Collaborative Research: Linking iron and nitrogen sources in an oligotrophic coastal margin: Nitrogen fixation and the role of boundary fluxes

该项目将调查地下水排放如何为西佛罗里达大陆架的沿海生态系统提供重要的营养物质。初步研究表明，地下水可能提供溶解有机氮（DON）和铁在该地区。在像西佛罗里达大陆架这样的沿海生态系统中，硝酸盐和铵的浓度非常低，DON是生物体可用的氮的主要形式。氮循环受到铁的有效性的强烈影响，因为铁对于光合作用和固氮都是必不可少的。本研究将探讨DON和铁的来源和组成，以及它们对海岸生态系统的影响。该小组将对近海地下水威尔斯、河流和河口沃茨进行采样，并在冬季和夏季对西佛罗里达大陆架进行两次考察。研究人员将参加K-12和外联活动，以提高对该项目和相关科学的认识。该项目将资助五个机构的六名研究生和八名本科生的工作，进一步推动NSF的教育和培训目标。由于对西佛罗里达大陆架（WFS）上无法解释的、紧密相关的DON和溶解铁浓度的初步观察，拟议的工作将量化海底地下水排放（SGD）产生的DON和铁到WFS的通量和同位素特征，并使用四个季节性近岸活动对近海地下水威尔斯井、河口、以及两次跨大陆架航行。这项工作将评估SGD是否刺激WFS上的固氮，以及刺激固氮进一步改变该地区DON和溶解铁化学的潜力。跨大陆架巡航将调查假设的最大SGD和束毛藻丰度的时期（6月），以及河流流量和SGD减少的时期（2月），从而比较两种不同的地球化学制度。DON和溶解铁的浓度和同位素组成，DON的分子组成，以及铁结合配体的浓度和组成将被表征。固氮速率和束毛藻属。将测量铁胁迫基因的丰度和表达。来自SGD和河流的DON和铁的通量将用镭同位素质量平衡进行量化。SGD对固氮和DON/配体生产的影响将受到限制与孵化的天然浮游植物群落与海底地下水的修正案。将检验两个假设：1）SGD是WFS上生物可利用的DON和溶解铁的主要来源，并且2）SGD-缓解铁胁迫改变了WFS上的优势束毛藻物种，增加了固氮速率并改变了DON和铁组成。总的来说，这项工作将建立海洋氮和铁循环之间的联系，并评估沿海投入的潜力，以改变水沿着WFS出口到大西洋之前。因此，这项研究将提供一个框架，以考虑这些边界通量在贫营养沿海系统和河流和SGD的相对重要性，作为氮和铁的来源，在其他类似的位置，如沿海系统在澳大利亚，印度和非洲，该奖项反映了NSF的法定使命，并通过使用基金会的学术价值和更广泛的影响审查标准。

This project will investigate how groundwater discharge delivers important nutrients to the coastal ecosystems of the West Florida Shelf. Preliminary studies indicate that groundwater may supply both dissolved organic nitrogen (DON) and iron in this region. In coastal ecosystems like the West Florida Shelf that have very low nitrate and ammonium concentrations, DON is the main form of nitrogen available to organisms. Nitrogen cycling is strongly affected by iron availability because iron is essential for both photosynthesis and for nitrogen fixation. This study will investigate the sources and composition of DON and iron, and their influence on the coastal ecosystem. The team will sample offshore groundwater wells, river and estuarine waters, and conduct two expeditions across the West Florida Shelf in winter and summer. Investigators will participate in K-12 and outreach activities to increase awareness of the project and related science. The project will fund the work of six graduate and eight undergraduate students across five institutions, furthering NSF’s goals of education and training. Motivated by preliminary observations of unexplained, tightly-correlated DON and dissolved iron concentrations across the West Florida Shelf (WFS), the proposed work will quantify the flux and isotopic signatures of submarine groundwater discharge (SGD)-derived DON and iron to the WFS, and evaluate the bioavailability of this temporally-variable source using four seasonal near-shore campaigns sampling offshore groundwater wells, estuarine, and riverine endmembers and two cross-shelf cruises. The work will evaluate whether SGD stimulates nitrogen fixation on the WFS, and the potential for the stimulated nitrogen fixation to further modify the chemistry of DON and dissolved iron in the region. The cross-shelf cruises will investigate hypothesized periods of maximum SGD and Trichodesmium abundance (June), and reduced river discharge and SGD (February), thus comparing two distinct biogeochemical regimes. The concentrations and isotopic compositions of DON and dissolved iron, molecular composition of DON, and the concentration and composition of iron-binding ligands will be characterized. Nitrogen fixation rates and Trichodesmium spp. abundance and expression of iron stress genes will be measured. Fluxes of DON and iron from SGD and rivers will be quantified with radium isotope mass balances. The impacts of SGD on nitrogen fixation and DON/ligand production will be constrained with incubations of natural phytoplankton communities with submarine groundwater amendments. Two hypotheses will be tested: 1) SGD is the dominant source of bioavailable DON and dissolved iron on the WFS, and 2) SGD-alleviation of iron stress changes the dominant Trichodesmium species on the WFS, increases nitrogen fixation rates and modifies DON and iron composition. Overall, the work will establish connections between marine nitrogen and iron cycling and evaluate the potential for coastal inputs to modify water along the WFS before export to the Atlantic Ocean. This study will thus provide a framework to consider these boundary fluxes in oligotrophic coastal systems and the relative importance of rivers and SGD as sources of nitrogen and iron in other analogous locations, such as coastal systems in Australia, India, and Africa, where nitrogen fixation and SGD have also been documented.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.