Contributions of rivers and lakes to the carbon balance of the United States during 1900-2019

Abstract:

Accurate accounting of carbon fluxes is essential for understanding the roles that different components of land surface system play in the regional carbon cycle. In the conterminous United States (CONUS), carbon cycling in terrestrial ecosystems has been intensively investigated, and carbon fluxes in inland water ecosystems alone were estimated using empirical models. However, no study has comprehensively quantified the regional carbon budget across both terrestrial and aquatic ecosystems. We developed a coupled land-aquatic system model to estimate the contributions of riverine C exports and inland water CO₂ degassing to the US carbon balance. Our results show that CO₂ emissions from rivers and lakes were comparable, and they both significantly increased since the 1960s driven by changes in climate and land use. The total aquatic CO₂ emissions were about three times larger than the sum of C exports which consist of dissolved organic carbon (DOC), particulate organic carbon (POC) and dissolved inorganic carbon (DIC). All of the three components of C exports significantly increased during 1900-2018, however, their primary drivers were different. Although aquatic CO₂ emissions and C exports to the coast were relatively small (about 5%) compared to terrestrial net primary production (NPP), they had a large influence on regional net biome production (NBP). Our study highlights the importance of placing aquatic C fluxes into the overall regional C budget.