Abstract
We studied the carbon flow of an allochthonous carbon source (sucrose) in a biofilm food web using stable isotope analysis and lipid biomarkers. Initial biofilms were grown for 2 weeks in a third order stream in Thuringia, Germany, and then incubated in replicate flow channels in climate‐controlled chambers. Either of two sucrose types, differing in their δ 13 C values, was added either immediately (2‐week‐old biofilm) or after a preincubation of 4 weeks (6‐week‐old biofilm). Although sucrose decrease rates were similar with both biofilms, 2‐week‐old biofilms showed a higher carbon uptake capacity cell −1 . The 2‐week‐old biofilm was characterized by low abundances of all trophic levels, which increased one to two orders of magnitude during sucrose consumption. The 6‐week‐old biofilm had higher abundances. Biofilm bacteria incorporated added sucrose carbon, but algae showed no significant carbon incorporation, although a part of this carbon should be mineralized to carbon dioxide by bacteria. Sucrose carbon was also incorporated into ciliates and possibly other protozoans. Grazing rates indicated that up to 23.3% of the sucrose carbon reached higher trophic levels in 2‐week‐old biofilms. Less sucrose carbon was transferred to higher trophic levels in 6‐week‐old biofilms, where similar carbon amounts might have been channelled via filter feeding from the water column to ciliates. Ciliate community composition seemed to be affected by highly abundant rotifers. Whereas total carbon flow in 2‐week‐old biofilms was controlled by bacteria capable of high carbon uptake rates, higher trophic levels were more important in 6‐week‐old biofilms.