Vertical size distribution of phytoplankton in the western Mediterranean Sea during early summer

Abstract

Size fractions of chlorophyll a (chl a) <10, <3 and <1 mu m were measured by fluorimetry in 2 areas of the Mediterranean Sea: the Ligurian Sea and the Algerian Basin. Most of the particulate matter containing chl a in the photic zone was very small: 80 to 100% passed through a 10 mu m mesh net; 30 to 80% and 20 to 60% passed through 3 and 1 mu m nuclepore membranes respectively. The chlorophyll biomass of each size class was maximum in the surface or subsurface chlorophyll maximum layer. During the summer stratification, the relative importance of the <10 mu m fraction in the water column, calculated as percentage of chlorophyll <10 mu m versus total chlorophyll, did not change; but the percentages of the <3 and <1 mu m fractions were maximum at the top and at the bottom of the photic zone and minimum in the chl a maximum layer, where phytoplankton >3 mu m represented more than 60% of the chl a biomass.