9504301 Cochlan This study will investigate the nitrogen dynamics of a natural planktonic community in response to iron enrichment. The stable isotope 15N will be used to quantify nitrate, ammonium and urea uptake in the equatorial Pacific Ocean, a high-nutrient, low-chlorophyll (HNLC) area, during the next mesoscale iron enrichment experiment (Iron Ex II) scheduled for May-June, 1995. The study will obtain accurate and quantitative data of the nitrogenous nutrition of the natural assemblage that will determination whether or not iron fertilization enhances new production. The size- spectrum of nitrogen utilization will be determined by using different pore-sized filters to collect particulate material after short-term incubations, since cell size is thought to be a major factor controlling the sinking flux of material from the surface waters (i.e. export production, which is related to new production). All nitrogen uptake rates of total and size- fractionated samples will be corrected for the effects of isotopic-dilution which may occur during the experimental incubation periods, a condition which otherwise could result in serious underestimation of calculated nitrogen uptake rates of urea and ammonium. The heterotrophic contribution to total community nitrogen uptake will be determined by utilizing size-fractionation techniques and specific metabolic inhibitors to distinguish between heterotrophic bacteria and autotrophic phytoplankton. Additionally, the radioactive isotope 3H will be used to determine the changes in heterotrophic bacterial production as a result of iron fertilization, and these data, together with estimates of bacterial abundance, will provide an estimate of growth rate for the heterotrophic assemblage. The nitrogen uptake rates determined for autotrophs and heterotrophs, together with quantification of dissolved organic nitrogen (DON) released during normal 15N-tracer incubations and its effect on traditional measures of uptake, will allow for accurate estimations of new and regenerated production in the experimental ecosystem. The experiments in this proposed nitrogenous nutrition study are a key component in the next mesoscale iron enrichment study, and their results will be critical for a comprehensive understanding of the ecosystem response to iron enrichment during Iron Ex II.
