13C and 15N uptake by marine phytoplankton—IV. Uptake ratios and the contribution of nitrate to the productivity of Antarctic waters (Indian Ocean sector)

Abstract

Significant latitudinal gradients in inorganic carbon and nitrate uptake were observed in the Indian sector of the Antarctic Ocean. Carbon uptake weas directly related to photosynthetically available radiation (PAR), while nitrate uptake was inversely related to PAR. This led to a direct relationship between carbon:nitrate uptake ratios and PAR both within and between stations. Field measurements as well as laboratory experiments on cultures of two diatoms isolated from the study area show that ammonium uptake decreases with time after addition of ammonium, under conditions of substrate saturation. This phenomenon, as well as other problems associated with measurements of ammonium uptake, makes it difficult to estimate the contribution of this compound to primary production from direct uptake measurements. More reliable estimates can be obtained from values of carbon:nitrate uptake ratios determined from double labeling experiments. These ratios are lower than the Redfield ratio south of the polar front, and indicate that nitrate can account for most of the primary production in offshore areas during the austral summer.