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Wetlands are a major source of DOC to streams, lakes, rivers, and estuaries (Childers et al., 2000; Kristensen et al., 2008; Mulholland & Kuenzler, 1979). DOC export rates depend on DOC concentrations in soil pore spaces, leaching that occurs directly into the water column (e.g., of plant litter), and flows of water through the wetland (Dinsmore et al., 2013; Jager et al., 2009; K. C. Petrone et al., 2007). The DOC concentrations in streams draining peat‐dominated catchments have been increasing (Freeman, Evans et al., 2001) as have DOC concentrations in many rivers and lakes (Evans et al., 2005; Monteith et al., 2007; Skjelkvåle et al., 2005). The DOC exported from tidal wetlands has distinctive optical properties such as high DOC‐specific absorption, low spectral slope, and high fluorescence that reflect its relatively high molecular weight and aromatic‐rich structure compared to estuarine‐derived DOC (Tzortziou et al., 2008), a property that can be used to observe DOC sourced from tidal wetlands using remote sensing (Cao et al., 2018).

Climate change and alterations in atmospheric chemistry have the potential to increase rates of wetland DOC export. Rising air temperatures increase wetland DOC concentrations and cause DOC to become enriched in phenolic compounds (Freeman, Evans, et al., 2001), thereby inhibiting DOC degradation in receiving systems (Freeman et al., 1990). Similarly, there is generally greater DOC export from tropical vs. boreal peatlands (Drösler et al., 2014). In boreal and alpine regions, melting permafrost is leading to higher DOC export from wetlands to aquatic systems (Frey & Smith, 2005), with evidence that this DOC is rapidly consumed by heterotrophic bacteria or degraded through photochemical mechanisms (T. W. Drake et al., 2015; Selvam et al., 2017). Rising atmospheric CO₂ concentrations increase plant productivity in peatlands and enhance DOC exudation from plants, contributing to increased rates of DOC export (Freeman, Fenner, et al., 2004). Similarly, salt marshes respond to elevated CO₂ with higher porewater DOC concentrations, but only in the plant communities that exhibit CO₂‐related increases in growth (C3 but not C4 plants; Keller, Wolf, et al., 2009; Marsh et al., 2005). There can be synergies between elevated CO₂ and warming that further increase DOC export (Fenner et al., 2007). The observed increases in DOC export can also be related to the recovery from acidification due to atmospheric deposition (Monteith et al., 2007), driven by the increased solubility of organic matter at higher pH (Evans et al., 2012; Pschenyckyj et al., 2020).

The export of DOC from peatlands is sensitive to water discharge (Dinsmore et al., 2013; Pastor et al., 2003), which can vary due to changes in precipitation, storage within the wetland, and/or losses to evapotranspiration. Since climate change is altering the frequency and severity of precipitation events (Hartmann et al., 2013), this could affect DOC export by changing the water balance or making export more flashy (Holden, 2005). Following large rain events, there are increased inputs of DOC to aquatic systems (Jager et al., 2009; Paerl et al., 2018) that can cause hypoxia and anoxia in downstream aquatic systems (Paerl et al., 1998). In colder climates, changes in the balance between snow and rain, plus earlier melting of the snowpack, can change the timing of DOC export (Billett et al., 2012).

The DOC exported from wetlands is generally “modern” in age (that is, post‐1950), which is consistent with shallow flow paths of water through surface soils (Billett et al., 2012; Evans et al., 2007; S. Moore et al., 2013; Raymond & Hopkinson, 2003). However, the recent origin of exported bulk DOC can mask inputs of smaller amounts of millennial‐aged DOC, which can be mineralized upon entry to the aquatic system (Dean et al., 2019). In aquatic systems, DOC from wetland and terrestrial systems is subject to microbial mineralization, photochemical oxidation, and flocculation in lakes, streams, rivers, and estuaries (Cole et al., 2007). Much of this processing occurs in freshwater lentic and lotic systems. The relatively short transit time from estuaries to the coastal ocean suggests that DOC exported from estuarine wetlands (e.g., salt marshes) is likely not metabolized within estuaries (Cai, 2011). Although the chemical structure of terrestrial DOC should make it resistant to decay – certainly in comparison to phytoplankton‐derived DOC – very little terrestrial DOC is found in the ocean (Blair & Aller, 2012; Cai, 2011; Hedges & Keil, 1995).