The reduction of plankton biomass induced by mesoscale stirring: a modeling study in the Benguela upwelling

Recent studies, both based on remote sensed data and coupled
models, showed a reduction of biological productivity due to
vigorous horizontal stirring in upwelling areas. In order to
better understand this phenomenon, we consider a system
of oceanic flow from the Benguela area coupled with a simple
biogeochemical model of Nutrient-Phyto-Zooplankton (NPZ) type.
For the flow three different surface velocity fields are
considered: one derived from satellite altimetry data, and the
other two from a regional numerical model at two different
spatial resolutions. We compute horizontal particle dispersion
in terms of Lyapunov Exponents, and analyzed their correlations
with phytoplankton concentrations. Our modelling approach
confirms that in the south Benguela there is a reduction of
biological activity when stirring is increased. Two-dimensional
offshore advection and latitudinal difference in Primary
Production, also mediated by the flow, seem to be the
dominant processes involved. We estimate that mesoscale
processes are responsible for 30 to 50% of the offshore fluxes
of biological tracers. In the northern area, other factors
not taken into account in our simulation are influencing the
ecosystem. We suggest explanations for these results in the
context of studies performed in other eastern boundary upwelling
areas.