scientific chapter |
Slattery M, Lesser MP
As shallow coral reefs worldwide are increasingly degraded by natural and anthropogenic stressors, mesophotic coral ecosystems (MCEs; ~30–150 m) represent a potential refugia that may act as seed banks for coral reef resilience. This is particularly true in the Caribbean Basin where phase shifts from coral dominance to either algal or sponge dominance have been reported with increasing frequency. However, coral reefs of the Bahamas and Cayman Islands are less impacted than other Caribbean reefs, and they offer an opportunity to assess the ecology of MCEs and the connectivity between shallow and mesophotic coral reefs. The MCEs of the Bahamas and Cayman Islands are composed of a shallow sloped upper zone (~30–60 m) and a vertical lower zone (60–100+ m). The upper MCE zone has similar biodiversity and percent cover to nearby shallow reefs, but the lower MCE zone is dominated by sponges, and it includes many species not found on shallow reefs. The ecological importance of the mesophotic sponges is unequivocal; they provide habitat for a variety of species and play a significant role in benthic-pelagic coupling via filtration of bacterioplankton, and their symbiotic microbes are sources of nitrogen cycling. Moreover, our data indicate that sponge diversity on MCEs in the Bahamas and the Cayman Islands is more similar to one another than the sponge diversity among shallow and mesophotic reefs in each region. Threats to shallow reefs (e.g., climate change and invasive species) may also impact MCEs; conservation will require mitigation of these stressors.