Impact of allochthonous organic carbon on microbial food web carbon dynamics and structure in Lake Ortrasket

Bacterial utilization of allochthonous organic carbon and the subsequent fa te of the bacterial production in the pelagic food web were estimated in La ke Ortrasket, a large humic lake in northern Sweden. Bacterial production r elied mainly on allochthonous DOC and exceeded primary production in the ep ilimnion. Bacterial productivity was clearly stimulated during high flow ep isodes, increasing the input of bacteria degradable organic material to the lake. Bacterioplankton were exploited mainly by mixotrophic flagellates wh ich probably used bacteria as a source of carbon, phosphorus and nitrogen. The possibly extremely low availability of inorganic P during periods with high bacterial production may have allowed the mixotrophs to outcompete obl igate autotrophs and help them become dominant phytoplankters during large parts of the summer. The results from Lake Ortrasket indicated that the tot al production depended on bacterial energy mobilization from allochthonous organic carbon compounds and that heterotrophic mobilized energy was linked via mixotrophs to higher levels in the food chain.