Effects of Atmospheric Precipitation Additions on Phytoplankton Photosynthesis in Lake Michigan Water Samples

Abstract The effects of incremental additions (0.1-50% v/v) of atmospheric precipitation on phytoplankton photosynthesis ( 14 C uptake) were tested in Lake Michigan water samples. Wet deposition was used in experiments I, III, and IV, and a melted snow core was used in experiment II. Additions of precipitation significantly reduced photosynthesis in the first three experiments, starting at about the 5-15% treatment level. No significant difference occurred in experiment IV, but photosynthesis was greater than in the control samples and this precipitation sample appeared to stimulate primary productivity. Soluble reactive phosphate, nitrate, and ammonia levels in the precipitation samples exceeded the lake water averages by factors of 10, 2, and 50, respectively. Silicon levels in precipitation were lower than the lake water, but likely did not limit photosynthesis. The additions of precipitation reduced pH very little and no consistent relationship was observed with reduced photosynthesis. Alkalinity was greatly reduced in the treated samples and special precautions were required in calculating photosynthetic rates as a result. Zn, Cd, and Cu levels in the precipitation samples greatly exceeded the lake water averages, producing a high potential for toxic effects on photosynthesis at treatment levels >10%. These experiments suggest that algal nutrients and toxic trace elements in atmospheric precipitation may competitively interact to either stimulate or depress phytoplankton photosynthesis in Lake Michigan water samples.