2:25 PM - 2:40 PM
[ACG38-03] Grazing impact on sediment-accumulating and stabilizing capacity of back-reef seagrass meadows
Keywords:Seagrass meadows, Sediment, Terrestrial suspended matter, Herbivory, Coral reefs, Buffering effects
Seagrass beds in shallow coastal marine habitats promote sediment accumulation and stabilize sediment by mitigating water flow and wave action. They effectively capture suspended particles and fix them in underlying sediment, and their well-developed root systems protect sediment from resuspension. Particularly, seagrass beds growing in the lagoon of fringing coral reefs prevent the intrusion of terrestrial soil materials carried by inflowing rivers, thus safeguarding corals from deterioration due to excessive sedimentation. However, in the coastal areas of Okinawa Archipelago, especially in the Yaeyama Islands, there has been severe damages of seagrass beds caused by intensive herbivory of green sea turtles (Chelonia mydas) in recent years. As a result, seagrasses have rapidly diminished from coral reef lagoons.
In this study, we conducted a case study on the seagrass beds of the Shiraho Coral Reef (24.3°–24.5°N 124.2°–124.3°E) on Ishigaki Island, which had suffered severe grazing by sea turtles since around 2020. By deploying multiple sediment traps (inner diameter: 50 mm; height: 120 mm), we aimed to: 1) confirm that the seagrass beds prevent resuspension and transfer of river-borne suspended soil into the coral habitat, and 2) investigate how the decline of seagrass beds due to herbivory affects the mobility of sediments. To estimate the physical conditions governing sediment mobility, we employed an improved version of the hydrodynamic model developed by Nakamura et al. (2018). This model was used to determine the average flow velocity and maximal bottom shear stress (BSTR) due to both advection and waves during the periods at the locations (50 m grid) of the trap deployment.
The main results are as follows.
(1) Materials collected by the sediment traps ware commonly low in organic matter content (<12% in dry weight (DW)) showing different carbon and nitrogen stable isotope compositions from suspended matter found in the water column, which suggested that trapped materials represented laterally transported resuspended and/or river-borne particles rather than sinking sestonic particles from the water column.
(2) Before 2018, when the seagrass beds were intact, the sediment flux recorded by the traps within the reef lagoon was generally <2.5 g DW m–2 h–1 with only few exceptions. In unvegetated sandy zone, the sediment flux showed a significant positive correlation with BSTR (p < 0.05). However, no such relationship was observed in seagrass beds (p > 0.3).
(3) Composition of the trapped materials within seagrass beds was dominated by acid-insoluble (i.e. terrestrial) mineral particles, accounting for up to 80% of DW, while outside the seagrass beds, carbonates were the main component. In particular, within the coral habitats, the fraction of terrestrial soil never exceeded 20%. Outside the seagrass beds, the fraction of terrestrial soil showed a significant positive correlation with advective water flow (p < 0.0001), while no such relationship was found in the seagrass beds (p > 0.5).
(4) After 2022, when intense grazing by sea turtles led to a significant decrease in average leaf length and shoot density of seagrasses, sediment flux within the seagrass habitat increased dramatically. Despite comparable BSTR (0.5 – 3.0 N m–2), sediment flux frequently exceeded 10 g DW m–2 h–1. Moreover, within the seagrass beds, a significant positive correlation between sediment flux and BSTR became evident (p < 0.001).
The above findings suggest that seagrass beds in coral reef lagoons effectively suppress sediment resuspension by physical forcing, preventing river-borne soil particles from reaching the coral habitat. Furthermore, such functionality of seagrass habitats has significantly degraded recently due to intensive grazing by green sea turtles.
Nakamura, T. and others (2018) Coral Reefs, 37, 37-53. doi:10.1007/s00338-017-1632-3
In this study, we conducted a case study on the seagrass beds of the Shiraho Coral Reef (24.3°–24.5°N 124.2°–124.3°E) on Ishigaki Island, which had suffered severe grazing by sea turtles since around 2020. By deploying multiple sediment traps (inner diameter: 50 mm; height: 120 mm), we aimed to: 1) confirm that the seagrass beds prevent resuspension and transfer of river-borne suspended soil into the coral habitat, and 2) investigate how the decline of seagrass beds due to herbivory affects the mobility of sediments. To estimate the physical conditions governing sediment mobility, we employed an improved version of the hydrodynamic model developed by Nakamura et al. (2018). This model was used to determine the average flow velocity and maximal bottom shear stress (BSTR) due to both advection and waves during the periods at the locations (50 m grid) of the trap deployment.
The main results are as follows.
(1) Materials collected by the sediment traps ware commonly low in organic matter content (<12% in dry weight (DW)) showing different carbon and nitrogen stable isotope compositions from suspended matter found in the water column, which suggested that trapped materials represented laterally transported resuspended and/or river-borne particles rather than sinking sestonic particles from the water column.
(2) Before 2018, when the seagrass beds were intact, the sediment flux recorded by the traps within the reef lagoon was generally <2.5 g DW m–2 h–1 with only few exceptions. In unvegetated sandy zone, the sediment flux showed a significant positive correlation with BSTR (p < 0.05). However, no such relationship was observed in seagrass beds (p > 0.3).
(3) Composition of the trapped materials within seagrass beds was dominated by acid-insoluble (i.e. terrestrial) mineral particles, accounting for up to 80% of DW, while outside the seagrass beds, carbonates were the main component. In particular, within the coral habitats, the fraction of terrestrial soil never exceeded 20%. Outside the seagrass beds, the fraction of terrestrial soil showed a significant positive correlation with advective water flow (p < 0.0001), while no such relationship was found in the seagrass beds (p > 0.5).
(4) After 2022, when intense grazing by sea turtles led to a significant decrease in average leaf length and shoot density of seagrasses, sediment flux within the seagrass habitat increased dramatically. Despite comparable BSTR (0.5 – 3.0 N m–2), sediment flux frequently exceeded 10 g DW m–2 h–1. Moreover, within the seagrass beds, a significant positive correlation between sediment flux and BSTR became evident (p < 0.001).
The above findings suggest that seagrass beds in coral reef lagoons effectively suppress sediment resuspension by physical forcing, preventing river-borne soil particles from reaching the coral habitat. Furthermore, such functionality of seagrass habitats has significantly degraded recently due to intensive grazing by green sea turtles.
Nakamura, T. and others (2018) Coral Reefs, 37, 37-53. doi:10.1007/s00338-017-1632-3