CONTRIBUTION OF SYNECHOCOCCUS SPP TO SIZE-FRACTIONED PRIMARY PRODUCTIVITY IN 3 WATER MASSES IN THE NORTHWEST ATLANTIC-OCEAN
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The distribution of phycoerythrin-rich Synechococcus spp. relative to eukaryotic algae and the contribution of Synechococcus spp. to in situ primary production were compared at a neritic front, in warm-core eddy 84-E, and at Wilkinson's Basin, during a cruise to the Northwest Atlantic Ocean in July/August 1984. Immunofluorescence analyses of Synechococcus strains demonstrated the restricted distribution of the tropical oceanic serogroup to the warm-core eddy, while strains of the neritic serogroup and those labelled by antiserum directed against a motile strain, were abundant in all three water masses. Although the majority of Synechococcus spp. cells were observed in the 0.6 to 1 m fraction, an increasing proportion of the total Synechococcus spp. cells were found in the 1 to 5 m fraction as nitrate concentrations increased near the base of the thermocline. From immunofluorescence analyses, we determined that the increasing proportion of larger Synechococcus spp. cells at depth was not the result of a change in strain composition, and may therefore be associated with increasing cell volume due to the enhanced nutrient supply. The contribution of the different size fractions to the total standing crop of chlorophyll and the in situ rate of photosynthesis was distincty different for the three water masses. At the neritic front, the larger photoautotrophs of the 1 to 5 m and >5 m fractions were the major contributors to chlorophyll concentrations and primary production. Synechococcus spp. appeared to provide only 6% of the dawn-to-dusk in situ primary production at the neritic front. In modified Sargasso water in the warm-core eddy, Synechococcus spp. contributed 25% to the in situ rate of integrated primary production. In this warm-core eddy, the 0.2 to 0.6 m fraction made a major contribution to the standing crop of chlorophyll and primary production that equalled or exceeded that of the larger sze categories. Furthermore, at the bottom of the euphotic layer, eukaryotes numerically dominated the 0.2 to 0.6 m fraction, which contributed 61% of the primary productivity. At Wilkinson's Basin, the Synechococcus spp.-dominated 0.6 to 1.0 m fraction made the greatest contribution to the standing crop of chlorophyll an primary production, while smaller photoautotrophs (0.2 to 0.6 m) accounted for little of the chlorophyll or photosynthetic rates measured over the euphotic layer. Largest numbers of Synechococcus spp. (2.9x108 cells l-1) occurred at the 18% isolume, coincident with a shoulder in the chlorophyll fluorescence profile and the site of maximum in situ primary productivity. At Wilkinson's Basin, Synechococcus spp. contributed 46% to the in situ photosynthesis integrated over the water-column. 1986 Springer-Verlag.
