Benguela upwelling system in South Africa- Drivers and implications of seasonal hypoxia using a suite of observations and models by Professor Samantha Siedlecki

Seminar Abstract

St. Helena Bay (SHB), a retentive zone in the productive southern Benguela Upwelling System off western South Africa, experiences seasonal hypoxia, and episodic anoxic events that threaten local fisheries. To understand the drivers of oxygen variability in SHB, we queried 25 years of dissolved oxygen (DO) observations alongside high-resolution wind data, hydrographic data, and dynamical data from a high-resolution model. At 70 m in SHB (mid-bay), upwelling-favorable winds provide replenishment of cold, oxygenated water in spring. Downwelling and deep mixing in winter ventilated mid-bay bottom waters, which returned to hypoxic conditions during wind relaxations and reversals. In the nearshore (20 m), hypoxia occurred specifically during periods of upwelling-favorable wind stress and was most severe in autumn. Upwelling persists from spring into autumn, yet dissolved oxygen (DO) inshore is only replenished at the onset of upwelling in spring.We query this dynamic from observations of DO in relation to a dynamical model of SHB. The increase in DO at the onset of upwelling coincides with the onshore entrainment of deep (≤ 295 m) water. DO decreases rapidly in summer through autumn, at which time source waters upwelling onshore become restricted to less oxygenated waters from shallower depths.  Using a statistical model, we extended basic the hydrographic observations to nitrate and DO concentrations and developed metrics to identify the accumulation of excess nutrients on the shelf and N-loss to denitrification, both of which were most prominent in autumn. A correspondence in the biogeochemical properties of hypoxic waters at 20 m to those at 70 m incriminates the latter as the source waters upwelled inshore in autumn. We find that wind-driven upwelling drives the replenishment of respired bottom waters in SHB with oxygenated water, albeit, importing of less oxygenated water later in the upwelling season, thus exacerbating inshore hypoxia later in the season. The subsurface residence time is longest in late summer through autumn, allowing for faster onset of hypoxia from respiration. We conclude that seasonally high respiration in conjunction with a shoaling of upwelling depth and a longer residence time instigate the development and persistence of hypoxic conditions in SHB in summer through autumn.