Can C₄ plants contribute to aquatic food webs of subtropical streams?

Abstract

Summary 1. Recent stable isotope studies have revealed that C4 plants play a minor role in aquatic food webs, despite their often widespread distribution and production. We compared the breakdown of C3 (Eucalyptus) and C4 (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 C4 species. Anisocentropus was observed to ingest C4 plant litter, particularly in the absence of other choices, and faecal material collected was clearly of C4 origin, as determined by stable isotope analysis. However, the stable carbon isotope values of the larvae did not shift away from their C3 signature in any of the feeding trials. 5. These data suggest that shredders avoid the consumption of C4 plants, in favour of native C3 species that appear to be of lower food quality (based on C : N ratios). Lower rates of consumption and lack of assimilation of C4 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 C4 crops such as sugarcane will undoubtedly have important consequences for stream ecosystem function.

Sources of organic matter and methylmercury in littoral macroinvertebrates: a stable isotope approach

“…This is in agreement with a recent study by Clapcott and Bunn (2003), which reported low contributions of C 4 plants to aquatic food webs, despite their widespread distribution and production. Thus, the high density of corn fields in the Lake St. Pierre watershed could have a lower than expected contribution to consumer assimilated diets.…”

Are Large Herbivores Vectors of Terrestrial Subsidies for Riverine Food Webs?

“…At the agricultural sites (Issey, Tenwek, Olbutyo and Issey-supplementary Table S2), the relative importance of C4 relative to C3 producers for most consumers was higher during the dry season. In contrast, a range of previous studies suggest that C4 producers contribute minimally as an energy source to consumer biomass as compared with C3 producers and autochthonous (algae and periphyton) producers (Clapcott and Bunn 2003;Abrantes and Sheaves 2010;Roach 2013).…”

Separation of algae from detritus for stable isotope or ecological stoichiometry studies using density fractionation in colloidal silica

“…Microscopic examination of the relative proportions of algae and detritus in FPOM samples can be deceptive because large algal cells are conspicuous whereas colloidal detritus is not. Many aquatic consumers appear to indiscriminately ingest this organic material, but studies of specific consumers show that they can selectively ingest or assimilate certain components of the FPOM (e.g., Raikow and Hamilton 2001;Clapcott and Bunn 2003), which can lead to marked differences between the isotopic ratios of consumers and their apparent food resources.…”