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

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 bacterial biomass production (BP) versus respiration (BR) is expressed by the bacterial growth efficiency (BGE = BP/(BP + BR) x 100). Studies on the dynamics of the BGE of bacterioplankton growing on natural 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 similar 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.