Bacterioplankton seasonal dynamics and their controlling factors

Bacteria are a very important component of the microbial food-web in the ocean. The efficiency of the microbial food-web influences the biogeochemical cycles of various elements. Thus the activity of the bacterial assemblage in the water column can substantially affect the biogeochemistry of the water. Primary production and its products have been suggested to be the major food source for bacterial growth. Furthermore, temperature is frequently reported to limit bacterial growth interactively with substrate availability. But there has also been some inconsistency in reports and further research is needed. Thus, the seasonal variation of bacterial abundance, biomass, production and community structure was investigated from November 2004 until September 2005 in Logy Bay, Newfoundland. In addition, growth experiments were carried out to characterise the response of the bacterial community to differing temperatures and substrate concentrations. Logy Bay is a coastal, cold-temperate to subpolar environment with water temperatures ranging from - -1.5° C to 16° C during the year. The bay is oligotrophic with a short spring phytoplankton bloom. The productivity of the bacterial assemblage appeared to be bottom-up controlled by dissolved organic matter released during primary production. Besides substrate control of bacterial activity, in situ temperature limited the production rates. Bacterial biomass was most likely top-down controlled by bacterivorous grazers. Coincident with the phytoplankton, the bacterial community changed suggesting that different bacterial groups are more competitive at high or low substrate concentrations. Besides the concentration, the composition of the ambient dissolved organic matter might be relevant for bacterial community structure changes