Mariculture-based carbon sequestration is an important pathway for coastal marine carbon dioxide removal (mCDR), because the continuous growth, harvest, and potential removal or storage of macroalgae and shellfish biomass can contribute to the transfer of carbon from the active coastal water column. However, the effectiveness and durability of this pathway are strongly constrained by local water-environmental conditions. Seasonal stratification and bottom-water hypoxia frequently occur in coastal aquaculture areas, which may suppress organism growth, increase mortality risk, accelerate organic matter decomposition, and destabilize biomass-based carbon sequestration. Therefore, hypoxia risk control is not only an ecological restoration issue, but also a key supporting process for maintaining the carbon-fixation continuity and operational reliability of mariculture-based mCDR.
This study proposes an engineering assessment framework for guide-plate artificial downwelling arrays as a hydrodynamic support module for coastal mCDR. Guide-plate artificial downwelling uses tidal-current energy to transport oxygen-rich surface water into bottom hypoxic layers through inclined underwater guide plates, thereby enhancing bottom oxygen supply and vertical exchange without relying primarily on external energy input. First, a hydrodynamic assessment model is developed based on two core variables: maximum downwelling height, Hm, and effective downwelling flow rate, Qd. The model is used to characterize the ability of a guide-plate device to deliver oxygen-rich water to the target hypoxic layer under different tidal current velocities, stratification intensities, guide-plate geometries, effective inflow areas, and deployment angles. Second, considering reversing flood–ebb tidal currents, wake-interference constraints are introduced to compare the stable downward transport capacity and robustness of four array layouts: random, regular, unidirectional-oriented, and bidirectional robust layouts. Finally, the hydrodynamic delivery capacity is linked to the environmental demand of mariculture-based mCDR by constructing a hypoxia-mitigation supply–demand ratio, which compares the downwelling flow required by the target hypoxic layer within a preset time scale with the effective downwelling flow provided by the guide-plate array. This ratio is used to evaluate whether different layouts can meet the hypoxia risk-control demand required to support biomass maintenance and carbon-sequestration stability. The proposed framework clarifies the role of guide-plate artificial downwelling as a water-environmental support strategy for coastal mCDR, providing decision support for hydrodynamic performance assessment, array layout evaluation, risk assessment, and subsequent observation-scheme development.