Shallow bays in the Caribbean, like Baie Orientale and Baie de L'Embouchure in Saint Martin, are often sheltered by coral reefs and covered by seagrass meadows. They provide valuable services as tourism and coastal protection. The ecosystems are linked through biological, chemical and physical processes. But they are under pressure due to sea-level rise. The response of one of the ecosystems to climate change could impact the other ecosystem.

In order to predict the impact of sea-level rise on the biogeomorphology in Baie Orientale and Baie de L'Embouchure, the hydrodynamic model Delft3D Flexible Mesh is applied. The effect of seagrass meadows and coral reefs on both flow and waves are captured with this model. In this way, the long term change in average hydrodynamic conditions due to sea-level rise is determined depending on the response of the ecosystems.

A wave-driven circulation is found in both bays with flows of 0.5 m/s over the reefs and currents of 0.2 m/s inside the bays. The hydrodynamic conditions are mainly determined by the reef height. Depending on the response of coral reefs to climate change and the amount of sea-level rise, the wave height inside the bays and the wave-induced currents increase. Under the worst-case scenario, where coral reefs degrade and seagrass meadows die, flow velocities increase by more than 100% in Baie de L'Embouchure and by 200% in Baie Orientale under a sea-level rise of 0.87 m. The significant wave height rises to 300% in Baie Orientale and doubles in Baie de L'Embouchure. But this increase of hydrodynamic stresses is not expected to lead to devastating damage to coral reefs and seagrass meadows. Instead, the response of coral reefs will be determined by changing water temperatures and ocean acidification. A shift in seagrass occurrence due to the changed hydrodynamics is expected.

The long term impact of sea-level rise on the biogeomorphology of Baie de L'Embouchure and Baie Orientale seems to be limited. The ability to mitigate the impact of sea-level rise is shown and the resilience of the ecosystems proved, which is very promising for other shallow Caribbean bays that are threatened by sea-level rise.