AME 44:127-141 (2006)  -  doi:10.3354/ame044127

ABSTRACT: Phytoplankton dynamics were investigated at the mesoscale in the northern part of the Strait of Sicily in July-August 1997 on fractionated samples (<3 and >3 µm) using HPLC pigment analysis and flow cytometry. Distribution, diversity and photoacclimation varied within the different water masses and features present at the time of sampling, including a surface filament of deep, cold water. Picophytoplankton (<3 µm) accounted for 80% of total chlorophyll on average, and was numerically dominated by cyanobacteria of the genus Prochlorococcus, with an average concentration of 5.2 × 10⁴ cells ml^–1. The biomass and pigment diversity of picophytoplankton was higher in the deep chlorophyll maximum (DCM) and was related to hydrological and biological features, whereas larger phytoplankton (>3 µm) appeared to respond to different cues. Chlorophyll pigment content per cell of Synechococcus spp., Prochlorococcus spp. or picoeukaryotes was estimated by coupling pigment data with flow cytometric counts. In Prochlorococcus spp., we found an average of 0.44 and 1.56 fg divinyl-chlorophyll a (dvchl a) cell^–1 in surface and DCM layers, respectively. In contrast, chl a content in the picoeukaryote group ranged between 17 and 168 fg chl a cell^–1, depending upon depth and water mass, which suggested strong photoacclimation and photoadaptation with depth. The relative contribution of each eukaryote pigment to one size class or the other changed through the water column, and reflected size segregation within single taxonomic groups.

KEY WORDS: Picophytoplankton · Biodiversity · Deep chlorophyll maximum · DCM · Pigments · HPLC · Flow cytometry · Mesoscale