Inorganic and Organic Nitrogen Uptake Dynamics in the Equatorial Pacific during the Second Mesoscale Iron Enrichment Experiment (Iron Ex II)

本研究将探讨天然浮游生物群落对铁富集的氮动态响应。在定于1995年5月至6月进行的下一次中尺度铁富集实验（iron Ex II）期间，将使用稳定同位素15N来量化赤道太平洋这一高营养、低叶绿素（HNLC）地区的硝酸盐、铵和尿素吸收。该研究将获得天然组合氮营养的准确定量数据，从而确定铁施用是否能提高新产量。短期孵卵后，通过使用不同孔径的过滤器收集颗粒物质，将确定氮利用的大小谱，因为细胞大小被认为是控制来自地表水的物质下沉通量的主要因素（即出口生产，这与新生产有关）。将对总分馏样品和粒度分馏样品的所有氮吸收率进行校正，以消除在实验孵化期间可能发生的同位素稀释的影响，否则这种情况可能导致尿素和铵的计算氮吸收率严重低估。利用粒径分馏技术和特定的代谢抑制剂来区分异养细菌和自养浮游植物，可以确定异养对群落总氮吸收的贡献。此外，放射性同位素3H将用于确定铁施肥导致的异养细菌产量的变化，这些数据与细菌丰度的估计一起，将提供对异养组合生长速度的估计。测定自养和异养生物的氮吸收速率，以及在正常的15n示踪剂孵育过程中释放的溶解有机氮（DON）的量化及其对传统吸收措施的影响，将有助于准确估计实验生态系统中新的和再生的产量。本氮营养研究的实验是下一个中尺度铁富集研究的关键组成部分，其结果对于全面了解铁Ex II期间生态系统对铁富集的响应至关重要。

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.