Terrestrial organic carbon has long been recognized a significant subsidy to aquatic ecosystems via an uptake of dissolved organic carbon by bacteria and subsequent consumption by organisms at higher trophic levels. However, the effect of organic carbon input from adjacent ecosystems on benthic food webs has not been addressed. In this study, we tested the importance to a benthic food web of allochthonous organic carbon deposited directly onto the sediment. We manipulated carbon input, light levels and fish presence in 24 enclosures in a lake in Northern Sweden. Corn starch was used as an organic carbon source. The isotopic 13C signal of corn (a C4 plant) deviates strongly from the signal of other carbon sources in the lake. We dispersed the starch closely over the bottom of the enclosures to simulate a pulse of particulate organic carbon to the sediment.
The starch addition resulted in increasing bacterial respiration in the sediment but no detectable change in bacterial production, possibly due to cropping by bacterivores. Macroinvertebrate abundance and biomass were not affected by the input of starch. However, we observed a clear shift in the isotopic signature of most surface dwelling herbivorous and all predatory invertebrates. We suggest that the starch provided a subsidy to the food web via incorporation by bacteria.