Temporal and spatial variability of chroococcoid cyanobacteria Synechococcus spp. specific growth rates and their contribution to primary production in the Sargasso Sea

The distribution, contribution to primary production and specific growth rates of the phycoerythrin-rich planktonic chroococcoid cyanobacteria Synechococcus spp. were studied in a transect along 70W in the Northwest Atlantic during April 1985. Large subsurface maxima were observed between 32"N and 26"N (3 to 15 April). Abundances of cyanobacteria increased nearly 10-fold at 35"N (3 to 5 April) over a 2 wk period. The largest concentrations of Synechococcus were observed north of the Subtropical Front in the Sargasso Sea (26"N). Primary production by Synechococcus varied between 3.4 and 11.8 fgC cell-' h-' with specific growth rates that ranged from 0.5 to 1.2 d-' as determined by epifluorescence track autoradiography. Their contribution to total primary production (as measured by differential filtration) ranged from 60 to 95 O/ O of the total.