Spatial and temporal variability of phytoplankton in the Gulf of Cádiz through remote sensing images

The temporal and spatial distribution of chlorophyll concentration in the Gulf of Cádiz (SW Spain) was analysed between 1998 and 2002 by remote sensing data from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS). Climatological and monthly averages showed the presence of more productive waters in the inner shelf of the basin, especially during spring and fall, according to the phytoplankton blooms observed. This pattern was further confirmed by the modes obtained with an Empirical Orthogonal Function (EOF) decomposition of weekly chlorophyll composite images performed for the whole period. The first EOF mode explained 20% of the variability and corresponded to the chlorophyll seasonality in the basin. The second EOF mode accounted for 10% of the variability and distinguished several regions with different oceanographic features in the area. Five zones could be identified among which, a coastal zone between Huelva and Cádiz, was found to show the highest chlorophyll concentration values. Local zonal winds, categorized as westerlies and easterlies, were coupled with differences in biological production. The former induced an increase in chlorophyll concentration whereas the latter caused a decrease in phytoplankton biomass. Rainfall and river discharge also affected markedly the chlorophyll concentration.