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

The effects of irradiance on nitrate uptake and subsequent dissolved organic nitrogen (DON) release were investigated using laboratory cultures of Phaeocystis sp. and phytoplankton assemblages dominated by Phaeocystis antarctica in the Ross Sea during cruises in November–December, 1994 and December 1995–January, 1996. 15N-labeled nitrate was used to determine nitrate uptake rates, and an ion retardation column (IRC) method was used to determine 15N-DON release rates (both the total as well as the low molecular weight (LMW) fraction). Nitrate uptake was strongly dependent on irradiance. The maximum specific nitrate uptake rate achieved by Phaeocystis cultures in short (<8 h) incubations was 0.0069 h-1, and maximum specific nitrate uptake rates by phytoplankton assemblages dominated by Phaeocystis antarctica in the Ross Sea were 0.0089 and 0.0033 h-1, respectively, for the two cruises. Photoinhibition of nitrate uptake was observed during both cruises, but was greatest during late December and January, although the mean daily irradiances for the two cruises were similar (49.9 vs. 49.5 mol m-2 d-1). From 8 to 19% of the nitrate assimilated by Phaeocystis cultures and natural phytoplankton assemblages was released as dissolved organic nitrogen. Total DON release rates increased with irradiance in the laboratory and the Ross Sea; however, LMW DON release rates were extremely low and appeared independent of irradiance in the Ross Sea. DON release was also highly correlated with nitrate uptake and phytoplankton growth rate. Release of dissolved organic nitrogen from Phaeocystis-dominated assemblages in general does not appear to play a substantial role in the nitrogen flux in the Ross Sea.