Spatial distribution of phytoplankton <13 μm in the Gulf of Cádiz in relation to water masses and circulation pattern under westerly and easterly wind regimes

Spatial distribution of both abundance and biomass of phytoplankton <13 μm were analysed by flow cytometry, distinguishing eukaryotic picoplankton, nanoplankton, cryptomonads, Synechococcus and Prochlorococcus. These groups were related to different circulation patterns observed in two successive mesoscale samplings in the Gulf of Cádiz. The first sampling was characterized by westerly-driven upwelling processes around Cape Santa Maria and a surface signal of the Huelva front. During the second sampling easterlies blew and warm waters covered the shelf between Cádiz and Cape Santa Maria although a reminiscent upwelling was still observed west of the cape. Under the first wind regime, lowest phytoplankton abundance was measured in the upwelling area probably due to time lag of phytoplankton response, whereas the highest abundance occurred associated with the Huelva front (T∼16 °C). During the second cruise and under easterly wind regime, higher phytoplankton abundance and biomass of all groups were found as compared to the first period. Phytoplankton accumulation was mainly associated with the 16 °C isotherm in upwelled waters and in warm shelf waters. In the mixing area between warm shelf waters and cold upwelled waters, phytoplankton size spectra were found to be less negative in the warm stratified side than in the cold mixed side. This unusual feature was attributed to the warming and nutrient enrichment of shelf water during each tidal cycle in river delta and marshes located around the Gulf of Cádiz, leading to elevated biomass around these structures. On TS diagrams, eukaryotic nanoplankton was associated with warm shelf water, eukaryotic picoplankton and Synechococcus with upwelled water, while Prochlorococcus reached highest abundances in deeper water (50–75 m) associated with the mixing line between North Atlantic Central Water and Surface Atlantic Water.