Abstract
Shallow tropical bays in the Caribbean, like Orient Bay and Galion Bay in Saint Martin, are often sheltered by coral reefs. In the relatively calm environment behind the reefs, seagrass meadows grow. Together, these ecosystems provide valuable ecosystem services like coastal protection, biodiversity hotspots, nursery grounds for animals and enhancing tourism and fisheries. However, sea-level rise imperils these ecosystems and the services they provide because of changing hydrodynamic conditions, with potential effects on the interdependencies between these ecosystems. By means of a hydrodynamic model that accounts for the interaction with vegetation (Delft3D Flexible Mesh), the impact of sea-level rise (0.87 m in 2100) is investigated for three scenarios of future reef development (i.e. keep-up, give-up and catch-up). If coral reefs cannot keep up with sea-level rise, the wave height and flow velocity increase significantly within associated bays, with the wave height doubling locally in case of eroding reefs in our model simulations. Since the presence of seagrass strongly depends on the hydrodynamic conditions, the response of seagrass to the future hydrodynamic conditions is projected using a habitat suitability model that is based on a logistic regression. The spatial character of the bays determines the response of seagrass. In Orient Bay, which is deeper and partly exposed to higher waves, the seagrass will likely migrate from the deeper parts to shallow areas that become suitable for seagrass because of the surf zone moving landward. In contrast, the conditions for seagrass worsen in Galion Bay for the catch-up and give-up scenario; due to the shallowness of this bay, the seagrass cannot escape to more suitable areas, resulting in significant seagrass loss. It is shown that healthy coastal ecosystems are able to limit the change in hydrodynamic conditions due to sea-level rise. Therefore, preserving these ecosystems is key for ensuring the resilience of shallow tropical bays to sea-level rise and maintaining their ecosystem services.
Highlights
In shallow tropical bays in the Caribbean, coral reefs and seagrass meadows are typically present
Healthy coral reefs can limit the change in hydrodynamic conditions, i.e. the mean wave height inside the bays increased less than 0.05 m in the keep-up scenario
Our study shows that if the coral reefs keep up with the rising sea level such that the water depth above the reefs will not increase, the hydrodynamic conditions will not change significantly; currents within the studied bays are almost exclusively wave-driven, and the wave height is primarily determined by the wave dissipation over the reefs
Summary
In shallow tropical bays in the Caribbean, coral reefs and seagrass meadows are typically present. These ecosystems are linked through biological, chemical and physical processes (Gillis et al, 2014; Saunders et al, 2014). In addition to their ecological value, they form a natural flood protection. Seagrass meadows are able to attenuate flow and waves (Fonseca and Cahalan, 1992; Ondiviela et al, 2014), and to trap and stabilize sediment (Christianen et al, 2013; Duarte, 2002; Orth et al, 2006). Seagrass meadows are able to attenuate flow and waves (Fonseca and Cahalan, 1992; Ondiviela et al, 2014), and to trap and stabilize sediment (Christianen et al, 2013; Duarte, 2002; Orth et al, 2006). James et al (2019b) showed the po tential role of seagrass in maintaining tropical beaches, which are important for the $32.0 billion tourist industry in the Caribbean (World Tourism Organization, 2019)
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