GROWTH DYNAMICS OF MARINE SYNECHOCOCCUS SPP IN THE GULF OF ALASKA

Abundance, specific growth rates and overall contribution to phytoplankton carbon and productivity of marine Synechococcus spp. were investigated in the Subarctic Pacific during SUPER-cruises III and IV in June and September 1987. Specific growth rates of the cyanobacteria were measured by the frequency-of-dividing-cells method. Vertical distribution of Synechococcus spp. varied seasonally, with cells evenly distributed in the mixed layer at the beginning of June, but concentrated in and below the pycnocline (40 to 60 m) in September. Laboratory experiments on strains isolated from the Gulf of Alaska indicate a duration of division of 10.5 h. The calculated instantaneous growth rate was maximum in June at the surface (0.7 d-1), but much lower in September in the mixed layer (about 0.25 d-1) and lowest below the pycnocline (about 0.16 d-1). In June near the surface, the contribution of Synechococcus spp. to phytoplankton carbon and total primary production was estimated to be 21 and 8 %, respectively. The contribution was lower in September in the surface but increasing with depth reaching maximum values at and below the 1 % light level (50 to 60 m depth) of 31 and 68 %, respectively. Growth rates determined in the field, as well as in the laboratory. are in the range of values found for Synechococcus spp. populations thriving at lower latitudes, indicating that the low temperatures of the Subarctic Pacific do not inhibit growth of Synechococcus spp.