Size-dependent C:N uptake by phytoplankton as a function of irradiance:: Ecological implications

Dual-labeling assays were performed on the phytoplankton community of Lake Biwa (Japan) to estimate the ratio of inorganic carbon to nitrate-N uptake as a function of cell size and irradiance. The assays were conducted during a period of change in underwater light and phytoplankton species composition associated with typhoon-induced mixing events. There were consistent qualitative differences in the C:N uptake characteristics of large (>2 mu m) versus small (<2 mu m) phytoplankton. For both fractions, the ratio of C:N uptake versus irradiance was well described by a log-log model; however, in the majority of assays, the slope of the relationship was positive for the >2-mu m fraction and negative for the small cells. This striking difference between the two fractions also corresponded to patterns in the C:N stoichiometry of the plankton. Surface samples of the >2-mu m fraction had a higher C:N ratio than deep populations; this pattern was not seen in the <2-mu m seston. Similarly, a decrease in water-column transparency associated with the typhoon events was accompanied by a significant correlative trend of increasing C:N ratios in the <2-mu m fraction and decreasing C:N ratios in the >2-mu m fraction. These observations imply that in aquatic ecosystems where nitrate plays an important role in the nitrogen economy of the phytoplankton, high-irradiance conditions favor maximum biomass production per unit of nitrogen uptake by large phytoplankton, and low-irradiance conditions favor a high biomass increment per unit of nitrogen uptake by small cells. These observations are consistent with the ecological distribution of large- versus small-cell phytoplankton in several types of freshwater and marine environments.