An Assessment of Phytoplankton Nutrient Deficiency in Northern Minnesota Acid-Sensitive Lakes

Abstract

The Northern Lakes and Forests ecoregion of Minnesota contains thousands of lakes, characterized by their sensitivity to acid rain, and their typically low productivity. Four acid- sensitive lakes were studied for 1988-1991 to determine if phytoplankton were deficient in nitrogen, phosphorus, or both N and P, and if nutrient input via atmospheric deposition could increase primary production. The relative accuracy of predictions based on growth response bioassays, physiological assays, and nutrient deficiency indices was also evaluated. Our results show that: (1) N enrichment generally caused a greater biomass response than P, although N+P almost always yielded the greatest effect and co-limitation was likely in two of the lakes; (2) predictions based on DIN:TP ratios generally agreed with the growth bioassays, TN:TP and DIN:SRP were not useful and could be misleading; and (3) atmospheric deposition could satisfy most of the daily algal N demand and increase the fertility of these lakes. These results suggest that although water quality protection based solely upon phosphorus may not protect against this non-point source of nutrients, without these control strategies, P-inputs would be expected to have a disproportionally greater impact on phytoplankton growth.