Effects of Inorganic Nitrogen Enrichment on Lake Huron Phytoplankton: An Experimental Study

Effects of inorganic nitrogen (N03) enrichment on phytoplankton were studied using a factorial design experiment. Using species assemblages from Lake Huron waters, the effects of NO] and total phosphorus (TSP) additions on species abundance and composition were monitored. The speciesnutrient relationship was determined by canonical correspondence analysis (CCA). In addition, divisional differences were related to nutrient additions using correspondence analysis (CA). These techniques were instrumental in developing species relationships to N03 and TSP gradients and, by partialling out the effect of TSP to determine the primary speciesNO] relationship. Nitrate-phosphorus (N:P) effects were also analyzed at the species and divisional levels. Gross changes in biomass, as represented by chlorophyll a concentrations, were not evident from N03 enrichment. However, species composition did change, particularly for a few species. Most evident were increases in relative abundance of Cyclotella comensis. Cyclotella stelligera, Fragilaria capucina, F. pinnata, and flagellates showed similar increased proportional abundance. Flagellates responded positively to both N03 additions and N:P interactions. Overall, there were not great changes at the level of major algal divisions. All major groups maintained their abundances relative to one another throughout the experimental period. The results from this study showed that increased NO] levels in the Great Lakes would affect species composition. INDEX WORDS: Lake Huron, phytoplankton, inorganic nitrogen.