The surface distribution of autotrophic and heterotrophic picoplankton was assessed in 24 transects perpendicular to the coast along the N and NW Iberian peninsula shelf in late winter and early spring 2002. Community structure was analyzed by flow cytometry (FC) and found to be strongly influenced by hydrography. Typical late winter conditions were found during the survey, characterized by the presence of the poleward Portugal coastal counter current (PCCC) in the west and an increasing stratification eastwards. Cyanobacteria (mostly Synechococcus) dominated at low chlorophyll a (Chl a) concentration whereas both the total and relative abundance of picoeukaryotes generally increased with total phytoplankton biomass. Differences in the cell size of most FC-defined picoplanktonic groups were also observed along the longitudinal and coastal–offshore gradients. The presence of Prochlorococcus (<103 cells mL−1) coincided with the core of the PCCC and its significant correlation with salinity suggests its possible use as a tracer of this current. Two groups of heterotrophic bacteria were distinguished according to their relative DNA content. High DNA bacteria dominated the community (60 ± 1% SE of total numbers), reaching maximum values in areas under riverine influence with presumed higher inputs of organic matter. Picoplankton biomass was dominated by heterotrophic bacteria in the western region (58 ± 3%) while autotrophic groups contributed on average 66 ± 2% in the southern Bay of Biscay. The heterotrophic bacteria to phytoplankton biomass ratio decreased significantly along the measured range. Yet showing regional differences, the estimated contribution of picophytoplankton to total algal biomass was high (mean 59 ± 4%), indicating the important role of small cells at the onset of the spring bloom in these temperate shelf waters.

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