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

合作研究：连接寡营养海岸边缘的铁和氮源：固氮和边界通量的作用

• 批准号: 2326719
• 负责人: Kristen Buck
• 金额: $ 54.91万
• 依托单位: Oregon State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2023
• 资助国家: 美国
• 起止时间: 2023-03-15 至 2025-04-30
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2326719&HistoricalAwards=false
• 关键词: Collaborative; Research; Linking; iron; nitrogen

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.

该项目将调查地下水排放如何为西佛罗里达陆架的沿海生态系统提供重要的营养物质。初步研究表明，地下水可能为该地区的沿海生态系统提供溶解有机氮（DON）和铁。硝酸盐和铵浓度非常低，DON 是生物体可利用的氮的主要形式。氮循环受到铁可用性的强烈影响，因为铁对于光合作用和固氮至关重要。该团队将对近海地下水井、河流和河口水域进行采样，并在冬季和夏季在西佛罗里达大陆架进行两次考察，并参加 K-12 和外展活动。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，以进一步推进 NSF 的教育和培训目标，其动机是对无法解释的、紧密相关的 DON 的初步观察。和整个西佛罗里达陆架 (WFS) 的溶解铁浓度，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，并使用这项工作将评估 SGD 是否会刺激 WFS 的固氮作用及其潜力。为了进一步改变该地区 DON 和溶解铁的化学性质，跨大陆架巡游将调查 SGD 和 Trichodesmium 丰度最大的时期（六月）以及河流流量和 SGD 减少的时期（二月），从而比较两个时期。 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。 Trichodesmium spp. 的丰度和铁胁迫基因的表达将通过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。将测试两个假设：1) SGD 是生物可利用 DON 和溶解铁的主要来源。 WFS，以及 2) 铁胁迫的 SGD 缓解改变了 WFS 上的优势毛藻物种，提高了固氮率并改变了 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，并评估沿海的潜力。因此，本研究将提供一个框架，以考虑寡营养沿海系统中的这些边界通量以及河流和 SGD 作为其他氮和铁来源的相对重要性。类似的地点，例如澳大利亚、印度和非洲的沿海系统，也记录了固氮和 SGD。该奖项反映了 NSF 的法定使命，并通过使用基金会的智力优点和更广泛的影响审查标准进行评估，被认为值得支持。