BASIN-WIDE DISTRIBUTIONS OF LIVING CARBON COMPONENTS AND THE INVERTEDTROPHIC PYRAMID OF THE CENTRAL GYRE OF THE NORTH-ATLANTIC OCEAN, SUMMER 1993

Total living carbon biomass of microplankton, nanoplankton and picopla nkton was estimated using a combination of dual beam flow cytometry an d epifluorescence microscopy for surface samples along a basin-wide tr ansect from 5 degrees S to 61 degrees N in the North Atlantic during t he summer of 1993. We used constant per cell carbon estimates for the 4 groups of picoplankton (Prochlorococcns, Synechococcus, eukaryotic p icophytoplankton and heterotrophic bacteria), and volume sensitive car bon estimates for the other groups in compiling the autotrophic and he terotrophic components. This yielded 2 regions where the autotrophic b iomass was greater (54% of total Living carbon) than the heterotrophic biomass, the tropical (5 degrees S to 24 degrees N) and subarctic (50 degrees to 61 degrees N) regions. In the subtropical region (25 degre es to 45 degrees N), however, heterotrophic biomass outweighed (59% of total living carbon) autotrophic biomass. Although this phenomenon ha s previously been reported from this and other oligotrophic areas, our findings indicate that the degree of heterotrophy has been overestima ted by reliance upon chlorophyll as a surrogate for autotrophic biomas s. The phytoplankton carbon to chlorophyll ratios we generate covary w ith 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 oli gotrophic 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 fr om the basin-wide survey predicts significantly higher bacterial abund ances for given chlorophyll concentrations than earlier reports sugges ted.