Distribution and size biomass structure of nanophytoplankton in the Strait of Gibraltar

The spatial distribution and size structure of microbial nanophytoplankton were analysed using flow cytometry and then related to water column structure and nutrient concentration in the Strait of Gibraltar. Microbial biomass increased as the Atlantic-Mediterranean interface (AMI) shoaled and mixing processes increased from SW to NE in the strait region. Prokaryotic phytoplankton (Prochlorococcus sp. and Synechococcus sp.) were more abundant on the Atlantic side of the strait. They showed, however, a contrasting distribution along a gradient from open, oligotrophic waters (Prochlorococcus sp. dominance) to shelf, nutrient-rich waters (Synechococcus sp. dominance). In contrast to prokaryotic picophytoplankton, eukaryotic nanophytoplankton showed the highest abundance on the Mediterranean side of the strait, affected by upwelling processes. The slope of the phytoplankton size-abundance spectra (P-SAS) smoothed as the AMI shoaled, due not only to increasing nanophytoplankton abundance but also to decreasing picophytoplankton (mainly Prochlorococcus sp.) abundance. The spatial distribution pattern and size structure of microbial phytoplankton indicate a transition from 'microbial food web' to 'grazing food web' dominating conditions from the SW to the NE of the Strait of Gibraltar.