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Modern Shelf Anoxia |
Sea Fisheries Research Institute, Private Bag X2, Rogge Bay, 8012 Cape Town, South Africa
Anoxia in continental shelf waters has previously been ascribed to lack of circulation, advection from a remote source and in situ formation. It appears that in situ formation resulting from degradation of organic-rich material derived from phytoplankton blooms is generally the controlling factor in coastal upwelling ecosystems such as the Benguela system. Here, spatial variation in accumulation of organic-rich shelf sediments and associated anoxia is closely related to the degree to which primary production is enhanced at sites ‘downstream’ of coastal upwelling centres. Consequently, at sites where there is strong seasonal variation in the input of recently upwelled water, there is temporal variability in the incidence of oxygen-deficient waters overlying the shelf and anoxic sediments.
The extent of phytoplankton blooms is also dependent on coupling between the benthic and pelagic zones and physical characteristics of the water column. This is supported by observations of phytoplankton abundance, sedimentation of particulate organic matter and benthic oxygen deficiency in the Benguela upwelling system, where the continental shelf is marked by oxygen-deficient bottom waters and anoxic sediments equatorward of three of the major upwelling centres where such coupling occurs.
This work reaffirms the view that primary production rather than anoxia exerts direct control on the accumulation of organic-rich sediments.
1 Current address: Oceanography Department, Texas A and M University, College Station TX77843, USA.
