On the role of physical processes on the surface chlorophyll variability in the Northern Mozambique Channel

Abstract

In the Indian Ocean regions under the influence of monsoons, two phytoplankton blooms characterize the seasonal cycle of surface chlorophyll, one during summer, and the other during winter. In the Northern Mozambique Channel, however, where the wind regime is an extension of the northern Indian Ocean monsoons, the annual cycle of chlorophyll concentrations is characterized by a single winter bloom. Wind stress and surface net heat flux modulate the seasonal cycle of the mixed layer depth with impacts on the surface chlorophyll. In order to evaluate the importance of these forcing fields on the seasonality of the mixed layer depth, and consequently, the surface chlorophyll variability, we used a suite of physical-biogeochemical model sensitivity experiments. Our results show that the seasonal cycle of surface chlorophyll is primarily modulated by the net heat flux while the wind field controls the amplitude. The winter bloom is triggered by negative surface heat fluxes, where cooling at the surface induces mixing and entrainment of nutrients at the base of the nutricline and light is not limiting. Winds enhance the winter bloom by uplifting additional nutrients and diluting subsurface chlorophyll into the surface layer. In the summertime, weaker wind stress and positive heat flux inhibit vertical mixing. As a consequence, the surface layer is depleted in nutrients and a deep chlorophyll maximum is formed. Analysis of top-down control on phytoplankton biomass reveals that zooplankton abundance increases in a near-linear proportion with phytoplankton biomass despite the deepening of the mixed layer depth. This suggests that the phytoplankton stock in the Northern Mozambique Channel is also controlled by grazing, given that zooplankton biomass is not directly affected by the deepening of the mixed layer during wintertime.