Estimation of the Physical Environments Influenced by Seagrasses in a Shallow Coastal Water Using a Numerical Model

The physical and bio-geochemical environments are altered by submerged marine macrophytes. Although the conventional studies have attempted to quantitatively understand the interaction of the environments with seagrasses, most of those focused on narrow meadow scales using flume experiments. In this study, we estimated the seagrasses drag effects to current fields in a shallow coastal water using a 3D-physical model.

 The model site is the Furen Lagoon, Japan, having a surface area of 57.4km². The lagoon has 4 river inflows and the greater part of the area except the non-vegetation area (deeper than ca. 3.0 m) is covered with seagrass meadows. The model used employs hydrostatic equilibrium and the horizontal resolution set at 100 m. The water column up to ca. 10 m is divided into 6 layers with a thickness of 1.0 to 1.25 m. To examine the canopy drag effect in the physical environment, we adopted the Manning’s roughness coefficient and carried out two numerical experiments with (Case SG) and without (Case NoSG) seagrasses meadow.

 In the northern area which has three river inflows, the model estimated lower salinity for “Case SG”, indicating that the high seagrass density in the northern area facilitate retaining of the fresh water from the rivers and interrupts the inflow of the saline water. The model also showed that the current field is attenuated in the same area by the canopy drag effect. In the non-vegetation area, however, “Case SG” presented higher salinity and current speed than the “Case NoSG”, indicating that the saline water inflows to the vegetation area (shallower than ca 3.0 m) is hindered by the canopy drag effect and that the water is mainly exchanged through unvegetated deeper channel. Finally, “Case SG” showed the better agreements with the observational data for the numerical physical environment.