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

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 (P_b) occurred in the photic zone during the productive season at the offshore stations. This suggested an uncoupling between P_b 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 P_b relative to autochthonous carbon fixation and low variance of P_b and bacterial abundance (N_b), suggesting an important contribution of terrestrial dissolved organic carbon to the carbon and energy supply. Bacterial per capita growth rate (r_c) was highest during spring, while P_b was highest during summer at all stations. The seasonal variation in P_b was mainly explained by variation in the r_c, rather than in N_b. A positive correlation of N_b with temperature, and a negative correlation with salinity, suggested that >61% of the seasonal variation in N_b was a consequence of the formation of a stratified photic zone with a higher carrying capacity. Temperature limitation of r_c only occurred in the stratified photic zone, suggesting that other growth factors were sufficient during this period. A density limitation of the maximum r_c 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