Inter-Research
Aquatic Microbial Ecology

ABSTRACT: Total living carbon biomass of microplankton, nanoplankton and picoplankton was estimated using a combination of dual beam flow cytometry and epifluorescence microscopy for surface samples along a basin-wide transect from 5* S to 61* N in the North Atlantic during the summer of 1993. We used constant per cell carbon estimates for the 4 groups of picoplankton (Prochlorococcus, Synechococcus, eukaryotic picophytoplankton and heterotrophic bacteria), and volume sensitive carbon estimates for the other groups in compiling the autotrophic and heterotrophic components. This yielded 2 regions where the autotrophic biomass was greater (54% of total living carbon) than the heterotrophic biomass, the tropical (5* S to 24* N) and subarctic (50* to 61* N) regions. In the subtropical region (25* to 45* N), however, heterotrophic biomass outweighed (59% of total living carbon) autotrophic biomass. Although this phenomenon has previously been reported from this and other oligotrophic areas, our findings indicate that the degree of heterotrophy has been overestimated by reliance upon chlorophyll as a surrogate for autotrophic biomass. The phytoplankton carbon to chlorophyll ratios we generate covary with chlorophyll with the highest ratios found in the most oligotrophic regions. Application of a constant carbon to chlorophyll ratio would therefore underestimate the contribution of autotrophs most in the oligotrophic regions. Nano- and microplankton contribute 40% of the total living carbon biomass of the subtropical region. Bacterial abundances covaried with phytoplankton biomass across all the trophic regions we sampled, however the regression we generated for this relationship from the basin-wide survey predicts significantly higher bacterial abundances for given chlorophyll concentrations than earlier reports suggested.

KEY WORDS: Living carbon . North Atlantic

Published in AME Vol. 10, No. 3 (1996) on June 27
Print ISSN: 0948-3055; Online ISSN: 1616-1564. Copyright © Inter-Research, Oldendorf/Luhe, 1996