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 70°W 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. MARINE ECOLOGY PROGRESS SERIES Mar. Ecol. Prog. Ser. INTRODUCTION Published May 5 Recent research has indicated that phycoerythrinrich chroococcoid cyanobacteria of the genus Synechococcus are a significant component of the picoplankton (Waterbury et al. 1979, Johnson & Sieburth 1979, 1982, Glover 1985). They are often the most abundant photoautotrophs in mesotrophic and oligotrophic regions, with cell concentrations ranging from lo3 to 105 cells ml-' (Waterbury et al. 1979, Johnson & Sieburth 1979, 1982, Krempin & Sullivan 1981, Campbell et al. 1983, Murphy & Haugen 1985). Photosynthetic rates of Synechococcus spp. are likewise thought to be high, but are uncertain because rate measurements are typically estimated by means of size fractionation, and the cyanobacteria lumped within the picoplankton (size fraction < 3 pm). This fraction often accounts for between 45 and 60 % of the total primary production (Li et al. 1983, Platt et al. 1983, Takahashi & Bienfang 1983, Glover et al. 1986, Prezelin et al. 1986). More specific fractionation for Synechococcus spp. support the presumed high specific Lamont-Doherty Geological Observatory Contribution No. 4279; Biowatt Contribution No. 05 growth rates; h o r v e v e ~ , iew studies nave investigated the specific photosynthetic rates of Synechococcus spp. in the field. The only available information about specific I4C-uptake by Synechococcus in natural samples using epifluorescence nuclear-track autoradiography has been reported by Douglas (1984) for marine environments, and Fahnenstiel et al. (1986) for lakes. These investigations have also confirmed high photosynthetic and growth rates for Synechocococcus spp. The high specific growth rates implicate these cyanobacteria as a significant component of the microbial food web (Iturriaga & Mitchell 1986). The present study documents the spatial and temporal variability of Synechococcus spp. in the Sargasso Sea in spring 1985. We also estimate growth rates from 2 independent isotopic techniques, document the changes in abundance occurring at the same site over a 2 wk period, and estimate population losses.

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