Metabolic state along a summer north–south transect near the Antarctic Peninsula: a size spectra approach

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To establish a metabolic state along a north–south transect in Antarctic waters, we approached community respiration (CR) from a combined perspective based on the metabolic theory of ecology (MTE) and the size-scaling of the whole planktonic community. A detailed analysis of a summer integrated multi-trophic normalized biomass size spectra (NBSS), from heterotrophic bacteria to zooplankton, was carried out. To acquire individual size data, different techniques were combined: flow cytometry for smaller fractions (<20 µm of equivalent spherical diameter), FlowCAM for larger nano- and microplankton and scanning and image analysis for the zooplankton fractions. The distribution of the NBSS was linear at all stations (R2 values: 0.87–0.93) but dome-shape features appeared related to phytoplankton cell distribution which are responsible for a large fraction of microbial respiration. Generally, the region showed an autotrophic budget south of the archipelago due to gross primary production (GPP) values up to 2804 mg C m−2 day−1, where salps could significantly contribute to the carbon export flux. Contrastingly, higher CR rates (>1000 mg C m−2 day−1) were found at the northern stations due to a higher phytoplankton respiration activity associated with increasing sea water temperatures and a higher presence of heterotrophic organisms (microheterotrophs, chaetognaths and copepods) resulting in a net heterotrophic metabolic state (GPP/CR < 1).

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