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

• Published in: Ecosystems (New York) 2014, Vol.17 (6), p.974-989
• Main Authors: Marwick, Trent R, Borges, Alberto Vieira, Van Acker, Kristof, Darchambeau, François, Bouillon, Steven
• Format: Article
• Language: English
• Subjects: Aquatic ecosystems; Aquatic sciences & oceanology; Biogeochemistry; biomass; Biomedical and Life Sciences; Carbon; Carbon cycle; carbon sinks; climate; combustion; Dry seasons; Ecology; Environmental Management; Fresh water; Freshwater; Geoecology/Natural Processes; Grasslands; Hydrology/Water Resources; Isotopes; landscapes; Life Sciences; Organic carbon; Organic matter; Photosynthesis; Plant Sciences; Rainy seasons; Riparian ecology; Riparian grasslands; Riparian soils; Riparian vegetation; River basins; Rivers; Sciences aquatiques & océanologie; Sciences du vivant; Sediments; Stable isotopes; stream channels; Streams; Vegetation; Vegetation cover; watersheds; Zoology
• ISSN: 1432-9840; 1435-0629; 1435-0629
• DOI: 10.1007/s10021-014-9772-6

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 C₄ grasslands by C₃-enriched riparian vegetation, and the distinct carbon stable isotope signature (δ¹³C) 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 δ¹³C signatures of bulk sub-basin vegetation (δ¹³CVEG) with those of riverine OC pools for a wide range of sites within two contrasting river basins in Madagascar. Although C₃-derived carbon dominated in the eastern Rianala catchment, consistent with the dominant vegetation, we found that in the C₄-dominated Betsiboka basin, riverine OC is disproportionately sourced from the C₃-enriched riparian fringe, irrespective of climatic season, even though δ¹³CVEG estimates suggest as much as 96% of vegetation cover in some Betsiboka sub-basins may be accounted for by C₄ biomass. For example, δ¹³C values for river bed OC were on average 6.9 ± 2.7‰ depleted in ¹³C compared to paired estimates of δ¹³CVEG. The disconnection of the wider C₄-dominated basin is considered the primary driver of the under-representation of C₄-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 δ¹³C signatures as proxies for past forest-grassland distribution and climate, as the C₄ component may be considerably underestimated due to its disconnection from riverine OC pools.