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

In spring 1994, within the ROSSMIZE research project, combined measurements of nitrogen (N-15) and carbon (C-14) uptake were made in the Ross Sea, pas sing from the McMurdo polynya to the ice-covered area in the north, in orde r to study the effect of environmental conditions (light availability, ice cover, vertical stability) on the coupling of N and C cycles. Nitrogen (nit rate 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; o nly in the inner part of the pack ice C:N uptake was lower than the balance d composition ratio. The high uptake ratios are ascribed to a larger C requ irement during early phases of bloom evolution and to a greater importance of nitrogen sources, such as urea and other dissolved organic compounds, wh ich were not measured in this study. In contrast, the lower C:N ratios in m ost of the pack-ice environment are ascribed to reduced photosynthesis in c omparison to nutrient assimilation at low irradiances and to an increased i mportance of bacterial processes.