Bacterioplankton production in humic Lake Ortrasket in relation to input of bacterial cells and input of allochthonous organic carbon

In order to compare riverine bacteria input with lake water bacterial produ ction and grazing loss with output loss, a bacterial cell budget was constr ucted for humic Lake Ortrasket in northern Sweden. The riverine input of ba cterial cells in 1997 represented 29% of the number of bacterial cells prod uced within the layer of the lake affected by inlet water. A large share of the in situ lake bacterial production was consumed by grazers, mainly flag ellates, which stresses the importance of bacteria as energy mobilizers for the pelagic food web in the lake. The bacterial production in Lake Ortrask et, which is almost entirely dependent on humic material as an energy sourc e, was clearly stimulated by high flow episodes which brought high amounts of little degraded material into the lake. During base flow condition the b acterial production in the inlet rivers was high, which led to an input of more degraded material to the lake. This material did not stimulate the lak e bacterial production. Internal factors that determined the utilization of the allochthonous DOC in the lake were the retention time and the exposure to light and high temperatures. Thus, the potential for in situ production of bacteria in Lake Ortrasket was to a large extent a function of how prec ipitation and runoff conditions affected terrestrial losses and river trans port of humic material.