Bacterial growth and grazing loss in contrasting areas of North and South Atlantic

Samples were collected from the top 200 m of the water column at 50 stations during two cruises in different, near equinoctial seasons on an Atlantic transect near the 20°W meridian between 50°N and 50°S. These samples were analysed to determine characteristics of the heterotrophic bacterial populations. Flow cytometry was used to enumerate these bacteria and determine their average size so as to calculate their biomass. Heterotrophic bacterial production, and the rate of grazing of these bacteria by heterotrophic nanoplankton in the main depth layers, were determined using 3H thymidine and 14C leucine techniques. The biomass of heterotrophic nanoplankton in these layers was determined using a glucosaminidase assay. Five provinces were distinguished along the transect and characterized by average values of all measured parameters. The relative composition and activity of the microbial community in the water columns within each province changed little between the two cruises. Lowest heterotrophic bacterial biomass of 1–2 mg C m–3 and production of 0.1–0.2 mg C m–3 day–1 were found in the northern and southern Atlantic gyres, and were relatively similar in both seasons. Biomass and production were 2–4 times higher in the northern and southern temperate waters, and in equatorial waters, than in the gyres and tended to show more seasonal variation. Production and biomass in the layer below the pycnocline were lower by 10–30% and about 50%, respectively, than values determined in the surface mixed layer, and varied less with latitude. Depth-integrated values of these two parameters were generally of similar size in the mixed water layer and the layer of the chlorophyll maximum and pycnocline, and tended to vary with season. The specific growth rate of heterotrophic bacteria was in the range 0.05 to 0.12 day–1 in the top mixed layer at all latitudes. In spite of the elevated temperatures, bacterial growth appears to be restricted by a shortage of nutrients so that the microbial community cycles very slowly, with a turnover time of the order of 1 week or more. The depth-integrated biomass of heterotrophic nanoplankton was generally about 100% of the heterotrophic bacterial biomass in the same water. Grazing by these nanoplankton at the rate measured could consume all of the new production of heterotrophic bacteria in all waters, and they probably control the populations of both heterotrophic and phototrophic bacteria.

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