PHYTOPLANKTON NUTRIENT DEFICIENCY AS RELATED TO ATMOSPHERIC NITROGEN DEPOSITION IN NORTHERN MINNESOTA ACID-SENSITIVE LAKES

The Northern Lakes and Forests ecoregion of Minnesota contains thousan ds of lakes, characterized by their sensitivity to acid rain and typic ally low productivity. Four acid-sensitive lakes were studied from 198 8 to 1991 to determine if phytoplankton were deficient in nitrogen, ph osphorus, or both N and P and if the nutrient input via atmospheric de position could increase primary production. The relative accuracy of p redictions based on growth response bioassays, physiological assays, a nd nutrient deficiency indices was also evaluated. Our results show th at (1) N enrichment generally caused a greater biomass response than P , although N + P almost always yielded the greatest effect and co-limi tation 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 depositio n could satisfy most of the daily algal N demand and increase the fert ility of these lakes. These results suggest that although water qualit y protection based solely on P may not protect against this non-point source of nutrients, without these control strategies, P inputs would be expected to have a disproportionately greater stimulation of phytop lankton growth.