Variability and controls of the ocean acidification metrics pH and pCO2 in a large embayment of an Eastern Boundary Upwelling System (EBUS)

An assessment was undertaken of the spatial and temporal variability of pCO2 and pH in St Helena Bay, a highly productive, wide-open bay located in the Benguela, a major eastern boundary upwelling system (EBUS). Mechanisms controlling their patterns of variability and their linkage to the distribution of oxygen are explored. The mean surface pCO2 was 385 μatm, below the mean atmospheric concentration of 410 μatm, indicative of a system serving as a CO2 sink. The corresponding mean pH of bay surface waters was 8.3. The greatest variation from these means was driven by deep winter mixing causing a seasonal reversal of the air-sea pCO2 gradient. Mean surface pCO2 was also elevated above mean atmospheric levels in the nearshore (<15 m depth), a likely consequence of higher microbial respiration in the upper water column due to regular resuspension of detrital material. The nearshore of the bay is also sometimes subject to high biomass dinoflagellate blooms referred to as red tides, and their decay leads to exceptional pCO2 concentrations. Discharge from the Berg River and the associated degradation of terrestrially derived organic matter also contributes to higher pCO2 in the nearshore particularly during winter when rainfall is highest. Through the water column the important role of photosynthesis in determining the vertical distribution of pCO2 is evident in that pCO2 is shown to be a function of both Chl a and PAR. In St Helena Bay nearly 50% of water column biomass is located below the community compensation depth (i.e. where respiration carbon loss exceeds gross community production) leading to rapidly declining O2 and increasing pCO2 concentrations with depth. Resulting conditions of bottom water hypoxia/anoxia are therefore accompanied by more corrosive waters (pH declining to ∼7.4) and conditions of severe hypercapnia (pCO2 exceeding 2000 μatm) subjecting marine life to multiple stressors. St Helena Bay and similar bays in EBUS are likely to provide extrema in the deoxygenation and acidification of coastal waters.