Oxygen-depleted bottom waters along the west coast of South Africa, 1950-2011

As part of the southern Benguela upwelling ecosystem, South Africa's west coast represents a very productive marine habitat. Decadal-scale environmental and ecosystem regime shifts have been documented based on wind and ocean temperature data, as well as biota. This study analyses the variability of oxygen-depleted (<2mLL(-1)) bottom waters on South Africa's west coast shelf using data from St Helena Bay, the most productive embayment in the ecosystem, from 1957, data from demersal and pelagic fisheries surveys since the 1980s, and monthly observations from a fixed transect off St Helena Bay from 2000 to 2011. Oxygen-depleted waters are generally restricted to bottom depths <150m, and occur regularly during austral summer and autumn, albeit with variable severity. The high spatial variability makes it impossible to define any single station as an indicator station for the extent of oxygen depletion. We provide estimates of the shelf area, 30-33 degrees S, covered by oxygen-depleted bottom water in austral summer and autumn, and show a parabolic relationship between oxygen depletion and upwelling strength. The results suggest that the cumulative upwelling divergence from the onset of the upwelling season (September) to the establishment of oxygen depletion (January) can be used as a proxy indicator for oxygen depletion in summer. Periods of better bottom ventilation are indicated for the second half of the 1960s to the early 1980s; more extensive oxygen depletion at the beginning of the 1950s, and the mid-1990s to the mid-2000s. Implications for environmental monitoring and marine living resources are discussed.