Impact of anthropogenic disturbances on carbon cycle changes in terrestrial‐aquatic‐estuarine continuum by using an advanced process‐based model

Abstract

Recent research has shown that inland water—including rivers, lakes, and groundwater—may play some role in carbon cycling, although the extent of its contribution has remained uncertain due to the limited amount of reliable data available. 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 used an advanced process‐based model derived by coupling a process‐based national integrated catchment‐based eco‐hydrology (NICE) model with various biogeochemical cycle models (NICE‐BGC). Generally, total nitrogen and phosphorus transports in overland flow were found to increase. In contrast, total suspended sediment in overland flow 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 across the water‐air interface decreased and particulate organic carbon (POC) increased in most rivers through variations in carbon budget, where excess nutrients might stimulate gross primary production of carbon‐rich algal biomass. 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 MARgins and CATchments Segmentation (MARCATS) segment numbers showed that the estimated total CO2 flux from the world's estuaries was 0.14 Pg C/year. Generally, the simulation 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.

Evaluation of global biogeochemical cycle in lotic and lentic waters by developing an advanced eco‐hydrologic and biogeochemical coupling model

“…The simulated concentrations and fluxes in NICE‐BGC showed higher uncertainties and biases in the estimation of seasonal, intra‐annual and inter‐annual variations of carbon cycle in the author's previous studies (Nakayama, 2017b, 2020, 2022). Though the result in the present study evaluated only the annual‐averaged fluxes, the values varied greatly depending on some rivers such as Nile, Yangtze and Yenisei rivers (Figures 4 and 5).…”

“…This means that the newly developed model incorporates the connectivity of the biogeochemical cycle accompanied by the hydrologic cycle between surface water and groundwater, hillslopes and river networks, and other intermediate regions (Nakayama, 2016). Recently, the author modified NICE‐BGC to include the effect of reservoirs (Nakayama & Pelletier, 2018), two parameters (soil organic carbon and carbon emission from the intermediate soil pool) to improve the accuracy of long‐term simulation (Nakayama, 2020) and the extension to the global estuaries (Nakayama, 2022) (Figure 2). While the coupled models such as NICE‐BGC might sometimes occur the propagation of errors, it is impossible to treat these errors in a formal way at this moment and is fair to discuss some of the potential errors and determination of the uncertainty range (the standard deviation), as described in this paper.…”

“…Flow diagram of the new development of NICE‐BGC coupled with LAKE2K in a stratified water quality model (Chapra & Martin, 2012). Dotted frames indicate the newly developed processes in NICE‐BGC coupled with LAKE2K to simulate the effect of global reservoirs and lakes on hydrologic and biogeochemical cycle changes in the extension of the author's previous paper (Nakayama, 2022; Nakayama & Pelletier, 2018). …”

“…Other studies have also indicated that carbon budgets are diverse in various basins/catchments and that dissolved organic carbon (DOC), dissolved inorganic carbon (DIC) and particulate organic carbon (POC) are also important for the evaluation of CO 2 flux and sediment storage in the global carbon cycle (Aufdenkampe et al, 2011;Cole et al, 2007;Raymond et al, 2013;Tranvik et al, 2009). In order to bridge the gap between vertical flux and horizontal carbon transport, the author has developed an advanced model coupling eco-hydrology and the biogeochemical cycle [National Integrated Catchment-based Eco-hydrology (NICE)-BGC] (Nakayama, 2017a(Nakayama, , 2020(Nakayama, , 2022, which incorporates complex coupling of the hydrologic-carbon cycle in terrestrial-aquatic-estuarine linkages. Although the model has been shown to simulate both horizontal transport to the ocean and vertical fluxes, including aquatic metabolism and terrestrially derived carbon together in major rivers, it did not include explicitly biogeochemical processes in lentic water.…”

Increased Terrestrial Carbon Export and CO2 Evasion From Global Inland Waters Since the Preindustrial Era

“…Yet, such processes largely determine how C cycling in aquatic ecosystems is affected by natural and anthropogenic disturbances. The comparison of the current C models (M. Li et al., 2019; Marescaux et al., 2020; Mayorga et al., 2010; Nakayama, 2022; Nakhavali et al., 2020; Saccardi & Winnick, 2021; H. Zhang et al., 2022) reveals their limited ability to differentiate small stream processes in simulating riverine C dynamics. The representation of C transport across the river‐lake‐reservoir continuum remains incomplete in these models.…”