Disproportionate Contribution of Riparian Inputs to Organic Carbon Pools in Freshwater Systems

A lack of appropriate proxies has traditionally hampered our ability to distinguish riverine organic carbon (OC) sources at the landscape scale. However, the dissection of C(4) grasslands by C(3)-enriched riparian vegetation, and the distinct carbon stable isotope signature (δ(13)C) of these two photosynthetic pathways, provides a unique setting to assess the relative contribution of riparian and more distant sources to riverine C pools. Here, we compared δ(13)C signatures of bulk sub-basin vegetation (δ(13)C(VEG)) with those of riverine OC pools for a wide range of sites within two contrasting river basins in Madagascar. Although C(3)-derived carbon dominated in the eastern Rianala catchment, consistent with the dominant vegetation, we found that in the C(4)-dominated Betsiboka basin, riverine OC is disproportionately sourced from the C(3)-enriched riparian fringe, irrespective of climatic season, even though δ(13)C(VEG) estimates suggest as much as 96% of vegetation cover in some Betsiboka sub-basins may be accounted for by C(4) biomass. For example, δ(13)C values for river bed OC were on average 6.9 ± 2.7‰ depleted in (13)C compared to paired estimates of δ(13)C(VEG). The disconnection of the wider C(4)-dominated basin is considered the primary driver of the under-representation of C(4)-derived C within riverine OC pools in the Betsiboka basin, although combustion of grassland biomass by fire is likely a subsidiary constraint on the quantity of terrestrial organic matter available for export to these streams and rivers. Our findings carry implications for the use of sedimentary δ(13)C signatures as proxies for past forest-grassland distribution and climate, as the C(4) component may be considerably underestimated due to its disconnection from riverine OC pools. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s10021-014-9772-6) contains supplementary material, which is available to authorized users.