<b>Butman</b>, D., R. Striegl, S. Stackpoole, P. del Giorgio, Y. Prairie, D. Pilcher, P. Raymond, F. Paz Pellat, and J. Alcocer, 2018: Chapter 14: Inland waters. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 568-595, https://doi.org/10.7930/SOCCR2.2018.Ch14.
Whether carbon fluxes from inland waters are increasing or decreasing at the national or continental scale remains unclear. Because carbon export from the terrestrial landscape is tightly linked to discharge, increases in discharge probably will lead to increases in carbon export (Mulholland and Kuenzler 1979). Current studies are arguing for an increase in discharge for many regions of North America, including the U.S. Midwest and New England; however, reductions in precipitation are predicted in the southern and western regions of the United States (Georgakakos et al., 2014). Human water use through irrigation also may be affecting the spatial variability of discharge, with lower discharge in regions of higher irrigation, an effect which may be mitigated by increases in precipitation (Kustu et al., 2011). However, future changes in precipitation that lead to regional drought will reduce the transfer of carbon from the terrestrial ecosystem into the aquatic environment, while simultaneously decreasing the total area of aquatic ecosystems. Other anthropogenic drivers also can impact fluxes. Evidence suggests that DIC fluxes have increased from the Mississippi River over time because of land-management practices associated with liming and irrigation for agriculture, as well as increases in precipitation across portions of the basin (Raymond et al., 2008; Tian et al., 2015). In the United Sates, about 30 Tg of lime are applied each year, resulting in a potential flux of 7.2 Tg of inorganic carbon per year in the form of bicarbonate, or an actual flux of approximately 5.4 Tg C per year, assuming that 25% is balanced by the export of products from weathering reactions other than carbonic acid (Oh and Raymond 2006). The total U.S. riverine flux of DIC is approximately 35 Tg per year (Stets and Striegl 2012). Thus, liming and fertilizer use may contribute about 15% of total river bicarbonate flux in the United States.
Calculations suggest that DOC export from the Mississippi River has increased since the early 1900s, primarily a result of land-cover change from forest and grasslands to managed agriculture (Ren et al., 2016). Tributaries to the Mississippi have been shown to have decreasing DOC as a result of wetland loss (Duan et al., 2017). However, DOC flux from the Mississippi River to the Gulf of Mexico did not change from 1997 to 2013 (Stackpoole et al., 2016). Changing concentrations of dissolved CO2 were identified in nine lakes in the Adirondacks, New York, where six showed significant increases and three showed significant decreases over 18 years (Seekell and Gudasz 2016). The rate of change in both the positive and negative direction was found to be in excess of 12 µatm per year, well outside the rate of increase in the atmosphere. Increasing trends in these lakes were attributed first to basin-scale recovery from acid precipitation, resulting in an increase in soil CO2 production in systems with little buffering capacity, where CO2 can be a large contributor of inorganic carbon exported from the catchment. Also attributed were changes in DOC concentrations, export, and remineralization rates within the lake environment (Burns et al., 2006; Seekell and Gudasz 2016). Globally, evidence indicates increases in the concentrations of organic carbon from a number of sources, a phenomenon termed the “browning” of waters. However, studies suggest that these increases are caused by regionally specific factors, including recovery from acid rain; increases in carbon export from soils; and the mobilization of permafrost carbon into stream systems (Evans et al., 2006; Lapierre et al., 2013; Monteith et al., 2007; Roulet and Moore 2006; Tank et al., 2016). Evidence also suggests that the active layer depth in permafrost soil has increased, mobilizing previously frozen carbon stocks (Neff et al., 2006). In addition, warming and related vegetation changes have increased DOC flux from the Mackenzie River to the Arctic Ocean (Tank et al., 2016). However, permafrost thaw and increased groundwater contribution to Arctic rivers also have been linked to increased mineralization of organic carbon in the subsurface and changes in the proportion of DOC and DIC exports in Alaska’s Yukon River basin (Striegl et al., 2005; Walvoord and Striegl 2007). Any decreases in organic carbon export, though, potentially may be offset by increased organic carbon runoff from vegetation change in low-lying regions (Dornblaser and Striegl 2015). The proportion of carbon mobilized under warming conditions that is mineralized to CO2 versus exported as DOC remains unknown. Furthermore, research indicates that permafrost thaw also has increased CH4 emissions since the 1950s as a result of degrading lake shorelines that contribute aged carbon (Walter Anthony et al., 2016). However, these emissions cannot be quantified at the national or continental scales.
Changes in aquatic carbon fluxes are linked directly to the residence time of water in both terrestrial and aquatic environments (Catalán et al., 2016). In particular, as precipitation increases, reducing water residence time, so do organic carbon fluxes from landscapes (Bianchi et al., 2013; Yoon and Raymond 2012). Knowing the contribution of groundwater versus surface water in streams is also important to understand CO2 fluxes from terrestrial systems (Hotchkiss et al., 2015). The removal of organic carbon in lakes, streams, and rivers is positively related to its residence time (Catalán et al., 2016; Vachon et al., 2016). The half-life of organic carbon in inland waters is about 2.5 years, much shorter than the decades to millennia required for soil systems to completely turn over (Catalán et al., 2016). Some studies hypothesize that increases in precipitation caused by an altered climate will move carbon that would be stored in soils into aquatic environments where remineralization may accelerate the return of organic carbon to the atmosphere as CO2 in high and temperate latitudes (Drake et al., 2015; Raymond et al., 2016). In addition, the installation or removal of dams will directly affect the quantity and form of carbon in aquatic environments by shifting water residence time, water surface areas, and sediment loads. Predicting how the overall carbon balance will shift across North America remains difficult because of complex interactions between inorganic and organic carbon within aquatic systems and the importance of anthropogenic change at the landscape scale (Butman et al., 2015; Lapierre et al., 2013; Regnier et al., 2013; Solomon et al., 2015; Tank et al., 2016).
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