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.