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 o rganic nitrogen (DON) release were investigated using laboratory cultu res of Phaeocystis sp. and phytoplankton assemblages dominated by Phae ocystis antarctica in the Ross Sea during cruises in November-December , 1994 and December 1995-January, 1996. N-15-labeled nitrate was used to determine nitrate uptake rates, and an ion retardation column (IRC) method was used to determine N-15-DON release rates (both the total a s 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 wa s 0.0069 h(-1), and maximum specific nitrate uptake rates by phytoplan kton assemblages dominated by Phaeocystis antarctica in the Ross Sea w ere 0.0089 and 0.0033 h(-1), respectively, for the two cruises. Photoi nhibition of nitrate uptake was observed during both cruises, but was greatest during late December and January, although the mean daily irr adiances 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 organi c nitrogen. Total DON release rates increased with irradiance in the l aboratory and the Ross Sea; however, LMW CION release rates were extre mely low and appeared independent of irradiance in the Ross Sea. DON r elease was also highly correlated with nitrate uptake and phytoplankto n 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. (C) 1998 Elsevier Science Ltd. All rights reserved.