<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.
Advances in the ability to manipulate large databases of carbon chemistry covering the United States, coupled with new methods for spatial analysis, have enabled new and robust estimates for carbon fluxes from inland waters in CONUS and Alaska. By identifying and including CO2 emissions, the U.S. fluxes of carbon are estimated to be approximately 193 Tg C per year. These fluxes are dominated by river and stream networks exporting up to 59.8 Tg C per year to the coast and emitting nearly 85.9 Tg C per year as CO2 to the atmosphere. Availability of data is limited from Mexican inland waters. Deep, tropical, warm monomictic lakes constitute carbon sinks primarily as POC, while shallow, tropical—and mostly eutrophic—lakes are sources of CO2 and CH4 to the atmosphere. Further data collection is needed to properly assess carbon cycling within inland waters at the national scale in both Canada and Mexico. However, based on estimates presented here, the carbon flux from inland waters is now understood to be four times larger than estimates presented in SOCCR1.
14.7.2 Key Knowledge Gaps and Current Opportunities
Peer-reviewed and detailed estimates are not currently available for carbon fluxes from inland waters within Mexico and Canada. Further collaboration is necessary among monitoring efforts in these countries and the United States to properly develop a spatially explicit inland water database on carbon concentration and carbon fluxes across North America. In addition, robust estimates of annual carbon fluxes for the Laurentian Great Lakes are not yet possible, a surprising limitation given their importance as the largest inland waters on Earth. Preliminary data suggest that these systems vary from a net carbon source to the atmosphere in Lake Superior, Lake Michigan, and Lake Huron to a net carbon sink in Lake Erie and Lake Ontario. By combining a box model analysis with a literature review of respiration, river inputs, and burial, McKinley et al. (2011) conclude that the Great Lakes efflux lies between 2.3 and 36 Tg C per year. If future research suggests emissions near 2.3 Tg C per year, then the emission of carbon as CO2 may be nearly balanced by carbon burial (Einsele et al., 2001). However, if new data suggest significantly higher emissions, such results would increase the importance of the Great Lakes with respect to total carbon fluxes from the United States and Canada. The Great Lakes are heavily affected by anthropogenic disturbance through nutrient enrichment and invasive species, with unknown impacts on carbon cycling.
Also unavailable is a comprehensive estimate for the contribution of CH4 to carbon emissions for inland waters of North America. Data on CH4 do not yet exist across space and time to properly scale to national and continental levels, though significant progress is being made (Holgerson and Raymond 2016; Stanley et al., 2016; Wik et al., 2016).
One major methodological advancement in past years is in situ probe systems (Baehr and DeGrandpre, 2004). Probes to measure aspects of the carbon cycle are becoming more accurate and affordable (Bastviken et al., 2015; Johnson et al., 2010), and the research community is advancing methodologies to process high-temporal datasets (Downing et al., 2012), identifying the role that storm events may play in carbon fluxes. The possibility now exists to instrument inland water systems along the aquatic continuum from when water emerges from the terrestrial interface to when it is exported to the coast or large inland lakes. Such instrumentation will facilitate understanding of the transformations of terrestrial carbon during transport to inland waters and the controls on this transport. However, deploying sensor systems alone is not enough to ensure the development of the data needed to reduce uncertainties. The inland water carbon cycle science community must learn from the efforts of organizations like the International Ocean Carbon Coordination Project to develop standard approaches and reference materials for study comparison and reproducibility. Furthermore, future research needs to take advantage of developments in both large- and small-scale data acquisition and should attempt nested watershed studies across scales to understand the carbon cycling within inland water environments. These studies, coupled with new methods to quantify surface waters at the global scale, particularly small streams and ponds, will help further constrain the importance of inland waters to the Earth biogeochemical system under a changing climate (Pekel et al., 2016).
At 193 Tg C per year, the fluxes of carbon through inland waters of the United States are significant. The scaled value of 507 Tg C per year for North America represents an estimate that requires further science to reduce uncertainties. In the context of the overall cycling of carbon among terrestrial, wetland, and aquatic environments, there are important methodological differences that must be considered when using the estimates of carbon flux from inland waters. The aquatic carbon fluxes presented herein are derived from the modeling of fluxes to the coast, lake sediments, and the atmosphere. The quantification of the lateral flux of carbon to estuarine systems is perhaps the most well constrained, as it is derived from long-term monitoring of water flow and decades of direct measurements of carbon concentration. The emission of CO2 from water surfaces is more uncertain. The difficulty of quantifying this emission is compounded by the ephemeral nature of small streams, along with a lack of detailed spatial information on their total length and surface area. As suggested in this chapter, small streams and ponds represent a large fraction of the CO2 emissions from inland waters to the atmosphere, important when scaling fluxes across the United States and the world. Furthermore, apportioning the carbon in an aquatic environment to its source (e.g., autochthonous versus allochthonous) currently is not possible. This gap in understanding removes an ability to differentiate, for example, soil respiration that simply has changed location into an aquatic ecosystem from in-stream respiration.
The importance of erosional fluxes of carbon to North American inland waters also cannot be properly assessed. The lateral transport of soil carbon and the concurrent fluxes of CO2 returning to the atmosphere in China suggest that upwards of 45 Tg C per year enter inland waters, thus representing a terrestrial carbon sink (Yue et al., 2016). However, this type of calculation does not fully account for replacement of carbon within soils, the remineralization of organic carbon during transport, direct inputs of inorganic carbon, or the lateral fluxes of dissolved carbon to the coast. Therefore, caution is warranted when including inland waters in a mass balance for total carbon accounting. To fully understand the role that inland waters play across the land-water continuum, studies must be conducted at the watershed scale, coupling terrestrial and inland water processes. These measurements will help constrain future modeling studies that require coupling between hydrology and biogeochemistry.
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