Integrating Discharge‐Concentration Dynamics Across Carbon Forms in a Boreal Landscape

Abstract

The flux of terrestrial carbon across land‐water boundaries influences the overall carbon balance of landscapes and the ecology and biogeochemistry of aquatic ecosystems. The local consequences and broader fate of carbon delivered to streams is determined by the overall composition of carbon inputs, including the balance of organic and inorganic forms. Yet, our understanding of how hydrologic fluxes across different land‐water interfaces regulate carbon supply remains poor. We used 7 years of data from three boreal catchments to test how different land‐water interfaces (i.e., forest, wetland, and lake) modulate concentration‐discharge (C‐Q) relationships for dissolved organic carbon (DOC), carbon dioxide (CO2), and methane, as well as the balance among forms (e.g., DOC:CO2). Seasonal patterns in concentrations and C‐Q relationships for individual carbon forms differed across catchments. DOC varied between chemostasis and transport limitation in the forest catchment, between supply limitation and chemostasis in the wetland catchment, and was persistently chemostatic in the lake outlet stream. Carbon gases were supply limited overall, but exhibited chemostasis or transport limitation in the forest and wetland catchments linked to elevated flow in summer and autumn. Unique C‐Q relationships for individual forms reflected the properties of different interfaces and underpinned changes in the composition of lateral carbon supply. Accordingly, DOC dominated the carbon flux during snowmelt, whereas gas evasion increased in relative importance during other times of the year. Integrating the C‐Q dynamics of individual carbon forms provides insight into the shifting composition of lateral export, and thus helps to predict how hydrologic changes may alter the fate of carbon supplied to streams.

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

“…For example, in small boreal catchments, DOC is the dominant carbon form of export (Campeau et al., 2018). The ratio between DOC and CO 2 concentration in streams can also change dramatically with discharge, especially from forested catchments, where DOC can become more dominant at high flows (Gómez‐Gener et al., 2021). Estimating terrestrial DOC and CO 2 inputs to inland waters at regional and global scales is a difficult task (Tank et al., 2018), although it remains critical for understanding inland water CO 2 emissions from terrestrial carbon inputs.…”

Watershed Classification Predicts Streamflow Regime and Organic Carbon Dynamics in the Northeast Pacific Coastal Temperate Rainforest

“…High frequency measurements in key watershed types could better resolve rapid DOC dynamics during early freshet (Voss et al., 2015) as well as summer and fall rain events (Fellman et al., 2009). Direct assessment of concentration‐discharge relationships across the diverse watershed types of this region could help tease apart the relative contributions of flow, temperature, and meltwater timing to DOC dynamics more generally (e.g., Gómez‐Gener et al., 2021; Winterdahl et al., 2014; Zarnetske et al., 2018). Furthermore, data from this region could be used to determine if DOC export is source limited across the extensive glacierized watershed types (e.g., Hood et al., 2020) in contrast to the more general pattern of transport limited DOC export from non‐glacierized watershed types (Hood et al., 2020; Zarnetske et al., 2018).…”

Long‐term changes in dissolved inorganic carbon across boreal streams caused by altered hydrology

“…This diluting effect contrasts with the hydrological influences on DOC, which across the full study period was generally weak and positive (Table 2). However, recent detailed studies on the DOC–discharge relationship within the KCS streams indicate that DOC has transitioned from being transport limited 10–20 yr ago to being largely chemostatic in later years as a result of changes in riparian pore‐water conditions (Fork et al 2020; Gómez‐Gener et al 2021). In contrast, no systematic changes in the DIC–discharge relation were identified across these streams during the current study period (Fig.…”

“…In addition, a large share of the total soil CO 2 derives from root and root-associated respiration further down in the soil profile (Högberg et al 2001). Due to the differences in soil concentration profiles between DIC and DOC, variability in the groundwater table position and the subsequent lateral export of water through different source layers creates opposing response patterns between soil exports of DOC and DIC with changes in runoff (G omez-Gener et al 2021). This view supports previous findings that riparian soil exports of these two C species are mechanistically uncoupled, with differences in dominating terrestrial source areas at the catchment scale (Winterdahl et al 2016;Campeau et al 2019).…”

“…However, although DOC and CO 2 are spatially related, evaluations over temporal scales suggest an uncoupling between these C components (Nydahl et al 2017). Furthermore, detailed catchment studies suggest that DOC and DIC are sourced differently in the landscape, such that their export responds differently to changes in hydrology (Winterdahl et al 2016;Campeau et al 2019) and shifts in land cover (G omez-Gener et al 2021). Whether the difference in hydrological sourcing between DOC and DIC also affects the long-term evolution of the different C forms in response to environmental changes remains unknown.…”