Evidence for the use of non‐detrital dissolved organic matter by microheterotrophs on plant detritus in a woodland stream

SUMMARY 1. Recent studies provide evidence for the use of exudates from living plants by epilithic microheterotrophs in streams. This study investigated the possible use of such non‐detrital sources of dissolved organic matter (DOM) by streatn microheterotrophs colonizing leaf litter. Biomass of bacteria and of fungi accumulating in situ on autumn‐shed leaves in flow‐through troughs from which light was excluded was compared to that accumulating on leaves in troughs open to natural illumination. 2. In experiments repeated at different times of year and in different stream sections, greater biomass of microheterotrophs consistently accumulated on the leaf detritus in troughs open to natural illumination. Differences in water temperature or in grazing of leaf surfaces by macroinvertebrates could not account for these consistent differences. Further, greater microheterotroph biomass accumulated on light‐ and dark‐incubated leaves in a stream section relatively open to sunlight, compared to corresponding leaves in a section heavily shaded by canopy and understorey vegetation. 3. These and other results suggest that, to some yet undetermined extent, detritus‐associated microheterotrophs use non‐detrital DOM. This conclusion is consistent with a priori predictions based on consideration of microbial energetics involved in the use of detrital versus non‐detrital DOM. 4. Studies of trophic pathways in streams and other aquatic habitats have failed to assess some potentially important sources of non‐detrital DOM. The ability of available techniques to assess the relative roles of detrital and non‐detrital sources of DOM is evaluated, and alternative approaches to this problem are suggested.