ROLES OF NUTRIENTS IN CONTROLLING GROWTH OF EPILITHON IN OLIGOTROPHICLAKES OF LOW ALKALINITY

The ability of nutrients to control photosynthesis was compared in epi lithon (the association on rock surfaces in the littoral zone) and phy toplankton of 13 low alkalinity lakes of the Experimental Lakes Area o f northwestern Ontario. The study included (1) surveys of lakes varyin g in nutrient concentrations; (2) experimental additions to lakes of c arbon and nitrogen (N), with or without phosphorus (P); and (3) experi mental additions to lakes of sulfuric and nitric acids. Nutrient contr ols of planktonic and epilithic a[gal photosynthesis differed consiste ntly. 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., a cidification) reduced epilimnetic concentrations of DIC. Expected incr eases in atmospheric carbon dioxide can, therefore, differentially aff ect the littoral and pelagic food webs in low DIC lakes. Epilithic pho tosynthesis in all study lakes was unrelated to N or P availability de spite apparent N and P deficiencies, based upon particulate nutrient r atios. Rates of epilithic respiration were, however, correlated with e pilimnetic concentrations of inorganic N.