Investigation of mechanisms leading to seasonal hypoxia in the Southern Benguela Upwelling System

东大西洋的南本格拉上升流系统（SBUS）是世界海洋中最肥沃的地区之一，拥有重要的经济渔场。不幸的是，SBUS水域会发生溶解氧严重耗尽的事件，这种情况也被称为季节性缺氧，已观察到这种情况会导致大量鱼类死亡。为了更好地了解引发严重缺氧事件的过程，该研究将结合实地观察（分析水样中溶解的硝酸盐、亚硝酸盐、铵、可溶性活性磷和硅酸，以及硝酸盐同位素比率，以确定上升流系统中营养物质的来源和归宿）和建模。这种综合方法是识别导致SBUS缺氧发展的过程的有力手段，并且从提议的研究中收集的机制可能会扩展到SBUS以外的其他上升流区域，如北本格拉、加利福尼亚和秘鲁上升流系统。为了拓展活动，研究生们将制作一部短片，讲述他们在南非的研究。这段视频在康涅狄格大学和开普敦大学的网站以及YouTube上都可以看到，它将成为向更广泛的观众传播科学的一种手段。作为该项目的一部分，将支持和培训两名研究生。这些学生将有机会与开普敦大学的南非合作者。Sarah Fawcett和Jennifer Veitch参与了这项研究。南本格拉上升流系统（SBUS）位于南非和纳米比亚海岸外，受到严重的季节性缺氧的影响，这对野生动物、渔业和国民经济产生了灾难性的影响。康涅狄格大学的研究人员认为，该地区缺氧事件的倾向与水文锋面近岸大陆架上的营养物质捕获程度有关。这反过来又影响了随后藻华的强度，以及当这些有机物质最终在陆架底部分解时随之而来的氧气需求。为了确认营养循环在调节缺氧事件中的作用，科学家们将利用观察和定量模拟的结合。对SBUS季度监测巡航收集的水样中溶解营养物和硝酸盐同位素比率的分析将用于评估营养循环在调节缺氧事件中的作用。同时，将建立一个理想的SBUS环流模型，以验证有关近岸营养捕获和意外缺氧的假设。具体而言，重点将放在风强度和周期性、陆架锋面结构和岸岸压力梯度在调节被困在陆架上的循环养分负担及其与缺氧的关联方面的潜在作用。最后，对SBUS的海洋环流和生物地球化学进行模拟，采用真实的大气、潮汐和海洋边界条件，对整个区域的三维环流和水文进行模拟。这一物理-生物地球化学耦合模型将被用于全面研究所提出的营养捕获机制，并确定其在年际缺氧强度调节中的作用，并以此为基础建立预测模型。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The Southern Benguela Upwelling System (SBUS) in the eastern Atlantic Ocean ranks among the most fertile region in the world ocean, host to economically important fishing grounds. Unfortunately, waters of the SBUS are subject to events wherein dissolved oxygen is severely depleted, a condition also known as seasonal hypoxia, which have been observed to cause substantial fish kills. To gain a better understanding of the processes triggering severe hypoxic events, the study will combine field observations (analyzing water samples for dissolved nitrate, nitrite, ammonium, soluble reactive phosphorus, and silicic acid, as well as nitrate isotopic ratios to identify the origin and fate of nutrients in upwelling systems) and modeling. This combined approach is a powerful means of identifying the processes that contribute to the development of hypoxia in the SBUS and the mechanisms gleaned from the proposed study are likely to extend beyond the SBUS to other upwelling regions, such as the Northern Benguela, California and Peru Upwelling Systems. For outreach activities, graduate students would create a short film on their research in South Africa. This film, made available on the University of Connecticut and the University of Cape Town websites and YouTube, would serve as a means of communicating the science to broader audiences. Two graduate students would be supported and trained as part of this project. These students would have the opportunity to work with the South African collaborators at the University of Cape Town, Drs. Sarah Fawcett and Jennifer Veitch, involved in the study. The Southern Benguela Upwelling System (SBUS), off the coasts of South Africa and Namibia, is subject to severe seasonal hypoxia which has been observed to have catastrophic impacts on wildlife, fisheries, and national economies. Researcher from the University of Connecticut posit that the propensity for hypoxic events in this region is linked to the extent of nutrient trapping on the shelf inshore of the hydrographic fronts. This, in turn, influences the intensity of subsequent blooms, and the consequent oxygen demand when this organic material is ultimately decomposed at the shelf bottom. To confirm the role of nutrient cycling in modulating hypoxic event, the scientists will utilize a combination of observations and quantitative simulations. Analyses of dissolved nutrients and nitrate isotope ratios from water samples collected on quarterly monitoring cruises in the SBUS will be used to assess the role of nutrient cycling in modulating hypoxic events. Concurrently, an idealized circulation model of the SBUS will be initiated to test the hypotheses surrounding inshore nutrient trapping and incident hypoxia. Specifically, the focus will be on the potential roles of wind intensity and periodicity, shelf frontal structure, and the alongshore pressure gradient in modulating the burden of recycled nutrients trapped on the shelf and its association with hypoxia. Finally, the ocean circulation and biogeochemistry of the SBUS will be modeled using a realistic hind-cast model forced with realistic atmospheric, tidal, and ocean boundary conditions to make hind-cast simulations of the 3-D circulation and hydrography throughout the domain. This coupled physical-biogeochemical model would be queried to fully investigate the proposed nutrient trapping mechanism and define its role in modulating the intensity of hypoxia inter-annually and from which a prognostic model can be developed.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.