Seasonal dynamics of bacterial growth efficiencies in relation to phytoplankton in the southern North Sea

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Abstract
The main function of heterotrophic bacterioplankton in marine carbon cycling is the conversion of dissolved organic carbon (DOC) into biomass and CO2. The relative importance of bac- terial biomass production (BP) versus respiration (BR) is expressed by the bacterial growth efficiency (BGE = BP/(BP + BR) × 100). Studies on the dynamics of the BGE of bacterioplankton growing on nat- ural DOC covering entire seasonal cycles are scarce. We measured BP and BR over a seasonal cycle in the southern North Sea at a total of 150 stations to determine seasonal variability in BGE. While BP varied over 1 order of magnitude over the seasonal cycle, BR varied only 2-fold. Cell-specific BP was related to primary production while BR was not. Mean BGE increased from 6 ± 3% in the winter to 25 ± 9% in the spring and summer. Depth-integrated BR was fairly stable over the seasonal cycle, averaging 57% of the particulate primary production. Based on the bacterioplankton respiration and the mean annual BGE of 20%, bacterioplankton organic carbon demand amounts to ∼70% of the particulate primary production in the southern North Sea, suggesting that autochthonous organic matter production is sufficient to fuel bacterioplankton carbon demand.

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