Award: OCE-1924270

We queried biological, chemical and physical dynamics in the Southern Benguela Upwelling System off the coast of South Africa to better understands factors that contribute to seasonal declines in dissolved oxygen that are detrimental to regional fisheries. Upwelling is a phenomenon wherein persistent winds along the shore promote the motion of cold, nutrient-rich water from deep toward the surface, fertilizing the surface and promoting phytoplankton growth that nourishes associated fisheries. We collected measurements of pertinent parameters and we began developing a regional ocean model of the region with our South African partners to be able to predict the onset of low oxygen conditions in the region. Intellectual merit: We uncovered significant and unexpected physical dynamics that contribute to the rapid seasonal onset of low oxygen conditions. Deeper waters become "trapped" inshore of 100 m depth during summer, limiting exchange with oxygenated water offshore. The waters that are upwelled to the surface inshore concurrently become restricted to less oxygenated waters from inshore during summer, such that waters at the shelf bottom are then replenished by low oxygen waters. These dynamics also help explain our observations that nutrient concentrations inshore increase below the surface as the upwelling season progresses, as nutrients accumulate in poorly-flushed water below the surface due to the decomposition of organic material produced at the surface (see Figure). The decomposition of organic material near the shelf bottom also results in the production of nitrous oxide, which we determined to be produced chiefly in the sediments. Nitrous oxide is a potent greenhouse gas that occurs naturally, whose production is cited to be high in upwelling systems. The funding also provided opportunities to conduct measurements in ancillary projects, including high resolution measurement of nitrogen nutrients in the North Pacific Subtropical Gyre. These data provided evidence that increased surface photosynthesis promoted by cyclonic eddies (swirling motion that causes nutrients to come to the surface) does not result in increased sequestration of carbon at depth (from the decomposition of organic), contrary to current notions. Rather, the material produced at the surface decomposes at relatively shallow depths below the sun-lit surface. Another set of opportunistic measurements of nitrogen nutrients from the Southern Ocean at the rarely visited Larsen Ice Shelf resulted in a novel way of estimating the export potential, namely, the fraction of plankton material produced at the surface that sinks to depths and is decomposed in deeper waters. Primary production at the Larsen Ice Shelf was found to far exceed rates measured northward in the open Southern Ocean. Broader impacts: Funding contributed to training of two graduate students at UConn. It also promoted a fruitful collaboration with our South African partners, including the development of a regional ocean model that will help fisheries managers and other stake holders predict the timing and severity of low oxygen conditions in the region. Last Modified: 01/13/2025 Submitted by: JulieGranger