Influence of Long-Term Oxygen Minimum Zone Variability on Biogeochemical Cycling in the Benguela Upwelling System

The Benguela Upwelling System (BUS) is one of four major Eastern Boundary Upwelling Systems with significant ecological and economical importance. It sustains high biological productivity and is characterised by a pronounced Oxygen Minimum Zone (OMZ). Despite extensive research, long-term OMZ variability and its biogeochemical impacts remain poorly constrained, particularly between the Northern (NBUS) and Southern (SBUS) subsystems. This study examines OMZ variability and its influence on Redfield stoichiometry (C:N:P = 106:16:1) to elucidate underlying biogeochemical processes.

Data from cruises along the A09 (NBUS) and A10 (SBUS) sections in 1991, 1992, 2003, 2011, 2017, and 2018 were analysed. Dissolved oxygen (DO), dissolved inorganic carbon (DIC), nitrate (N), and phosphate (P) data were obtained from the CLIVAR and Carbon Hydrographic Data Office (CCHDO), supplemented with three datasets from Flohr et al. (2014). Only stations within 200 km of the coastline were included, and OMZ vertical extent was assessed using three DO thresholds (20, 60, and 120 µmol kg^-1).

Results reveal contraction of the OMZ core in the NBUS, with the 20 and 60 µmol kg^-1 thresholds contracting from 301.4 m to 172.2 m and from 871.4 m to 521.9 m, respectively, between 1991 and 2011, while the 120 µmol kg^-1 threshold expanded from 373.0 m in 2008 to 550.7 m in 2018, likely associated with reduced inflow of South Atlantic Central Water (SACW). In the SBUS, the 120 µmol kg^-1 threshold expanded slightly from 270.3 m to 295.8 m between 2003 and 2011, while DO concentrations remained above this level, indicating a well-oxygenated water column. In the NBUS, C:N ratios increased from 4.94 to 6.19 in the upper 200 m and from 0.09 to 1.88 in the 200-500 m layer from 1991-2011, while in the SBUS, ratios increased from 5.66 to 5.77 and from 3.75 to 5.33 in the same layers from 1992-2011, indicating enhanced N loss. In the NBUS, C:P ratios decreased from 116.91 to 107.66 in the upper 200 m and ranged from 5.22 to 27.41 in the 200-500 m layer, while N:P ratios decreased from 20.78 to 17.12 and from 9.48 to 8.82, over the same depths from 1991-2011, indicating increased P accumulation. In contrast, in the SBUS, C:P ratios increased from 91.20 to 96.70 in the upper 200 m and from 60.83 to 81.70 in the 200-500 m layer from 1992-2011, indicating reduced P accumulation. 

These findings indicate that, despite contraction of the OMZ core (20 and 60 µmol kg^-1) and expansion of the 120 µmol kg^-1 threshold during 1991-2011, and reduced SACW inflow moderating hypoxia in the upper 500 m, the development of the OMZ with its vertical variability continues intensifying denitrification and/or anammox, resulting in N loss and enhanced P release from sediments in hypoxic waters, leading to P accumulation in the NBUS. OMZ variability exerts a stronger influence on both nitrogen and phosphorus cycling in the NBUS, while nitrogen cycling is more strongly affected than phosphorus cycling in the SBUS. This study highlights long-term oxygen variability as a key bottom-up driver shaping biogeochemical cycling and ecosystem functioning across the BUS.