Controls on Terrestrial Carbon Fluxes in Simulated Networks of Connected Streams and Lakes

Abstract

Globally, significant amounts of carbon are transferred from terrestrial systems to inland waters (Drake et al., 2018), yet only a small portion of this transiting material reaches the ocean (Battin et al., 2009;Cole et al., 2007). Most retained terrestrial carbon evades the atmosphere as carbon dioxide (CO 2 ) from the surface of streams and lakes, representing a significant proportion of terrestrial net atmospheric carbon uptake (Butman et al., 2016;Casas-Ruiz et al., 2021;Wallin et al., 2013). The magnitude of these inland water C fluxes from regional to global scales is generally assessed from individual studies of isolated streams and lakes (e.g., Butman & Raymond, 2011;Raymond et al., 2013). While such approaches yield reasonable estimates that highlight the quantitative importance of inland waters in the carbon cycle, they are inadequate for understanding and projecting carbon fluxes at scales that integrate networks of hydrologically connected streams, rivers and lakes.The fate of terrestrial carbon in inland waters, whether it is emitted to the atmosphere or exported downstream, is mainly driven by the recipient system's efficiency in mineralizing terrestrial dissolved organic carbon (DOC) and evading CO 2 . The capacity of inland waters to evade terrestrial CO 2 is a function of turbulence, which drives gas exchange rates between surface waters and the atmosphere. While gas exchange with the atmosphere in streams and lakes is usually fast enough relative to water residence time to allow a majority of excess CO 2 to evade within system boundaries (Öquist et al., 2009), the transport of dissolved CO 2 can also occur between streams and lakes (Vachon et al., 2021). For terrestrial DOC, however, mineralization in aquatic ecosystems depends on its inherent degradability (hereafter reactivity) and local environmental conditions such as temperature and water residence time (Vachon et al., 2021). For example, if water residence time is sufficiently short, mineralization of terrestrial DOC in a given system may be limited, and a large fraction will be transferred downstream (Raymond et al., 2016;Wollheim et al., 2018). As a result, small streams with relatively short water residence time preferentially export terrestrial organic carbon (Hotchkiss et al., 2015;Winterdahl et al., 2016) (Figure 1a). In contrast, systems with longer water residence time, such as larger rivers and lakes, generally mineralize a larger fraction of terrestrial organic carbon (McCallister & del Giorgio, 2008;Vachon et al., 2017) (Figure 1b). Hydrological connectivity between streams and lakes complicates this simple framework for inland water carbon processing and emissions. Because streams and lakes play different biogeochemical roles in networks (Vachon et al., 2021), such connections may constrain the carbon processes and fluxes that can operate locally. For example, the forms, reactivity, and timing of carbon supplied to streams can vary greatly depending on the presence