Numerical modelling of phytoplankton bloom in the upwelling ecosystem of Cabo Frio (Brazil)

The phenomenon of phytoplankton bloom during upwelling events in the coastal region of Cabo Frio (Brazil), is investigated using a 112 ocean model including the physics of coastal waters and the biological changes of the marine primary biomass. The vertical structure of the coupled, physical–biological model is described by an active layer overlaying a deep inert layer where the pressure gradient is set to zero. The physical model describes the changes of momentum, mass and heat in the dynamic layer and is forced by wind acting at the surface. The biological model describes the changes of its three components (nutrients, phytoplankton and zooplankton) and is forced by the primary biomass concentration injected into the upper layer as a consequence of upwelling favorable winds and by the light producing the photosynthesis of the phytoplankton. Coupling is established by the horizontal-velocity, layer-thickness and upwelling velocity fields from the physical solution. The equations are solved numerically in space and time by the finite difference method. Model results forced by transient winds are arranged initially for a standard run causing coastal upwelling. During the spin-up of winds, the nutrient, phytoplankton and zooplankton concentrations are injected into the upper layer by upwelling. Consequently a maximum of nutrients takes place (generated by the nutrient injection), decreasing from the increasing assimilation by phytoplankton and by reduction of nutrients supply (spin-down stage of winds). One day after the maximum of nutrients takes place, the phytoplankton bloom occurs with amplitude and duration similar to that observed in this region. This bloom is very short due the increase of grazing by zooplankton. The results of the standard run, reproduce observed features of physical and biological fields during upwelling events; in particular, the time dependent pattern of the short-lived phytoplankton bloom, suggesting that the physical–biological model presented here, contains much of this important time-dependent phenomenon. Experiments indicate that the change of transient wind duration affects the time response of the biological components, changing the instant of the maximum of nutrients concentration, but keeping unchanged the instant of maximum of phytoplankton. The reduction of the constant of optimum photosynthesis rate affects the bloom of phytoplankton delaying it and reducing its amplitude. Experiments indicate that the maximum grazing rate is an important factor in limiting the persistence and amplitude of the phytoplankton bloom.