Figure 1.
Location and depth-profile of the experimental site.
(A) Aerial photo of the field site showing the locations of the stations, the seagrass bed on the reef flat in the subtidal nearshore area (light blue), and the coral drop off (transition to dark blue). See [36] for a more elaborate map. Waves are coming predominantly from the north (right). (B) Depth profile at increasing distance from the beach. Location of stations are indicated including their mean water depths.
Figure 2.
The effect of seagrass presence on sediment stabilization.
Sediment levels in unvegetated gaps compared to levels in the seagrass meadow at T0 for two treatments: gaps exposed to waves (black circles) or exposed to waves reduced by wave bunkers (white circles). Seagrass stabilizes sediment both (A) directly after a storm and (B) 4 weeks after a storm. The inlay shows the setup of a bunker to reduce wave energy to seagrass and unvegetated gaps behind (left of) the bunkers. Significant differences between stations are indicated by different letters, and between wave exposed and wave-reduced plots by stars.
Figure 3.
Effect of canopy length on sediment stabilization.
(A) Turtle exclosure. (B) Difference in sediment bed level between grazed and ungrazed seagrass strips for the three stations (A, B, C) after 2 months protection by the turtle exclosure. The difference in leaf length of the canopy in turtle exclosures was a factor 2.6 longer (117.8±16.6 mm) than in grazed meadows (45.8±11.6 mm).
Table 1.
Summary of the measured significant wave height (Hs), peak wave period (Tz) and bed shear stress (BSS) along a cross-shore seagrass profile (Fig. 1).
Figure 4.
Conceptual model showing how erosion is decreased along a nearshore seagrass bed with a minimal canopy due to the combination of increased critical shear stress and resulting shallowness.
Sediment erosion occurs when bed shear stress (force per unit area of the flow acting on the bed) exceeds a critical bed shear stress (τb > τcrit). (A) A typical depth gradient of a nearshore habitat where waves break above the coral reef, are then further reduced in the surf zone and “swash” onto the beach. Sediment stabilization by seagrass (green line) increases sediment bed levels compared to a situation with seagrass (yellow). (B) As a consequence of the reduction of the water depth by sediment stabilization of seagrass (green line), more wave energy is attenuated while travelling towards the shore compared to unvegetated areas (yellow), and less wave energy can reach the shore in the surf zone. This highlights the importance of seagrass with respect to coastal defense. (C) In the grazed seagrass meadow with short leaves and low-biomass, the low structural complexity of shoots in combination with the relative high root and rhizome biomass increases the critical bed shear stress that is needed for erosion (τcrit.).