplankton bloom (when production rates are maximal) and a dirty rainfall event could increase or decrease the quantity of organic matter produced. This could substantially alter annual productivity rates since greater than half of the annual productivity occurs during bloom periods in the spring and fall. To test this hypothesis a series of experiments are being conducted to measure the change in primary production rate (/sup 14/C uptake) in water samples containing the natural assemblage of Lake Michigan phytoplankton treated with various additions of filtered rainwater. Samples treated with 5.0% rainwater showed production rates that averaged approximately 10% higher than those of the control samples, while an increase to 10% rainwater resulted in productivity levels equal to those of the control samples. However, further incremental additions of rainwater progressively reduced primary production, and the rate in the 50% rainwater treatment averaged <50% of the rate in the control samples. These preliminary findings suggest additions of rainwater less than 10% stimulate primary production. Phosphates in the rainwater may be the stimulating agent.more » Toxic constituents, such as trace elements or changes in pH caused by acidic pollutants in the rainwater, may be responsible for the observed reductions in productivity rate at higher dilution ratios. It appears that the stimulating effect of low-level rainwater additions are masked by the toxic effects of rainwater additions in excess of 10%.« less