Suppressing and enhancing effects of mesoscale dynamics on biological production in the Mozambique Channel

• Mesoscale dynamics can induce both suppressive and enhancing effects on coastal production.
• The enhancing effect relies on eddy-driven nutrient supply into the mixed layer.
• The suppressive effect is related to the eddy-driven off-shelf transport of coastal productive waters.
• The mechanism is strongly dependent on the circulation patterns.

We used a coupled physical–biogeochemical model to investigate how the strong eddy activity typical of the Mozambique Channel affects biological production. A numerical experiment was carried out, in which mesoscale dynamics were suppressed by cancelling the nonlinear terms for horizontal momentum in the Naviers–Stokes equation.Mesoscale dynamics were found to be responsible for (1) increased offshore production in the Mozambique Channel as a result of net eddy-induced offshore transport of nutrient-rich coastal waters; (2) decreased shelf production along the central Mozambican and south-west Madagascar coast caused by a reduction in nutrient availability related to the net eddy-induced lateral transport of nutrients; (3) increased coastal production along the northern Mozambican coast caused by eddy-induced nutrient supply.The model results also showed an intensification and shallowing of the subsurface production, related to increased upper layer nutrient concentrations caused by eddy activity.In addition, by driving the detachment of the East Madagascar Current at the southern tip of the island, inertial processes intensify the southern Madagascar upwelling and causes offshore diffusion of the upwelled waters.These results emphasize the complex role played by eddy activity and, more generally, inertial processes on marine ecosystems in this region.