EFFECTS OF INORGANIC NITROGEN ENRICHMENT ON LAKE-HURON PHYTOPLANKTON - AN EXPERIMENTAL-STUDY

Effects of inorganic nitrogen (NO3) enrichment on phytoplankton were s tudied using a factorial design experiment. Using species assemblages from Lake Huron waters, the effects of NO3 and total phosphorus (TSP) additions on species abundance and composition were monitored. The spe cies-nutrient relationship was determined by canonical correspondence analysis (CCA). In addition, divisional differences were related to nu trient additions using correspondence analysis (CA). These techniques were instrumental in developing species relationships to NO3 and TSP g radients and, by partialling out the effect of TSP to determine the pr imary species-NO3 relationship. Nitrate-phosphorus (N:P) effects were also analyzed at the species and divisional levels. Gross changes in b iomass, as represented by chlorophyll a concentrations, were not evide nt from NO3 enrichment. However, species composition did change, parti cularly for a few species. Most evident were increases in relative abu ndance of Cyclotella comensis. Cyclotella stelligera, Fragilaria capuc ina, F. pinnata, and flagellates showed similar increased proportional abundance. Flagellates responded positively to both NO3 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 resul ts from this study showed that increased NO3 levels in the Great Lakes would affect species composition.