The aim of this study was to obtain information on the importance of allochthonous versus autochtonous sources of nutrition for benthic communities of littoral and profundal sites of Lake Constance (Upper lake) on the basis of the content of stable isotopes (13C, 15N). For this purpose, isotope signatures of different potential sources and of consuments were analyzed during one vegetation period at two profundal and two littoral stations. These stations differed strongly in the respective impact of allochthous supply (close and remote to river inflows).The obtained 13C-values (average -28 ) suggested a dominance of algal epiphytes and/or terrestrial plant detritus as main sources of nutrition at both sampling sites (close and remote to river inflows). The isotope data gave also little evidence for a high importance of macrophytes as source of nutrition. The 15N values of the consuments were mostly 3 - 4 heavier than the sources. This hints to a dominantly direct utilization of the sources in one trophic level. For littoral filtering invertebrates (Cladocerans und Dreissena polymorpha) the isotope signatures showed a preferential utilization of lake seston with an increased percentage of allochthonous material at the station close to a river inflow. In contrast, nutrition of littoral copepods was much more based on sources of phytoplankton origin with a dominant utilization in two trophic levels.At the profundal stations a significantly higher percentage of allochthonous organic sedimentation was observed for the station close to river inflow, while the amount of sedimentation of autochthonous organic matter was of similar magnitude at both stations. The 13C-values were significantly heavier at the station close to river inflow (average -29 ) compared to the station remote to inflow (average -32 ) which also suggests an increased contribution of terrestrial organic matter at the station close to inflow. Additionally, the 15N signatures showed a trend to lighter values at the station remote to inflow. This also mirrors an increased percentage of young organic material originating from actual phytoplankton sedimentation. An increased contribution of allochthonous organic sedimentation resulted in a significant stimulation of microbial activities (respiration, growth rates, methane production). This means, that the sedimenting allochtonous POC is by no means completely refractory and thus allows a significant additional microbial secondary production which becomes available for benthic consuments.The 13C-values obtained for the dominant groups of profundal benthic invertebrates were similar to those of the sedimenting material at the station remote to inflow, while they were significantly lighter at the station close to inflow. Remarkably, the difference to sedimenting POC was much more pronounced for chironomids (average 11 ) than for tubificids (5 ). These results suggest that a significant part of allochthonous organic sedimentation is converted to methane wich subsequently is utilized as carbon source for methane oxidizing bacteria (MOB). The very light 13C-values biogenic methane (In summary, the results of this study suggest that more than a negligible part of the allochthonous inflow of organic matter is available for microbial utilization, resulting thus in an additional microbial production. Via this conversion into microbial biomass, allochthonous POC can represent a significant food source for littoral and profundal heterotrophic communities. As is suggested by existing patterns of horizontal distribution of benthic invertebrates, this pathway appears to be even dominant for the profundal communities of Lake Constance
