Temporal variations in organic carbon species and fluxes from the Chena River, Alaska

Water samples were collected biweekly from the Chena River, Alaska, during 2005–2006 for analysis of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), total dissolved carbohydrate (TCHO), including monosaccharide (MCHO) and polysaccharide (PCHO), particulate organic carbon (POC) and its isotopic compositions, and Si(OH)4. Carbon species exhibit strong temporal variations with elevated DOC, POC, and TCHO but depleted DIC and Si(OH)4 during the spring freshet and decreased DOC, POC, and TCHO but elevated DIC and Si(OH)4 concentrations under winter ice. Organic matter is mostly derived from surface soil leaching, whereas DIC and Si(OH)4 are associated with groundwater and mineral layer leaching. On average, DIC was the predominant carbon species, accounting for 77% 6 13% of the total carbon pool, whereas DOC and POC comprised 19% 6 10% and 4% 6 4%, respectively. However, DOC became the dominant carbon species during the spring freshet. TCHO comprised 15% 6 4% of DOC with higher CHO : DOC ratios during spring runoff and summer. Within the TCHO pool, MCHO was the predominant CHO component (89% 6 10%), leaving 11% 6 10% as PCHO. The particulate organic matter source during the summer drought season was mostly autochthonous, with low POC : chlorophyll a and C : N but high POC : suspended particulate matter ratios and depleted d13C values. The annual yields of DOC, DIC, and POC from the basin are 133 6 8, 361 6 7, and 27 6 4 3 103 mol C km22, corresponding to an export flux of 6.9 6 0.4, 18.8 6 0.4, and 1.4 6 0.2 3 108 mol C yr21, respectively. Anticipated changes in hydrological and biogeochemical cycles in high-latitude watersheds undergoing climate warming will likely be reflected in the chemical and phase speciation of carbon and other elements. Carbon plays a fundamental role in Earth’s climate and is a key component in the exchange between the biosphere, hydrosphere, and geosphere. A large pool of carbon is currently locked in permafrost in the form of organic carbon that is available for biogeochemical cycling if the permafrost melts (Guo et al. 2007). The introduction of this carbon to aquatic environments and the atmosphere through permafrost degradation could have major ramifications on the global carbon cycle and the climate system. Northern rivers are major conduits for the transport and export of terrigenous organic carbon to the Arctic Ocean. Recent environmental changes and amplified warming in the Arctic have been shown to have a profound effect on northern watersheds, resulting in increasing river discharge (Peterson et al. 2002), permafrost degradation (Jorgenson et al. 2001), coastal erosion (Guo et al. 2004a; Stein and Macdonald 2004), and vegetation change (McGuire et al. 2006). However, the biogeochemical consequences of these ecosystem changes remain poorly understood. The response of riverine organic carbon composition and fluxes to ongoing climate and environmental changes in the north is still a matter of debate (e.g., Frey and Smith 2005; Striegl et al. 2005). In addition, the composition and fluxes of riverine particulate organic carbon (POC) and dissolved inorganic carbon (DIC) have received little attention (Striegl et al. 2007), despite the fact that they may contribute substantially to the total C flux to the ocean (Stein and Macdonald 2004). Recent studies have shown that dissolved organic carbon (DOC) from northern rivers is mostly contemporary and is thus derived from recently fixed terrestrial biomass (Benner et al. 2004; Guo and Macdonald 2006; Raymond et al. 2007; Striegl et al. 2007), whereas POC is old and is believed to be derived from deep soils and upper permafrost through river bank erosion (Guo and Macdonald 2006; Guo et al. 2007). Nevertheless, most studies on the carbon biogeochemistry of northern watersheds have focused, so far, on the summer growing season, with sparse sampling during the rest of the year due to remoteness and extreme weather conditions, especially during winter and snowmelt. The snapshot sampling strategy has been shown to miss important flow regimes in biogeochemical studies of northern watersheds. Furthermore, major river basins have large drainage areas and a variety of tributaries with distinctly different permafrost, hydrology, vegetation cover, land use, and water chemistry (Brabets et al. 2000). As a result, the information obtained is an ambiguous mixture of basin-wide signals, and specific processes in different watersheds cannot be revealed using the overall downstream signature. In contrast, small watersheds may exhibit more sensitivity to climate change and associated local 1 Corresponding author ([email protected]). Acknowledgments We gratefully thank Mindy Juliana, Chuanhao Xu, and Laura Johnson for field sampling assistance and Associate Editor George Kling and two anonymous reviewers for constructive comments. This work was supported in part by the National Science Foundation (EAR#0554781 and ARC#0436179), the International Arctic Research Center, the University of Southern Mississippi, and the U.S. Army Alaska. Limnol. Oceanogr., 53(4), 2008, 1408–1419 E 2008, by the American Society of Limnology and Oceanography, Inc.