11:00 AM - 11:15 AM
[ACG53-08] The function of seagrass beds in estuaries to trap terrestrial materials and their potential for utilization
Keywords:Red soil and cray, Sediment trap , Seagrass, Metal elements
It is well known that the runoff of terrestrial soil (red soil) from land development in tropical and subtropical island regions has a negative impact on coral communities. The three-dimensional complex structure of seagrass beds reduces the amount of physical disturbance, and traps fine particles, including red soil, and deposits them on the seabed. This study investigated the dynamics of red soil deposition in seagrass beds in the Ngiwal estuary and adjacent coral reef in the Republic of Palau, where red soil runoff is a problem, during rough sea conditions (December 2023) and calm sea conditions (September 2024).
To confirm the situation of red soil sedimentation, surface sediment samples were collected from the seabed at 13 locations. In addition, several sediment traps were placed to investigate the amount of sedimentation flux in the seagrass beds and adjacent bare ground. In addition, plaster balls were also installed nearby to measure the amount of physical disturbance. The contents trapped in the sediment traps and the surface sediment were measured for particle size using sieve and laser diffraction and for multiple elements using ICP-OES and elemental analyzer. As a result, Al, Si, Ti, V, Mn, and Fe showed high concentrations with increasing silt fractions near the estuary. Conversely, Ca concentrations increased with increasing distance from the estuary, as did sand fraction particles. The particles trapped in the sediment trap can be broadly grouped into three categories: (1) particles transported from the river, (2) particles transported after resuspension at other locations, and (3) particles resuspended locally. In this study, these categories were applied to examine whether autochthonous particles were predominant, referring to particle size and elemental composition changing with the distance from the estuary.
There was no difference in the amount of red soil (Al, Si, Ti, Mn, V, Fe) downward flux between the seagrass beds and bare ground during calm sea conditions, while those fluxes were higher in the seagrass beds than those in bare ground during rough sea conditions. Therefore, it was found that seagrass beds play an important role in preventing the spread of red soil in rough sea conditions. The fact that the median grain size of the particles trapped by the sediment trap was larger in the seagrass bed than in the bare ground was consistent with previous studies, suggesting that the seagrass beds were actively trapping suspended particles. Therefore, in order to conserve coral communities, it is also important to conserve the seagrass beds distributed in the coral reefs.
In addition, the impact of red soil deposition on the elemental composition of seagrass was also investigated. Seagrass (Enhalus acoroides) leaves and roots were also collected at the same locations where the sinking particles and surface sediments were collected, and the concentrations of multiple elements were compared between the sediment and seagrass. The results showed that the concentrations of Mn and Fe, which are useful for plants growth, were higher at sites with higher red soil loadings than at sites with lower red soil loadings. This suggests that in coral reef sediments dominated by carbonate minerals with low trace metal content, the function of seagrass to trap red clay may have beneficial effected on their own growth.
To confirm the situation of red soil sedimentation, surface sediment samples were collected from the seabed at 13 locations. In addition, several sediment traps were placed to investigate the amount of sedimentation flux in the seagrass beds and adjacent bare ground. In addition, plaster balls were also installed nearby to measure the amount of physical disturbance. The contents trapped in the sediment traps and the surface sediment were measured for particle size using sieve and laser diffraction and for multiple elements using ICP-OES and elemental analyzer. As a result, Al, Si, Ti, V, Mn, and Fe showed high concentrations with increasing silt fractions near the estuary. Conversely, Ca concentrations increased with increasing distance from the estuary, as did sand fraction particles. The particles trapped in the sediment trap can be broadly grouped into three categories: (1) particles transported from the river, (2) particles transported after resuspension at other locations, and (3) particles resuspended locally. In this study, these categories were applied to examine whether autochthonous particles were predominant, referring to particle size and elemental composition changing with the distance from the estuary.
There was no difference in the amount of red soil (Al, Si, Ti, Mn, V, Fe) downward flux between the seagrass beds and bare ground during calm sea conditions, while those fluxes were higher in the seagrass beds than those in bare ground during rough sea conditions. Therefore, it was found that seagrass beds play an important role in preventing the spread of red soil in rough sea conditions. The fact that the median grain size of the particles trapped by the sediment trap was larger in the seagrass bed than in the bare ground was consistent with previous studies, suggesting that the seagrass beds were actively trapping suspended particles. Therefore, in order to conserve coral communities, it is also important to conserve the seagrass beds distributed in the coral reefs.
In addition, the impact of red soil deposition on the elemental composition of seagrass was also investigated. Seagrass (Enhalus acoroides) leaves and roots were also collected at the same locations where the sinking particles and surface sediments were collected, and the concentrations of multiple elements were compared between the sediment and seagrass. The results showed that the concentrations of Mn and Fe, which are useful for plants growth, were higher at sites with higher red soil loadings than at sites with lower red soil loadings. This suggests that in coral reef sediments dominated by carbonate minerals with low trace metal content, the function of seagrass to trap red clay may have beneficial effected on their own growth.