Effects of additions of DOC on pelagic biota in a clearwater system: Results from a whole lake experiment in northern Sweden

An oligotrophic clearwater lake, initially characterized by a pronounced do minance of autotrophic phytoplankton and mostly by one species, the green a lga Botryococcus, was subject to additions of dissolved organic carbon in t he form of white sugar (sucrose) during two consecutive years. The hypothes is tested was that it is organic carbon per se, and not other possible effe cts of humic substances, that determines the differences in structure of th e planktonic ecosystem between humic and clearwater lakes. The additions of DOC resulted in a significant increase in bacterial biomass and a decrease in the biomass of autotrophic phytoplankton. The biomass of mixotrophic an d heterotrophic flagellates instead increased significantly, whereas no eff ects were found to propagate to higher trophic levels. As a result of the c hanges among biota, total planktonic biomass also decreased to a level typi cal of nearby humic lakes. We suggest that it is the carbon component of hu mic material and its utilization by bacterioplankton that determines the st ructure and function of the pelagic food web in humic lakes.