Cochlan, W. Romberg Tiburon Center for Environmental Studies, San Francisco State University, cochlan@sfsu.edu
Bronk, D. Department of Marine Science, University of Georgia, dbronk@arches.uga.edu

 
PLANKTONIC AMMONIUM UPTAKE IN THE ROSS SEA, ANTARCTICA: KINETICS OF UPTAKE AND POTENTIAL INHIBITORY EFFECTS ON NITRATE UTILIZATION
 
As part of the JGOFS Antarctic Environment Southern Ocean Process Study (AESOPS), the kinetics of ammonium uptake by phytoplankton assemblages in the Ross Sea were determined using N-15 tracer techniques. Parallel experiments were conducted simultaneously to evaluate the potential suppresive effects of ammonium on nitrate uptake. Experiments were run in the central Ross Sea (Station O: 76.5 deg S, 176 deg E) during austral spring (early November 1996; complete ice coverage) and summer (late January 1997; ice-free conditions). Ambient nitrate concentrations were considently elevated, and biomass increased from 0.72 to 4.1 ug/L as the bloom developed between November and January. Maximum specific uptake rates (Vmax) of ammonium increased five-fold from spring to summer (0.001 to 0.005 h-1) and the half-saturation constant (Ks) increased fifteen-fold (0.15 to 2.21 uM). The three-fold decrease in the initial slope of the Michaelis-Menton plot for ammonium uptake in summer suggests that these Antarctic phytoplankton assemblages become less efficient at utilizing low ammonium concentrations as the ambient concentrations increase during the season. However, the potential suppressive effect of elevated ammonium (1-2 uM) on nitrate uptake increased from 30-40% to 70-80% between the two studies. During both seasons, nitrate was the dominant nitrogen form used for growth.
 
Day: Wednesday, Feb. 3
Time: Poster
Location: Sweeney Center
 
Code: SS31WE0641S