Carbon export from mangrove forests to the oceans partly acts as a sink for atmospheric CO₂, exceeding the rate of carbon burial in mangrove soils. Primary production in ecosystems adjacent to mangroves may prevent degassing and enhance further carbon export from mangroves to the oceans. In this study, we continuously monitored carbonate chemistry parameters (pCO₂, dissolved inorganic carbon (DIC), total alkalinity (TA)) and dissolved organic carbon (DOC) in a tidal flat adjacent to a fringe mangrove forest over a spring-neap tidal cycle. Mean pCO₂ during the entire period was 923 ± 318 μatm, and the export of TA, DIC, and DOC from the mangroves to the ocean was 36 ± 26 mmol m⁻² d⁻¹, 42 ± 39 mmol m⁻² d⁻¹, and 10 ± 9 mmol m⁻² d⁻¹, respectively. Semi-monthly pCO₂ variations in the mangrove front were controlled by the tidal level during spring tide and by photosynthesis and respiration on the tidal flat during neap tide. This means that during neap tide, photosynthesis on the tidal flat offset the increase in pCO₂ caused by the porewater export from the mangrove soil. The DIC/TA export ratio in this study was 1.17 ± 0.08, which was lower than the global average of 1.41 ± 1.39, indicating that the tidal flat adjacent to the mangrove forest may act as a buffer zone to mitigate the increase in pCO₂, resulting in much of the exported DIC being stored in the ocean.

Hypoxia and blue tide are the most significant environmental issues in Tokyo Bay as they have been damaging fisheries for a long time. Although studies on modeling these two associated phenomena have been conducted for decades, the scarcity of relevant observational datasets has greatly hindered the progress, and no study has successfully reproduced the entire processes of blue tide or predicted the time and place of its outbreak. To address the problems from limited data, this study proposed a relatively conventional benthic flux model and developed a novel method that converts the total organic carbon content into the fluxes of sediment oxygen consumption and sulfide release to represent the spatial differences in benthic fluxes. A pelagic sulfur model with only three key chemical reactions of blue tide that includes the disproportionation of elemental sulfur was proposed. The method significantly improved the results of dissolved oxygen in bottom water, as seen by the root mean square error decreasing by 15.9% and 18.9% in two simulations with largely different forcings. The sulfur model also accurately predicted the outbreaks of blue tides in each simulation, which is significant to the stakeholders as it facilitates the forecast of blue tides in Tokyo Bay.