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 station s during two cruises in different, near equinoctial seasons on an Atlantic transect near the 20 degrees W meridian between 50 degrees N and 50 degrees S. These samples were analysed to determine characteristics of the heterot rophic bacterial populations. Flow cytometry was used to enumerate these ba cteria and determine their average size so as to calculate their biomass. H eterotrophic bacterial production, and the rate of grazing of these bacteri a by heterotrophic nanoplankton in the main depth layers, were determined u sing H-3 thymidine and C-14 leucine techniques. The biomass of heterotrophi c 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 ac tivity of the microbial community in the water columns within each province changed little between the two cruises. Lowest heterotrophic bacterial bio mass of 1-2 mg C m(-3) and production of 0.1-0.2 mg C m(-3) day(-1) were fo und in the northern and southern Atlantic gyres, and were relatively simila r in both seasons. Biomass and production were 2-4 times higher in the nort hern and southern temperate waters, and in equatorial waters, than in the g yres and tended to show more seasonal variation. Production and biomass in the layer below the pycnocline were lower by 10-30% and about 50%, respecti vely, than values determined in the surface mixed layer, and varied less wi th 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 th e top mixed layer at all latitudes. In spite of the elevated temperatures, bacterial growth appears to be restricted by a shortage of nutrients so tha t the microbial community cycles very slowly, with a turnover time of the o rder of 1 week or more. The depth-integrated biomass of heterotrophic nanop lankton was generally about 100% of the heterotrophic bacterial biomass in the same water. Grazing by these nanoplankton at the rate measured could co nsume all of the new production of heterotrophic bacteria in all waters, an d they probably control the populations of both heterotrophic and phototrop hic bacteria.