Abstract
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.Electronic supplementary materialThe online version of this article (doi:10.1007/s10021-014-9772-6) contains supplementary material, which is available to authorized users.
Highlights
The riparian zone plays a key role in regulating the delivery of solutes to the adjoining inland surface waters (Remington and others 2007; Ranalli and Macalady 2010; Bouwman and others 2013)
In the Betsiboka basin, the d13C of the soil OC (d13CSOC) ranged between -25.8& and -12.0&, whereas SOC ranged from a pure C3 (-28.2&) to mixed C3:C4 (-16.9&) origin in the Rianila basin
1964; Benstead and Pringle 2004), and could potentially skew riverine organic carbon (OC) d13C values towards the C3 end-member. With their strong partitioning of C3 and C4 vegetation at the landscape scale, subtropical and tropical C4-rich grasslands provide the optimum environment to further explore the relative delivery of terrestrial C from different landscape units to river networks, as well as the investigation of trophic linkages between aquatic consumers and riparian carbon sources
Summary
The riparian zone plays a key role in regulating the delivery of solutes to the adjoining inland surface waters (Remington and others 2007; Ranalli and Macalady 2010; Bouwman and others 2013). In freshwaters with negligible algal production, the predominant driver of riverine OC d13C signatures is the proportion of organic matter (OM) derived from terrestrial vegetation following the C3 photosynthetic pathway (woody plants and trees, temperate grasses; d13C $ -27&) compared to the less fractionating C4 photosynthetic pathway (largely tropical and subtropical grasses; d13C $ -13&) (Hedges and others 1986; Bird and others 1994). Others have employed stable isotope techniques to reconstruct paleo-vegetation distribution and paleo-climate regime from OM buried within various environments, including lacustrine and fluvial deposits (Cerling and others 1988; Mora and others 2002) as well as nearshore (Santschi and others 2007) and offshore (dos Santos and others 2013) marine deposits Such studies often assume no inherent differences in sediment entrainment or transport between C3- and C4-derived carbon from source to sink (Wynn and Bird 2007). The predominantly perennial connection of the C3-enriched riparian zone to active (surface and ground-) water channels, relative to the seasonally parched and disconnected C4-dominated grasslands, has been identified as a key driver of riverine organic C pools within subtropical and tropical savannah/ grassland ecosystems globally (for example, Congo [Mariotti and others 1991]; Amazon [Bird and others 1992]; Australian rivers [Bird and Pousai 1997]; Cameroonian rivers [Bird and others 1994, 1998])
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