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

Abstract

ABSTRACT: The main function of heterotrophic bacterioplankton in marine carbon cycling is theconversion of dissolved organic carbon (DOC) into biomass and CO 2 . 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 cyclein the southern North Sea at a total of 150 stations to determine seasonal variability in BGE. While BPvaried over 1 order of magnitude over the seasonal cycle, BR varied only 2-fold. Cell-specific BP wasrelated to primary production while BR was not. Mean BGE increased from 6 ± 3% in the winter to25 ± 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 andthe mean annual BGE of 20%, bacterioplankton organic carbon demand amounts to ∼70% of theparticulate primary production in the southern North Sea, suggesting that autochthonous organicmatter production is sufficient to fuel bacterioplankton carbon demand.KEY WORDS: Bacterioplankton · Bacterial production · Bacterial respiration · Bacterial growthefficiency · Primary production · North Sea