AME 20:245-260 (1999)  -  doi:10.3354/ame020245

Bacterioplankton intra-annual variability: importance of hydrography and competition

Johan Wikner1,2,*, Åke Hagström3

1Department of Microbiology, Umeå University, 901 87, Umeå, Sweden
2Umeå Marine Sciences Centre, Umeå University, Norrbyn, 910 20, Hörnefors, Sweden
3Department of Marine Science, Kalmar University, Box 905, 398 29, Kalmar, Sweden

ABSTRACT: Field data from a 1.5 yr intensive study of 1 coastal (0 to 20 m) and 2 offshore stations (0 to 100 m) in the northern Baltic were analysed. Specific interest was paid to the difference in the spatiotemporal variation of bacterioplankton and its controlling factors. Less than 31% of the annual bacterial biomass production (Pb) occurred in the photic zone during the productive season at the offshore stations. This suggested an uncoupling between Pb and phytoplankton carbon fixation, which was further supported by the lack of a significant correlation between these variables in the photic zone. The basin with high allochthonous loading and long residence time showed high Pb relative to autochthonous carbon fixation and low variance of Pb and bacterial abundance (Nb), suggesting an important contribution of terrestrial dissolved organic carbon to the carbon and energy supply. Bacterial per capita growth rate (rc) was highest during spring, while Pb was highest during summer at all stations. The seasonal variation in Pb was mainly explained by variation in the rc, rather than in Nb. A positive correlation of Nb with temperature, and a negative correlation with salinity, suggested that >61% of the seasonal variation in Nb was a consequence of the formation of a stratified photic zone with a higher carrying capacity. Temperature limitation of rc only occurred in the stratified photic zone, suggesting that other growth factors were sufficient during this period. A density limitation of the maximum rc was observed at all stations during autumn and winter in both depth layers, suggesting competition to be of periodic importance. Bacterioplankton with a low r (intrinsic growth rate) and high K (carrying capacity) strategy dominated when sedimenting particles were a major resource in the aphotic zone, while the opposite strategy dominated during winter at low cell densities, when dissolved substrates were the major resource.

KEY WORDS: Estuarine · Pelagic · Bacteria · Growth · Abundance · Seasonal · Temperature · Salinity · Control · Competition

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