Picoplankton community structure along the northern Iberian continental margin in late winter-early spring

The surface distribution of autotrophic and heterotrophic picoplanktonwas assessed in 24 transects perpendicular to the coast alongthe N and NW Iberian peninsula shelf in late winter and earlyspring 2002. Community structure was analyzed by flow cytometry(FC) and found to be strongly influenced by hydrography. Typicallate winter conditions were found during the survey, characterizedby the presence of the poleward Portugal coastal counter current(PCCC) in the west and an increasing stratification eastwards.Cyanobacteria (mostly Synechococcus) dominated at low chlorophylla (Chl a) concentration whereas both the total and relativeabundance of picoeukaryotes generally increased with total phytoplanktonbiomass. Differences in the cell size of most FC-defined picoplanktonicgroups were also observed along the longitudinal and coastal–offshoregradients. The presence of Prochlorococcus (<10³ cells mL^–1)coincided with the core of the PCCC and its significant correlationwith salinity suggests its possible use as a tracer of thiscurrent. Two groups of heterotrophic bacteria were distinguishedaccording to their relative DNA content. High DNA bacteria dominatedthe community (60 ± 1% SE of total numbers), reachingmaximum values in areas under riverine influence with presumedhigher inputs of organic matter. Picoplankton biomass was dominatedby heterotrophic bacteria in the western region (58 ±3%) while autotrophic groups contributed on average 66 ±2% in the southern Bay of Biscay. The heterotrophic bacteriato phytoplankton biomass ratio decreased significantly alongthe measured range. Yet showing regional differences, the estimatedcontribution of picophytoplankton to total algal biomass washigh (mean 59 ± 4%), indicating the important role ofsmall cells at the onset of the spring bloom in these temperateshelf waters.