Phytoplankton uptake of 15N and 14C in the Ross Sea during austral spring 1994

In spring 1994, within the ROSSMIZE research project, combined measurements of nitrogen (15N) and carbon (14C) uptake were made in the Ross Sea, passing from the McMurdo polynya to the ice-covered area in the north, in order to study the effect of environmental conditions (light availability, ice cover, vertical stability) on the coupling of N and C cycles. Nitrogen (nitrate and ammonium) and carbon uptakes were measured under simulated in situ conditions. The obtained results revealed, in most situations, much higher C:N uptake ratios than the Redfield ratio for phytoplankton composition; only in the inner part of the pack ice C:N uptake was lower than the balanced composition ratio. The high uptake ratios are ascribed to a larger C requirement during early phases of bloom evolution and to a greater importance of nitrogen sources, such as urea and other dissolved organic compounds, which were not measured in this study. In contrast, the lower C:N ratios in most of the pack-ice environment are ascribed to reduced photosynthesis in comparison to nutrient assimilation at low irradiances and to an increased importance of bacterial processes.