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. We apply a coupled modelling system composed of astate-of-the-art hydrodynamical model and a low-complexity biogeochemicalmodel to an idealized Iberian Peninsula upwelling system to identify themain drivers of dissolved-oxygen variability and to study its response tochanges in the duration of the upwelling season and in the phytoplankton growthregime. We find that the export of oxygenated waters by upwelling frontturbulence is a major sink for nearshore dissolved oxygen. In oursimulations of summer upwelling, when the phytoplankton population is generallydominated by diatoms whose growth is boosted by nutrient input, netprimary production and air–sea exchange compensate dissolved-oxygendepletion by offshore export over the shelf. A shorter upwelling durationcauses a relaxation of upwelling winds and a decrease in offshore export,resulting in a slight increase of net dissolved-oxygen enrichment in thecoastal region as compared to longer upwelling durations. When phytoplanktonis dominated by groups less sensitive to nutrient inputs, growth ratesdecrease, and the coastal region becomes net heterotrophic. Together with thephysical sink, this lowers the net oxygenation rate of coastal waters, whichremains positive only because of air–sea exchange. These findings help indisentangling the physical and biogeochemical controls of dissolved oxygenin upwelling systems and, together with projections of increased duration ofupwelling seasons and phytoplankton community changes, suggest that theIberian coastal upwelling region may become more vulnerable to hypoxia anddeoxygenation.

Effects of upwelling duration and phytoplankton growth regime on dissolved-oxygen levels in an idealized Iberian Peninsula upwelling system