Lynch, A.

The intensity of major storm events generated within the Atlantic Basin is projected to rise with the warming of the oceans, which is likely to exacerbate coastal erosion. Nature-based flood defence has been proposed as a sustainable and effective solution to protect coastlines. However, the ability of natural ecosystems to withstand major storms like tropical hurricanes has yet to be thoroughly tested. Seagrass meadows both stabilise sediment and attenuate waves, providing effective coastal protection services for sandy beaches. To examine the tolerance of Caribbean seagrass meadows to extreme storm events, and to investigate the extent of protection they deliver to beaches, we employed a combination of field surveys, biomechanical measurements and wave modelling simulations. Field surveys of seagrass meadows before and after a direct hit by the category 5 Hurricane Irma documented that established seagrass meadows of Thalassia testudinum remained unaltered after the extreme storm event. The flexible leaves and thalli of seagrass and calcifying macroalgae inhabiting the meadows were shown to sustain the wave forces that they are likely to experience during hurricanes. In addition, the seagrass canopy and the complex biogeomorphic landscape built by the seagrass meadows combine to significantly dissipate extreme wave forces, ensuring that erosion is minimised within sandy beach foreshores. The persistence of the Caribbean seagrass meadows and their coastal protection services during extreme storm events ensures that a stable coastal ecosystem and beach foreshore is maintained in tropical regions.

Msc. thesis

Tropical seagrass meadows are vital coastal ecosystems threatened by both storm activity and human interference. Intense hurricane activity in the subtropical Atlantic has been projected to increase by the late 21st  century, having uncertain consequences for seagrass ecosystems in the region. This study investigated the responses of seagrass meadows to hurricane-related nearshore processes in the current and future storm climates. Further, the impacts of human interference and the sensitivity of meadow formation were studied. This was achieved by coupling the morphodynamic model XBeach and the agent-based seagrass community model SMEAGOL. The development of Thalassia testudinum  meadows was simulated with cycles of 40 hours of morphodynamics followed by one year of growth, over a one-dimensional cross-shore transect across Galion Bay, St Martin. Both seagrass leaves and the biogeomorphic landscapes they form are signicant in attenuating wave energy. Relative wave attenuation by Thalassia  meadows is reduced under the storm surge and waves generated by a category 5 hurricane. Removal of the seagrass canopy by changes in the water quality results in a signicant reduction in wave attenuation and coastal protection potential. Untouched Thalassia  meadows in shallow, reef-protected habitats are unlikely to experience widespread loss as a result of increasing storm activity in the future. The tipping point, if it exists, is certainly above four direct category 5 hurricane strikes in a year. A meadow fragmented in exposed areas by propeller scarring or other human interferences is unlikely to recover, and renders the meadow vulnerable to further damage in the event of a storm. Regeneration of Thalassia  meadows by sexual reproduction is a slow and sensitive process irrespective of the storm climate, remaining susceptible to complete reversal by storm impact for decades. As such, meadows removed by a combination of hurricanes and human activity may never recover.