Abstract. We apply a coupled modelling system composed of a
state-of-the-art hydrodynamical model and a low-complexity biogeochemical
model to an idealized Iberian Peninsula upwelling system to identify the
main drivers of dissolved-oxygen variability and to study its response to
changes in the duration of the upwelling season and in the phytoplankton growth
regime. We find that the export of oxygenated waters by upwelling front
turbulence is a major sink for nearshore dissolved oxygen. In our
simulations of summer upwelling, when the phytoplankton population is generally
dominated by diatoms whose growth is boosted by nutrient input, net
primary production and air–sea exchange compensate dissolved-oxygen
depletion by offshore export over the shelf. A shorter upwelling duration
causes a relaxation of upwelling winds and a decrease in offshore export,
resulting in a slight increase of net dissolved-oxygen enrichment in the
coastal region as compared to longer upwelling durations. When phytoplankton
is dominated by groups less sensitive to nutrient inputs, growth rates
decrease, and the coastal region becomes net heterotrophic. Together with the
physical sink, this lowers the net oxygenation rate of coastal waters, which
remains positive only because of air–sea exchange. These findings help in
disentangling the physical and biogeochemical controls of dissolved oxygen
in upwelling systems and, together with projections of increased duration of
upwelling seasons and phytoplankton community changes, suggest that the
Iberian coastal upwelling region may become more vulnerable to hypoxia and
deoxygenation.
This research has been supported by the IDEX UNITI – University of Toulouse (TEASAO IDEX UNITI – Univer- sity of Toulouse)