Seasonal distribution of picoplanktonic cyanobacteria ofSynechococcustype in the Eastern Skagerrak

Abundance of picoplanktonic chroococcoid cyanobacteria of Synechococcus type, inorganic nutrient levels and chl a were measured on 13 occasions from September 1989 to September 1990 in the eastern Skagerrak. Phycoerythrin-rich cyanobacteria (PEC) cells were present throughout the year but were most abundant in summer, when they also constituted the largest part of the phytoplank-ton biomass. Maximum abundance occurred in June when 2 x 108 cells 1-1 were recorded. PEC abundance was positively correlated with temperature and negatively correlated with nutrient concentrations. Our results support the general view that picophytoplankton are quantitatively most important under oligotrophic conditions and at high temperatures. PEC constituted a substantial part of the total phytoplankton biomass in the Skagerrak from May to September.