ROLES OF NUTRIENTS IN CONTROLLING GROWTH OF EPILITHON IN OLIGOTROPHIC LAKES OF LOW ALKALINITY

The ability of nutrients to control photosynthesis was compared in epilithon (the association on rock surfaces in the littoral zone) and phytoplankton of 13 low alkalinity lakes of the Experimental Lakes Area of northwestern Ontario. The study included (1) surveys of lakes varying in nutrient concentrations; (2) experimental additions to lakes of carbon and nitrogen (N), with or without phosphorus (P); and (3) experimental additions to lakes of sulfuric and nitric acids. Nutrient controls of planktonic and epilithic a[gal photosynthesis differed consistently. Phosphorus limited planktonic algal photosynthesis. In contrast, dissolved inorganic carbon (DIC) limited epilithic photosynthesis in both perturbed and unperturbed lakes because diffusive resistance kept the effective supply of DIC below the level needed for optimal growth. Epilithic photosynthesis was lowered when lake disturbances (e.g., acidification) reduced epilimnetic concentrations of DIC. Expected increases in atmospheric carbon dioxide can, therefore, differentially affect the littoral and pelagic food webs in low DIC lakes. Epilithic photosynthesis in all study lakes was unrelated to N or P availability despite apparent N and P deficiencies, based upon particulate nutrient ratios. Rates of epilithic respiration were, however, correlated with epilimnetic concentrations of inorganic N.