Estimation of carbon cycle changes in river-estuarine continuum by using advanced earth system model

Inland waters including rivers, lakes, and groundwater are suggested to act as a transport pathway for water and dissolved substances, and play some role in continental biogeochemical cycling (Cole et al., 2007; Battin et al., 2009). Quantifying the physical and chemical connections between land and associated fresh and coastal waters is critical for understanding the dynamics of carbon cycle in aquatic ecosystems. Recently, process-based National Integrated Catchment-based Eco-hydrology (NICE) model (Nakayama and Watanabe, 2004) was developed to couple with various biogeochemical cycle models in biosphere, aquatic ecosystems, and carbon weathering, etc. in global major river basins (NICE-BGC) (Nakayama, 2017; Nakayama and Pelletier, 2018). NICE-BGC also included the feedback between soil organic content and overland carbon fluxes, and succeeded to simulate inter-annual variations of carbon cycle in a terrestrial-aquatic continuum greatly affected by the extreme weather patterns (Nakayama, 2020). To evaluate global changes in the carbon cycle due to anthropogenic factors, such as application of fertilizer and manure, in major rivers including 130 tidal estuaries over an 18-year period (1998-2015), the present study expanded NICE-BGC to estuary in land and ocean margins where it is generally considered to be net heterotrophic ecosystems and show significant supersaturation of CO₂ (Frankignoulle et al., 1998; Regnier et al., 2013). The new model used Dirichlet boundary condition at the downstream of global major rivers by using some variables (water temperature, salinity, dissolved oxygen, nutrient, alkalinity, and pH, etc.) in coastal ocean. The simulated result showed that total nitrogen and phosphorus fluxes in overland flow were found to increase with nutrient application. In contrast, total suspended sediment decreased in some regions because the vegetation was able to expand to cover the ground, resulting in less erosion. NICE-BGC simulated the difference in carbon budget in major rivers with and without nutrient application. Generally, CO₂ degassing above water decreased and particulate organic carbon (POC) increased in most rivers through variations in carbon budget, reflecting various hydrologic and biogeochemical conditions. The simulated result also showed that the estuarine carbon cycle was sensitive to intense anthropogenic disturbances reflected by nutrient load, seawater temperature, increases in sea level, and ocean acidification. Extension of previous studies only by categorizing MARCATS segment numbers showed that the estimated total CO₂ flux from the world’s estuaries was 0.14 Pg C/yr. The simulation generally showed that incorporation of the nutrient cycle into the terrestrial-aquatic-estuarine continuum improved estimates of net land flux and carbon budget in inland waters, thus emphasizing that the effect of estuarine inland water should be explicitly included in the global carbon model to minimize the range of uncertainty.