Summer carbon budget simulation in a lagoon ecosystem with submerged aquatic vegetation

As international efforts against global warming advance, blue carbon has gained attention as a CO₂ sink. This study quantitatively evaluated the carbon budget of an eelgrass (Zostera marina) ecosystem in Komuke Lagoon, a restricted coastal lagoon on the northeastern coast of Hokkaido, Japan. Based on outdoor experiments conducted in 2018 and 2023, a new DIC equation was constructed that incorporates photosynthetic responses to photon flux density and water temperature. Furthermore, a box model integrating river inflow, seawater exchange with the open ocean, air-water CO₂ flux, bottom dilution, and absorption by eelgrass was developed to reproduce the seasonal variations of DIC in the lagoon. The results demonstrated that excluding eelgrass led to a summer-validated overestimation, projected on an annual basis, of DIC by a maximum of approximately 300 μmol kg^-1, indicating that eelgrass significantly affects DIC dynamics. Additionally, ignoring variations in Total Alkalinity (TA) in the simulation resulted in an underestimation of pCO₂ by up to 150 μatm, potentially leading to a several-fold overestimation of atmospheric CO₂ uptake. Analysis of the summer-validated carbon budget, integrated over the simulated calendar year 2018, indicated that CO₂ supply from the open ocean via tides was the most dominant factor, contributing approximately 66%. While eelgrass absorbed about half of the CO₂ supplied to the lagoon, atmospheric CO₂ uptake accounted for only about 5.4% of the total. This study demonstrated that for carbon budget evaluation in highly enclosed shallow coastal areas, it is important to utilize modeling that considers flow and water exchange processes associated with river inflow and tides, as well as TA variations. Simulations for 2013 and 2019 were additionally performed as reproducibility and inter-annual consistency checks against available summer observations; their full-year DIC trajectories are reported only as model projections, and the detailed carbon-budget partitioning reported here refers exclusively to 2018.