Nitrogen uptake kinetics in the Ross Sea, Antarctica

The first estimates of uptake kinetic parameters for NH4+, NO3-, and urea in the Ross Sea, Antarctica were measured on three cruises during austral late winter-early spring 1996 (pre-bloom), late spring 1997 (bloom development), and summer 1997 (bloom decline). Nitrogen (N) uptake experiments were conducted with water collected at the 50% light penetration depth using trace-metal clean protocols and N-15 tracer techniques. At all sites, ambient NO3- concentrations ranged from 5.8 to 30.5 mug-at N l(-1) and silicic acid concentrations were greater than 62.0 mug-at Si l(-1). The following trends were observed. First, based on maximum uptake rates (V-max). apparent N utilization followed the order NO3- > NH4+ > urea during the pre-bloom and bloom development cruises. During the summer cruise, as the bloom was declining, the apparent order of utilization was NH4+ > NO3 > urea. Second, evidence for possible repression of NO3- uptake by elevated NH4+ concentrations was only observed at one site. Third, the kinetic parameters of NH4+ uptake rates corrected for isotope dilution were compared with the kinetic parameters determined from uncorrected rates. In this comparison, the measure of substrate affinity, alpha (alpha = V-max/K-S) increased by an average of 4.6-fold when rates were corrected for isotope dilution, but values of V-max remained unchanged. Fourth, using bacterial production data. the magnitude of bacterial N uptake was estimated. Assuming that all bacterial N demands were met with NH4+, the estimated bacterial portion of NH4+ uptake ranged from <1%, when the ratio of bacteria to autotrophic biomass was low, to 35%, when bacterial abundance and biomass were highest. Finally, dramatic changes in NH4+ uptake capacity were observed at one station (Stn, O), where kinetic parameters were measured during all three cruises. We hypothesize that a mutualistic relationship exists between phytoplankton and heterotrophic bacteria, and that the creation of microzones of high NH4+ concentrations contributed to the changes seen at this station. (C) 2001 Elsevier Science Ltd. All rights reserved.