The Power of Three: Coral Reefs, Seagrasses and Mangroves Protect Coastal Regions and Increase Their Resilience

Study Number

102

Author

Greg Guannell, Katie Arkema, Peter Ruggiero, Gregory Verutes

Abstract

Natural habitats have the ability to protect coastal communities against the impacts of
waves and storms, yet it is unclear how different habitats complement each other to reduce
those impacts. Here, we investigate the individual and combined coastal protection services
supplied by live corals on reefs, seagrass meadows, and mangrove forests during both non-storm and storm conditions, and under present and future sea-level conditions. Using idealized profiles of fringing and barrier reefs, we quantify the services supplied by these habitats using various metrics of inundation and erosion. We find that, together, live corals, seagrasses, and mangroves supply more protection services than any individual habitat or any combination of two habitats. Specifically, we find that, while mangroves are the most effective at protecting the coast under non-storm and storm conditions, live corals and seagrasses also moderate the impact of waves and storms, thereby further reducing the vulnerability of coastal regions. Also, in addition to structural differences, the amount of service supplied by habitats in our analysis is highly dependent on the geomorphic setting, habitat location and forcing conditions: live corals in the fringing reef profile supply more protection services than seagrasses; seagrasses in the barrier reef profile supply more protection services than live corals; and seagrasses, in our simulations, can even compensate for the long-term degradation of the barrier reef. Results of this study demonstrate the importance of taking integrated and place-based approaches when quantifying and managing for the coastal protection services supplied by ecosystems.

Main Results and Conclusions

• It is more effective to use more than one type of coastal habitat as protection against non-storm and storm conditions than a single habitat type.
  • “…a single-habitat approach to coastal protection overlooks the possibility that structurally different natural systems on a seascape can together supply higher levels of protection services by progressively moderating the impacts of hydrodynamic processes…” (2)
• Coral reefs independently provide protection against waves during non-storm conditions.
  • “…live reefs provide the same amount of wave attenuation in present and future scenarios (Fig 4, Table 3). In contrast, dead reefs under present conditions become bare and smooth in the future, and the height of the transmitted maximum and mean offshore wave heights increase fivefold to 0.90 and 0.81 m, respectively.” (9)
  • “Under present day sea-level conditions, if the corals on the barrier reef are dead, nearshore waves are 0.19 m on average, or 35% higher than nearshore wave height values obtained if corals on the reef were alive.” (10)
  • “And as sea level rises, the far field impacts of coral reef degradation are even more pronounced [83]: nearshore waves and bed shear stresses in a future dead and bare barrier reef profile are, on average, respectively 3.5 and 13 times higher than what would have been observed if the reef were still alive.” (10)
• Coral reefs and seagrasses together provide greater protection during non-storm conditions.
  • “In both the barrier and fringing reef profiles, once waves pass the reef, they encounter and propagate through seagrass meadows that further attenuate the incoming waves, resulting in lower wave heights and bed stresses nearshore than the no live habitat and the coral only scenarios.” (11)
  • “In the barrier reef lagoon, seagrass meadows reduce average non-storm waves in the nearshore to 0.07 m, or half the mean wave height of the coral only scenario…” (11)
  • “Even without live corals on either reef, seagrasses still reduce waves and bed shear stresses nearshore by more than 60% to 0.07 m and 0.06 N/m2, compared to 0.19 m and 0.48 N/m2 with a dead reef and no seagrass…” (11)
  • “These results indicate that together, live corals and seagrasses provide more protection benefits in the barrier reef profile than either of those habitats alone.” (11)
  • “When operating together, they reduce nearshore wave height by nearly 40% (0.13 m vs. 0.22 m, Fig 6A) and bed shear stress by more than 60% (0.26 N/m2 vs. 0.7 N/m2…” (12)
• Coral reefs, seagrasses, and mangroves together provide protection in non-storm conditions.
  • “In both reef profiles, mangroves attenuate all non-storm waves by approximately 70% of the nearshore wave height, to only 2–3 cm, regardless of the presence of corals or seagrass offshore…” (13)
  • “In the barrier reef profile, in the absence of any live habitat, nearshore waves at the shoreward edge of the submerged forest are, on average, 0.1 m high (Fig 7B, “No Live Habitat”), which results in approximately 0.6 m3/m of mud scoured from the submerged forest (Fig 7D, “No Live Habitat”). In the presence of live corals and seagrasses, average wave height at the shoreward edge of the submerged forest and mud scour volumes in the forest decrease to 0.05 m and 0.0 m3/m
• Mangroves alone provide the most amount of storm protection.
  • “Examining the role of the different habitats, we find that in both profiles mangroves alone systematically reduce inundation levels and mud bed scour volumes, while live corals and seagrasses play a relatively minor role…” (14)
  • “…mangroves alone decrease the inundation level at the landward edge of the mud bed by more than 35% (2.22 m down to 1.48 m in the barrier reef, and 0.86 m to 0.48 m in the fringing reef).” (14)