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将重要的营养物质输送到西佛罗里达大陆架的沿海生态系统。初步研究表明，地下水可能同时提供溶解有机氮(DON)和铁。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是生物体可利用的主要氮素形式。氮循环受到铁供应的强烈影响，因为铁对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、组成及其对海岸生态系统的影响。该团队将对近海地下水井、河流和河口水域进行采样，并在冬季和夏季进行两次横跨西佛罗里达大陆架的探险。调查人员将参加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，促进NSF的教育和培训目标。在对西佛罗里达大陆架(WFS)上不明原因的、密切相关的DON和溶解铁浓度的初步观察的推动下，拟议的工作将量化海底地下水排放(SGD)衍生的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采样近海地下水井、河口和河流端部以及两个跨陆架巡航来评估这一时间可变来源的生物有效性。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和溶解铁的化学作用的可能性。跨陆架巡航将调查假设的SGD和Trichodesum丰度最大的时期(6月)，以及河流流量和SGD减少的假想时期(2月)，从而比较两种不同的生物地球化学制度。将表征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。固氮率和毛霉菌。将测量铁胁迫基因的丰度和表达。来自SGD和河流的DON和铁的通量将使用镭同位素质量天平进行量化。SGD对固氮和DON/配体生产的影响将受到自然浮游植物群落的孵化和海底地下水改良剂的限制。将检验两个假说：1)SGD是WFS上生物有效DON和溶解铁的主要来源，2)SGD-缓解铁胁迫改变了WFS上优势的毛霉物种，增加了固氮率，改变了DON和铁的组成。总体而言，这项工作将建立海洋氮和铁循环之间的联系，并评估沿海投入物在出口到大西洋之前修改WFS沿线水的潜力。因此，这项研究将提供一个框架，以考虑贫营养化海岸系统中的这些边界通量，以及河流和SGD作为其他类似地点的氮和铁来源的相对重要性，例如澳大利亚、印度和非洲的沿海系统，在这些地方也记录了固氮和SGD。这一裁决反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.