¹³C and ¹⁵N Uptake by Phytoplankton in the Antarctic Upwelling Area: Results from the Antiprod I Cruise in the Indian Ocean Sector

Abstract

¹³C and ¹⁵N uptake measurements were carried out in the Indian sector of the Antarctic Ocean along a 66º30'E. transect. Integrated values ranged from 0.41 to 1.00 and from 0.03 to 0.12 mg-atom m^-2 h^-1 for inorganic carbon and nitrogen respectively. These low values were in contrast to the high nutrient concentrations and the total irradiation available. Nitrate-nitrogen concentrations for example were mostly above 20µg-atom l^-1 and on-deck total irradiation integrated over the entire incubation period was on average 10 MJ m^-2. Integrated values for particulate matter varied between 337.5 and 694.4 mg-atom m^-2 for carbon and between 48.2 and 92.2 mg-atom m^-2 for nitrogen. Carbon to nitrogen composition ratios of the particulate matter have been compared with the uptake ratios of the same elements. While euphotic zone profiles of the former ratio exhibited only small changes (6.0-8.1, all stations included), corresponding uptake ratios varied over a wide range (0.1-25.1). Generally, the latter ratio decreased with decreasing available radiation.

Response curves of specific uptake rate for carbon and nitrogen to increasing available radiation were found to be hyperbolic in shape and yielded half-saturation constants ranging between 0.9 and 3.9% of surface quantum irradiance, or between 4.9 and 16.2 × 10²¹ quanta m^-2 h^-1, if expressed in absolute units. Specific uptake rate for nitrate was directly related to ambient water temperature along a north-south cross section, but this relationship may have been an indirect one as other factors were possibly involved (ammonium concentration for example).

Maximum growth rates as doublings per day expected from water temperature were about an order of magnitude higher than the measured actual specific uptake rates for inorganic carbon and nitrogen. This was also observed in a culture of natural phytoplankton. However, measured growth rates obtained from cell counts in the same culture agreed very well with the calculated maximum expected growth rates. Phytoplankton cells from the shipboard culture continued to divide while decreasing the nutrient cell content and without a corresponding increase in specific nutrient uptake rate. The lack of intensive nutrient utilization reflected the field situation and the limiting factor could not be identified in the culture or in the field.