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 c onducted during a period of change in underwater light and phytoplankt on species composition associated with typhoon-induced mixing events. There were consistent qualitative differences in the C:N uptake charac teristics of large (>2 mu m) versus small (<2 mu m) phytoplankton. For both fractions, the ratio of C:N uptake versus irradiance was well de scribed by a log-log model; however, in the majority of assays, the sl ope of the relationship was positive for the >2-mu m fraction and nega tive for the small cells. This striking difference between the two fra ctions also corresponded to patterns in the C:N stoichiometry of the p lankton. Surface samples of the >2-mu m fraction had a higher C:N rati o than deep populations; this pattern was not seen in the <2-mu m sest on. 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 r atios in the >2-mu m fraction. These observations imply that in aquati c ecosystems where nitrate plays an important role in the nitrogen eco nomy of the phytoplankton, high-irradiance conditions favor maximum bi omass production per unit of nitrogen uptake by large phytoplankton, a nd low-irradiance conditions favor a high biomass increment per unit o f nitrogen uptake by small cells. These observations are consistent wi th the ecological distribution of large- versus small-cell phytoplankt on in several types of freshwater and marine environments.