Variability in the size - fractioned distribution of phytoplankton across the Catalan front of the north-west Mediterranean

Author(s): Delgado M, M Latsa, M Estrada


The size distribution of the autotrophic pico- and nanoplankton community was studied by means of epifluorescence microscopy in two cruises carried out in May 1989 and February 1990, along two transects in the Catalan Sea (north-western Mediterranean). The contribution of each size class to the total autotrophic carbon biomass (PC) was assessed in the central station of each transect. In the first cruise, the water column was well-stratified, and the maximum phytoplankton concentrations were found at 40–60 m depth, coinciding with the deep chlorophyll maximum (DCM). In the second cruise, which took place during a period of weak stratification, the vertical distribution of the smallest organisms was fairly homogeneous within the euphotic zone, while the largest forms, composed mainly of diatoms, presented a subsurface maximum. Synechococcus spp. and nanoplanktonic microalgae of 2–5 μm contributed 20 and 45%, respectively, of the total autotrophic carbon at the central station of May 1989; while the >20 μm size class (composed mainly of diatoms) accounted for 55% of the autotrophic carbon at the central station of February 1990. Nevertheless, the biomass present in eukaryotic cells <20 μm was similar in both cruises (0.789 and 0.706 g C m−2 in May 1989 and February 1990 respectively). There were few consistent trends regarding preferential abundance of any particular size fraction in the shallower or deeper parts of the euphotic zone, though in general, the proportion of Synechococcus tended to be higher in the DCM than in the upper layers.

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