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

Frenette, Jean-Jacques, Warwick F. Vincent, Louis Legendre

Limnol. Oceanogr., 43(6), 1998, 1362-1368 | DOI: 10.4319/lo.1998.43.6.1362

ABSTRACT: 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 µm) versus small (<2 µm) 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-µm 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-µm fraction had a higher C : N ratio than deep populations; this pattern was not seen in the <2-µm 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-µm fraction and decreasing C : N ratios in the >2-µm 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.

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