Can C 4 plants contribute to aquatic food webs of subtropical streams?

Summary 1. Recent stable isotope studies have revealed that C 4 plants play a minor role in aquatic food webs, despite their often widespread distribution and production. We compared the breakdown of C 3 ( Eucalyptus ) and C 4 ( Saccharum and Urochloa ) plant litter in a small rain forest stream and used laboratory feeding experiments to determine their potential contribution to the aquatic food web. 2. All species of litter broke down at a fast rate in the stream, although Urochloa was significantly faster than Eucalyptus and Saccharum . This was consistent with the observed higher total organic nitrogen of Urochloa compared with the other two species. 3. The breakdown of Urochloa and Saccharum was, however, not associated with shredding invertebrates, which were poorly represented in leaf packs compared with the native Eucalyptus . The composition of the invertebrate fauna in packs of Urochloa quickly diverged from that of the other two species. 4. Feeding experiments using a common shredding aquatic insect Anisocentropus kirramus showed a distinct preference for Eucalyptus over both C 4 species. Anisocentropus was observed to ingest C 4 plant litter, particularly in the absence of other choices, and faecal material collected was clearly of C 4 origin, as determined by stable isotope analysis. However, the stable carbon isotope values of the larvae did not shift away from their C 3 signature in any of the feeding trials. 5. These data suggest that shredders avoid the consumption of C 4 plants, in favour of native C 3 species that appear to be of lower food quality (based on C : N ratios). Lower rates of consumption and lack of assimilation of C 4 carbon also suggest that shredders may have a limited ability to process this material, even in the absence of alternative litter sources. Large scale clearing of forest and vegetation for C 4 crops such as sugarcane will undoubtedly have important consequences for stream ecosystem function.