95-30732 Bronk This research project is part of the US Joint Global Ocean Flux Study (JGOFS) Southern Ocean Program aimed at (1) a better understanding of the fluxes of carbon, both organic and inorganic, in the Southern Ocean, (2) identifying the physical, ecological and biogeochemical factors and processes which regulate the magnitude and variability of these fluxes, and (3) placing these fluxes into the context of the contemporary global carbon cycle. The broad continental shelf of the Ross Sea is characterized by relatively high biomass with large phytoplankton blooms in the austral spring-summer. This region has been designated as an area for intensive process studies as part of the U.S. JGOFS comprehensive investigative program of carbon and biogenic fluxes in the Southern Ocean. As part of that effort, this study is designed to assess the rates of new and regenerated production in the Ross Sea continental shelf regime during the productive growing season in Antarctica (spring, summer, autumn). The overall objectives of this study are to obtain accurate and quantitative estimates of the nitrogenous nutrition of the planktonic assemblages in this dynamic system, 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). A suite of core measurements will be made to estimate new and regenerated production over relevant spatial and temporal scales. New production in the Ross Sea is hypothesized as a function of the evolution of the ecosystems' development and thus regulated by: the hydrodynamic properties of the water column (i.e. vertical stability from ice melt), the interaction of macronutrients (ammonium and urea), and the availability of the micronutrient iron. Core measurements 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; with measurements at seven depths ranging from 100% to 1% of surface irradiance; during morning, mid-day and night. The degree of isotopic dilution will be quantified during nitrogen uptake experiments to more accurately determine rates of uptake, and to estimate rates of microheterotrophic nitrogen regeneration. In addition to the core measurements additional experiments will answer specific questions about the system, and will refine our estimates of new and regenerated production.
