Nutrient limitation of bacterioplankton and phytoplankton in humic lakes in northern Sweden

1. Two small humic lakes in northern Sweden with concentrations of dissolve d organic carbon (DOC) between 15 and 20 mg L-1 were fertilized with inorga nic phosphorus (P) and inorganic nitrogen (N), respectively. A third lake w as unfertilized and served as a control. In addition to this lake fertiliza tion experiment, data from different regional surveys were used to assess t he role of different limiting factors. 2. The P fertilization had no effects on bacterioplankton or phytoplankton, while phytoplankton were significantly stimulated by N fertilization. Inor ganic nutrient limitation of bacterioplankton was a function of DOC concent ration in water of the investigated region and nutrient-limited bacteria we re found only in lakes with DOC concentrations less than around 15 mg L-1 3. The fertilization experiments demonstrated that the DOC-rich experimenta l lakes contained a bioavailable pool of P that was not utilized to its ful l potential under natural conditions. The overall mobilization of energy (b acterioplankton plus phytoplankton) in the experimental lakes was restricte d by lack of inorganic N.