Bacterioplankton intra-annual variability: importance of hydrography and competition

Field data from a 1.5 yr intensive study of 1 coastal (0 to 20 m) and 2 off shore stations (0 to 100 m) in the northern Baltic were analysed. Specific interest was paid to the difference in the spatiotemporal variation of bact erioplankton and its controlling factors. Less than 31% of the annual bacte rial biomass production (P-b) occurred in the photic zone during the produc tive season at the offshore stations. This suggested an uncoupling between P-b and phytoplankton carbon fixation, which was further supported by the l ack 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 terr estrial dissolved organic carbon to the carbon and energy supply. Bacterial per capita growth rate (r(c)) was highest during spring, while P-b was hig hest during summer at all stations. The seasonal variation in P-b was mainl y explained by variation in the r(c), rather than in N-b. A positive correl ation of N-b with temperature. and a negative correlation with salinity, su ggested 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. Tem perature limitation of r(c) only occurred in the stratified photic zone, su ggesting that other growth factors were sufficient during this period. A de nsity limitation of the maximum r(c) was observed at all stations during au tumn and winter in both depth layers, suggesting competition to be of perio dic importance. Bacterioplankton with a low r (intrinsic growth rate) and h igh K (carrying capacity) strategy dominated when sedimenting particles wer e a major resource in the aphotic zone, while the opposite strategy dominat ed during winter at low cell densities, when dissolved substrates were the major resource.