W.P. Cochlan & D.A. Bronk; San Francisco State University and University of Georgia

"New and Regenerated Production in the Southern Ocean: Ross Sea Study (U.S. JGOFS)"

Our study is designed to determine the nitrogenous nutrition of planktonic assemblages in the Ross Sea continental shelf regime during the productive growing season (spring, summer, autumn), and to understand the factors which control the magnitude and variability of primary production and the vertical flux of biogenic material from the euphotic zone (i.e., export production). Core measurements will include determining the uptake rates of four nitrogen substrates (nitrate, nitrite, ammonium, and urea) using the ¹⁵N tracer technique; at stations distributed along transects from the ice edge (marginal ice zone), across the shelf, slope, and into the open ocean. We will quantify the degree of isotopic dilution during our nitrogen uptake experiments to more accurately determine rates of uptake, and to estimate rates of microheterotrophic nitrogen regeneration. Supplementing the core measurements are a number of experiments designed to answer specific questions about the system, and to refine our estimates of new and regenerated production. First, we will measure rates of dissolved organic nitrogen (DON) release and correct our nitrogen uptake rates for loss of ¹⁵N label the DON pool; this loss of ¹⁵N label can result in significant underestimation of nitrogen uptake and thus new and regenerated production. Second, we will determine what size fractions of the planktonic community are responsible for the measured uptake, and how the size spectrum of utilization varies with both substrate and season; cell size is a major factor controlling the sinking of material from the surface ocean. Third, we will estimate the bacterial contribution to total community nitrogen uptake by utilizing size fractionation techniques and enumeration of bacterial abundance by epifluorescence microscopy; significant bacterial uptake of inorganic nitrogen would result in an overestimate of new and/or regenerated production. Fourth, we will perform experiments designed to estimate the suppressive effects of ammonium and urea on nitrate uptake, and thus on new production in the region. These measurements will ensure a greater understanding of the seasonal dynamics of new and regenerated production in the Southern Ocean, and will permit an accurate assessment of the importance of the Ross Sea continental shelf region to the overall flux of carbon in the Southern Ocean.