Basin-scale structures governing the position of the deep fluorescence maximum in the Gulf of Cádiz

Physical and biological properties of the water column in the Gulf of Cádiz area were examined during a survey in May 2001. The sampling region was dominated by the presence of a central warm core and anticyclonic circulation in the open ocean, upwelling processes nearby both Cape Santa María and Cape San Vicente, and a coastal counter current along the continental shelf. These features determined the stratification of the water column and thereby nutrient distribution. A strong positive correlation between a particular isopycnal (with a density of 26.6 kg m−3) and the depth of the Deep Fluorescence Maximum (DFM) throughout the whole basin was found. This isopycnal marked the interface between the deepest limit of the Surface Atlantic Water (SAW) and the shallowest limit of the North Atlantic Central Water (NACW) and was characterized by nutrient concentrations non-limiting for phytoplankton growth. According to this analysis, the isopycnal can be considered as a nutrient tracer in the Gulf, determining the depth at which the DFM is invariably located, with the DFM intensity depending upon advective and diffusive fluxes arising around the interface.