SIGNIFICANCE OF PICOCYANOBACTERIA IN THE RED-SEA AND THE GULF OF ADEN

The distribution of phycoerythrin-rich cyanobacteria was studied for the first time in the central Red Sea and the adjacent Gulf of Aden in February/March 1987. Cell numbers ranged from 4.6 to 367 x 10(3) cells mL-1 with an average concentration of 25.1 +/- 21.2 x 10(3) cells mL-1 in the upper 100 m of the water column. We found considerable small-scale variations both in space and time. The average cell number varied by a factor of 2 from day to day, and the vertical profile changed. Size fractionated chl a and primary productivity measurements revealed that picoplankton (0.2 - 2.0-mu-m) was the dominant phytoplankton size fraction and contributed 76 - 77% of the total to both parameters. Based on cellular chl a calculations, we derived that cyanobacteria prevailed among the autotrophic picoplankton at the majority of stations where their concentration exceeded 45 x 10(3) cells mL-1. Thus, the autotrophic picoplankton and especially cyanobacteria are an important carbon source in the epipelagial of the oligotrophic-Red Sea.