The effects of irradiance on nitrate uptake and dissolved organic nitrogen release by phytoplankton in the Ross Sea

Laboratory and field experiments have been conducted to examine the effects of irradiance on nitrate uptake and subsequent dissolved organic nitrogen (DON) release by Phaeocystis sp. and phytoplankton in the Ross Sea, Antarctica. Laboratory experiments were performed under controlled light and temperature conditions using unialgal cultures of Phaeocystis sp. obtained originally from McMurdo Station, Antarctica. The field experiments were carried out under in situ light and temperature conditions in the Ross Sea in November to December, 1994 (Cruise RSP^{2-94) and December, 1995 to January, 1996 (Cruise RSP2-95) . 15N-labeled nitrate was used to determine nitrate uptake rates, and a modified ion retardation column (IRC) method (Bronk and Gilbert 1991) was used to determine total and low molecular weight (LMW) DON release rates. The results indicate that nitrate uptake by phytoplankton in both the laboratory and the Ross Sea was strongly dependent on irradiance. The maximum nitrate uptake rate achieved by Phaeocystis sp. in the laboratory was 0.0071 h-1 in 0.5 - 8 h incubations. The maximum nitrate uptake rates by phytoplankton in the Ross Sea for the two cruises were 0.0089 h-1 and 0.0033 h-1, respectively. Photoinhibition of nitrate uptake at high irradiance levels was observed in both laboratory experiments and field experiments in the Ross Sea. An average of 8 to 19% of the nitrate taken up by Phaeocystis sp. and phytoplankton in the Ross Sea was released as DON (total DON) which resulted in a significant underestimate of new production. Total DON release rates increased with irradiance in the laboratory and the Ross Sea. However, the effects of irradiance on DON release were not as strong as those on nitrate uptake. In addition, while the LMW DON release by Phaeocystis sp. in the laboratory was high and dependent on irradiance, the LMW DON release rates in the Ross Sea were extremely low and were not dependent on irradiance. Estimates of new production in Antarctic waters clearly need to include DON release, and by quantifying this flux a more complete understanding of the region's role in global carbon cycles can be achieved.}