Analysis of mixing and biogeochemical effects induced by tides on the Atlantic–Mediterranean flow in the Strait of Gibraltar through a physical–biological coupled model

The output of a two-layer hydrodynamic model along a west–east section of the Gibraltar Strait is used to estimate tidal induced mixing between the Mediterranean and Atlantic water layers and to simulate the effects of mixing processes on biogeochemical fluxes and the pelagic community of the area. The hydrodynamic model is used to estimate interfacial mixing and water advection which drive the dynamics of the pelagic community. The model was run for 13 months, in order to analyse the effect of annual modulations in tidal amplitude on mixing. Incorporation of a third intermediate layer leads to a significant improvement in the model results, showing the necessity for a three layer circulation scheme when modelling biogeochemical processes in the Strait of Gibraltar. Pelagic processes are modelled using a simple Nutrient–Phytoplankton–Zooplankton (NPZ) model. The intense physical mixing and advection in the channel are the main influence on plankton dynamics in the area. It is found that residence times within the channel are so short that phytoplankton communities cannot grow appreciably during their transit. As a consequence, the use of a more sophisticated biogeochemical model does not lead to significant changes in the results obtained. According to the model, mixing over the Camarinal Sill causes an average of 16% of the out-flowing nutrients to be returned back to the Mediterranean. This fraction varies between 4% and 35% as a function of the tidal amplitude. The comparison of the model results with field data suggests that in order to obtain an accurate simulation of the plankton ecosystem dynamics in the strait, it is necessary to take into account the full horizontal flow, as recirculation and coast-channel interactions seems to be very important processes in explaining the biological patterns in the area.