Numerical analysis of the impacts of the vegetation drag force on net primary production and carbon flow in shallow-waters using a hydrodynamic and ecological intercoupling model

Seagrass meadows can affect the transport and accumulation of organic carbon (OC) in shallow-waters through the vegetation drag force (VDF), and thereby affect the carbon cycle. Many studies of seagrass VDF have focused only on the physical effects of VDF on hydrodynamics; hence, there is no understanding of the effects of VDF-induced changes of the current field on the carbon cycle. We therefore conducted field observations and developed a new carbon cycle model able to consider ecological-physical interactions by coupling a hydrodynamic model with a lower trophic level ecosystem model to evaluate the impact of VDF on the carbon cycle, including net primary production (NPP) and carbon transport. Application of the model to a lagoon where seagrass meadows occupied the majority of the area showed that VDF could reduce NPP by 19-43 % across the vegetated area by restricting the inflow of nutrient-rich, high-salinity water and thereby limiting primary production inside the vegetated area. However, VDF could increase NPP at the edge of the vegetated area by a factor of 2.0-4.4 relative to the interior of the vegetated area. The reduction of NPP throughout the vegetated area by VDF reduced the deposition of particulate OC by approximately 50 % and increased the offshore transport of carbon from the lagoon by 10-39 %. These results indicated that the enhanced current velocity and mass transport through the deep channel, which was unvegetated did not enhance the accumulation of OC within the vegetated area but rather allowed OC produced within the edge of the vegetated area to be transported efficiently to the offshore. The results of this study are expected to contribute to the quantification of blue carbon sequestration in coastal areas and to technology for selecting suitable sites for transplantation of seagrass by taking into account the impact of VDF on the carbon cycle.