ELEMENTAL RATIOS AND THE UPTAKE AND RELEASE OF NUTRIENTS BY PHYTOPLANKTON AND BACTERIA IN 3 LAKES OF THE CANADIAN SHIELD

The dynamics of carbon (C), nitrogen (N), and phosphorus (P), elementa l ratios, and dark uptake/release of N and P in bacterial and phytopla nkton size fractions were studied during summer 1992 in three lakes of contrasting food web structure and trophic status (L240, L110, L227). We wished to determine if phytoplankton and bacteria differed in thei r elemental characteristics and to evaluate whether the functional rol e of bacteria in nutrient cycling (i.e., as sink or source) depended o n bacterial elemental characteristics. Bacterial contributions to tota l suspended particulate material and to fluxes of nutrients in the dar k were substantial and varied for different elements. This indicated t hat some techniques for assaying phytoplankton physiological condition are compromised by bacterial contributions. C/N ratios were generally less variable than C/P and N/P ratios. Both elemental ratios and biom ass-normalized N and P flux indicated that phytoplankton growth in eac h lake was predominantly P-limited, although in L227 these data reflec t the dominance of N-fixing cyanobacteria, and N was likely limiting e arly in the sampling season. In L227, phytoplankton N/P ratio and biom ass-normalized N flux were negatively correlated, indicating that flux data were likely a reasonable measure of the N status of the phytopla nkton. However, for L227 phytoplankton, P-flux per unit biomass was a hyperbolic function of N/P, suggesting that the dominant L227 cyanobac teria have a limited uptake and storage capacity and that P-flux per u nit biomass may not be a good gauge of the P-limitation status of phyt oplankton in this situation. Examination of N-flux data in the bacteri al size fraction relative to the N/P ratio of the bacteria revealed a threshold N/P ratio (similar to 22.1 N/P, by atoms), below which, bact eria took up and sequestered added N, and above which, N was released. Thus, the functional role of bacteria in N cycling in these ecosystem s depended on their N/P stoichiometry.