Abstract

The Benguela Upwelling System in the southeast Atlantic Ocean is of crucial socio-economic importance due to its high productivity. However, predicting its response to global change and understanding past changes are still great challenges. Here, we compile data obtained from a research cruise and an oceanographic mooring to demonstrate that a topographically steered nutrient trapping zone develops in a narrow belt along the coast during the main upwelling season in austral spring and summer in the southern Benguela Upwelling System. High nutrient concentrations within this zone increase the impact of upwelling on the productivity of the southern Benguela Upwelling System, but the efficient nutrient trapping operates at the expense of decreasing oxygen concentrations. This enhances the probability of anoxic events emerging toward the end of the upwelling season. However, at the end of the upwelling season, the front that separates the coastally trapped waters from open shelf waters weakens or even collapses due to upwelling cessation and the reversing current regime. This, in addition to a stronger vertical mixing caused by winter cooling, fosters the ventilation of the nutrient trapping zone, which reestablishes during the following upwelling season. The postulated intensification of upwelling and changes in the ecosystem structure in response to global warming seem to reduce the nutrient trapping efficiency by increasing offshore advection of surface waters and plankton blooms. The intensified upwelling and resulting lower biological oxygen consumption appears to mask the expected impacts of global warming on the oxygen minimum zone (OMZ) in the southern Benguela Upwelling System. In contrast to other OMZs, including those in northern Benguela Upwelling Systems, the OMZ in the southern Benguela Upwelling System reveals so far no detectable long-term decrease in oxygen. Thus, the nutrient trapping efficiency seems to be a critical feature mitigating global change impacts on the southern Benguela Upwelling System. Since it is topographically steered, regional impacts on the nutrient trapping efficiency appear also to explain varying responses of upwelling systems to global change as the comparison between southern and northern Benguela Upwelling System shows. This emphasizes the need for further and more comparable studies in order to better understand the response of Eastern Boundary Upwelling Systems and their ecosystem services to global change.

Highlights

  • Coastal upwelling regions associated with Eastern Boundary Upwelling Systems and the monsoon-driven circulation in the Arabian Sea are the ocean’s most productive ecosystems (Carr, 2001; Messié et al, 2009)

  • CTD data obtained from station 7 exhibits well-oxygenated surface waters with oxygen concentrations of about 200 μM down to 80 m water-depth and hypoxic bottom water OMZ with oxygen concentrations down to 50

  • Further south in St Helena Bay, time series observations within the upper 40 m in March 2011 showed that a diurnal cycle with deepening and shoaling of surface mixed layer during the night is not always present (Pitcher et al, 2014)

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Summary

Introduction

Coastal upwelling regions associated with Eastern Boundary Upwelling Systems and the monsoon-driven circulation in the Arabian Sea are the ocean’s most productive ecosystems (Carr, 2001; Messié et al, 2009). Sub-Antarctic Mode Water loaded with preformed nutrients feeds the BUS and is further enriched with regenerated nutrients within the upwelling system (Figure 2) This occurs due to the export of organic matter from the euphotic zone into subsurface waters by offshore advecting upwellingdriven plankton blooms and its remineralization within the upwelling source waters (Dittmar and Birkicht, 2001; Tyrrell and Lucas, 2002). The remineralization of exported organic matter above and below the upwelling source waters, as well as the burial of organic matter in sediments, is considered as a vertical nutrient loss

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