<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.
A more complete accounting of aquatic carbon has been a major advance in aquatic carbon cycle science, specifically the inclusion of CO2 emissions from rivers and lakes to the atmosphere. Additionally, publications of high-resolution inventories of lake and river surface areas have enabled researchers to more accurately scale up local hydrology and chemistry datasets to regional and continental scales. One of the most important results from these new and rigorous assessments is the documentation of regional variability across Arctic, boreal, temperate, subtropical, and tropical ecosystems in North America.
14.3.1 Carbon Fluxes from U.S. Waters
Contemporary total inland water carbon fluxes from CONUS and Alaska were estimated with comparable datasets and methodologies (Butman et al., 2016; Stackpoole et al., 2016). Total aquatic carbon fluxes represent the sum of 1) lateral transport of DIC and total organic carbon (TOC) from river systems to the coast, 2) CO2 emissions from rivers and lakes, and 3) carbon burial in sediments. Although burial in lake sediments also has been considered storage at the continental scale, this report considers burial as the removal of carbon from the aqueous environment and thus adds burial to the total flux (see Equation 14.1).
The estimated total carbon flux from inland waters in CONUS is 147 Tg C per year (5% and 95%: 80.5 and 219 Tg C presented in Butman et. al., 2016). In Alaska, it is 44.5 Tg C per year (31.4 and 52.5 Tg C presented in Stackpoole et al., 2016). These estimates combine for a total flux of about 193 Tg C per year, as presented in Table 14.1. Carbon yields, which represent fluxes normalized by land surface area, are 18.6 g C per m2 per year in CONUS and 29 g C per m2 per year in Alaska. The higher value for Alaska is most likely related to the higher water surface area found across the state. Combined and weighted by area, the average yield for CONUS and Alaska is 20.6 g C per m2 per year.
Rivers dominate total carbon fluxes from inland waters in CONUS and Alaska. Coastal carbon export is 41.5 Tg C per year (5% and 95%: 39.4, 43.5 Tg C) for CONUS and 18.3 Tg C per year (16.3, 25.0 Tg C) for Alaska. River CO2 emissions are 69.3 Tg C per year (36.0, 109.6 Tg C) and 16.6 Tg C per year (9.0, 26.3 Tg C), respectively.
Carbon burial in lakes and reservoirs is 20.6 Tg C per year (9.0, 65.1 Tg C) in CONUS and 1.9 Tg C per year (1.3, 2.8 Tg C) in Alaska, lower than the respective river fluxes to the coast. Lake emissions are 16.0 Tg C per year (14.3, 18.7 Tg C) in CONUS and 8.2 Tg C per year (6.1, 11.2 Tg C) in Alaska. Lake CO2 losses to the atmosphere roughly equal the magnitude of carbon buried in lake sediments in CONUS, but lake CO2 emissions are much greater relative to carbon burial rates in Alaska.
14.3.2 Carbon Fluxes from Canadian Waters
The Canadian climate and terrestrial landscape are highly heterogeneous, from temperate rainforests to Arctic desert. The transport and processing of carbon in Canada’s inland waters are correspondingly variable. Although lake or river carbon cycling has been studied in several regions, significant gaps remain in this report’s assessment of country-wide carbon transport and transformation in aquatic systems. The terrestrial carbon export rate to aquatic networks varies from <1 g C per m2 per year to >20 g C per m2 per year for both organic and inorganic fractions, though their relative importance is region- specific (Clair et al., 2013). A recent estimate for all the drainage basins in Canada suggests that 18.2 Tg of organic carbon is exported to the coast each year (Clair et al., 2013). Although DIC is the dominant form of carbon export from terrestrial systems in the Prairie provinces, Manitoba, Saskatchewan, and Alberta (Finlay et al., 2010), the balance shifts toward co-equality in Southern Quebec catchments (Li et al., 2015) and to a dominance of organic carbon in the boreal zone (Molot and Dillon 1997; Roulet and Moore 2006). The combined organic and inorganic lateral flux from land to the coast is currently unavailable.
While the vast majority of Canadian lakes and rivers are supersaturated in CO2 and CH4 relative to the atmosphere and thus act as sources (Campeau et al., 2014; del Giorgio et al., 1997; Prairie et al., 2002; Teodoru et al., 2009), alkaline and eutrophic systems can act, at least temporarily, as carbon sinks (Finlay et al., 2010). Generally, however, Canadian lakes are net heterotrophic through the degradation of incoming DOC (Vachon et al., 2016), with emission rates of CO2 and CH4 from lakes typically varying as an inverse function of lake size (Rasilo et al., 2015; Roehm et al., 2009) and positively with organic matter inputs (del Giorgio et al., 1999). Lakes of northern Quebec have accumulated more carbon per unit area than their surrounding forest soils but less than surrounding peatlands (Heathcote et al., 2015). Lake bathymetric shape and exposure to oxygen are the primary determinants of carbon accumulation and of the efficiency of burial relative to the carbon supply (Ferland et al., 2014; Teodoru et al., 2012). At the whole-landscape scale, lake sediments account for about 15% of the accumulated carbon (Ferland et al., 2012).
14.3.3 Carbon Fluxes from Mexican Waters
Extensive data on carbon stocks and fluxes do not yet exist for Mexico, but a summary exists of several individual small-scale datasets about Mexican inland water carbon fluxes (Alcocer and Bernal-Brooks 2010). The state of knowledge presented herein regarding carbon cycling in the inland waters of Mexico focuses on lake GHG emissions and burial. Given the tectonic activity of Mexico, there has been an interest in understanding how the carbon emissions of volcanic lakes evolve across space and time. Carbon dioxide emissions from the lake inside El Chichón volcano, Chiapas, reportedly range from 0.005 to 0.016 Tg C per year, or 72,000 to 150,000 g C per m2 per year (Mazot and Taran 2009; Perez et al., 2011). More recently, research on Lake Alchichica showed that, on average, surface water pCO2 was below atmospheric pCO2 for 67% of the year, with an average surface water pCO2 of 184 microatmospheres (µatm; Guzmán-Arias et al., 2015). These findings suggest that deep, tropical, and warm monomictic lakes have the potential to take up atmospheric CO2 through primary production and preserve most of the POC deposited to the sediments, creating important carbon sinks. Emissions of CH4 may be as important as emissions of CO2 across regions of Mexico. Although few studies have evaluated the CH4 emissions from Mexican inland waters, the CH4 flux from six Mexican lakes is estimated to be about 1.3 ± 0.4 Tg CH4 per year, which constitutes 20% of Mexico’s CH4 emissions (Gonzalez-Valencia et al., 2013). The total CH4 flux from 11 aquatic ecosystems in Mexico City was 0.004 Tg CH4 per year, 3.5% of the CH4 emissions of the city (Martinez-Cruz et al., 2016). Fully quantifying the importance of anthropogenic inputs of CH4-producing organic materials through waste streams is critical for better constraining these fluxes at the national scale.
Other research on inland water carbon dynamics in Mexico has focused on reservoirs. The CO2 emissions of the Valle de Bravo reservoir, Estado de Mexico, calculated through the photosynthesis and respiration balance, was 0.34 g C per m2 per year (Valdespino-Castillo et al., 2014). Carbon burial has been studied in a few Mexican lakes. A 3-year study determined that the well-characterized system of Lake Alchichica, Puebla, has a carbon burial rate of 25.6 ± 12.3 g C per m2 per year (Oseguera-Pérez et al., 2013).
14.3.4 Carbon Fluxes from the Great Lakes
As previously suggested, a comprehensive assessment of carbon fluxes does not yet exist for all of the Laurentian Great Lakes. The best estimates for individual component carbon flux values for the Great Lakes come from Lake Superior. Primary production is estimated to be 5.3 to 9.7 Tg C per year, while respiration is estimated to be significantly greater at 13 to 83 Tg C per year (Cotner et al., 2004; Sterner 2010; Urban et al., 2005). External inputs of 0.68 to 1.03 Tg C per year (Cotner et al., 2004) of organic carbon are too small to account for this imbalance between primary production and respiration, suggesting significant sources of external DIC. However, modeling work suggests that previous respiration estimates were biased high because of spatial heterogeneity and found a much lower value of 5.5 Tg C per year (Bennington et al., 2012). Estimates do not yet exist for the balance between the amount of organic carbon buried in sediments versus the amount exported through rivers or emitted as CO2 and CH4. However, total carbon burial across all lakes may be on the order of 2.7 Tg C per year, with an areal sink of 15 g C per m2 per year since 1930 (Einsele et al., 2001). Additional research is needed to constrain the fluxes of carbon from the Great Lakes.
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