We investigated Puerto Rico’s mesophotic coral ecosystems to better understand them and their relationship to shallower reefs. They are off the southwestern coast at depths of 50-100 m. We found extensive and biologically diverse mesophotic coral ecosystems harboring unique species and species now rare on shallow reefs. Groupers, snappers and reef sharks are common at mesophotic depths. If managed well, these deeper fish stocks may replenish shallow reefs.
Why We Care
With the overall health of shallow coral reefs in decline, it is important to understand the value and role of mesophotic coral ecosystems in tropical and subtropical waters. These ecosystems are found most commonly at depths from 30-40 m to deeper than 100 m in the Caribbean. The dominant structural habitat in the mesophotic zone can be corals, sponges, and algae. Considered as extensions of shallow coral reef ecosystems, mesophotic coral ecosystems are thought to have biological, physical, and chemical connectivity with shallow coral reefs. Although these ecosystems harbor many of the species found in their shallower water counterparts, they are often colonized by a high number of native species of fishes and invertebrates. It has been hypothesized that mesophotic coral ecosystems can serve as sources to reseed or replenish degraded shallow-water coral ecosystem species.
What We Did
- characterized the community composition and structure of the mesophotic coral ecosystems
- determined the potential connectivity of these mesophotic ecosystems to shallower reefs, and
- assessed the vulnerability of mesophotic coral ecosystems to anthropogenic stresses such as, sedimentation and deep sewer outfalls.
The primary study site off La Parguera, Puerto Rico was selected because it is an area of low anthropogenic and natural stress. The second study area off Ponce represented an area with high anthropogenic stress including river inflow, a deep sewer outfall at 120 m in depth, and several dredge dump sites. The Ponce site allows us to compare mesophotic coral ecosystems that are exposed to such stressors as sedimentation, turbidity, and eutrophication to La Parguera, a site with little human impact.
This project is part of the Deep Coral Reef Ecosystem Studies program. It is led by Richard Appeldoorn, University of Puerto Rico, Mayaguez. Project partners at the University of Puerto Rico, Mayaguez include David Ballantine, Ivonne Bejarano, Michael Nemeth, Ernesto Otero, Francisco Pagan, Hector Ruiz, Nikolaos Schizas, Wildford Schmidt, Clark Sherman, Ernesto Weil, and Paul Yoshioka.
What We Found and Management Implications
We found that mesophotic coral ecosystems (at depths greater than 50 m) are:
- Unique and diverse communities – Fish and benthic communities within mesophotic coral ecosystems are diverse, although less so than their shallow water counterparts. The management implication is that mesophotic coral ecosystems should be protected to maintain biodiversity.
- Patchily distributed – As with shallow coral reefs, mesophotic coral ecosystems do not occur everywhere; they are restricted to sites where conditions allow for their development. Over a 12 mile stretch off La Parguera, five sites had well-developed mesophotic coral ecosystems. Thus, management activities must target actual or predicted locations.
- Well-connected to shallow-water fishes, but not to the shallow coral communities – For corals, connectivity between shallow and mesophotic coral communities is minimal since there is little overlap in species composition below 40 m. However, for fishes, the potential population connectivity between shallow and mesophotic fish stocks is high. The most common form of connectivity is through ontogenetic migration from shallow or nearshore nursery habitats (mangroves, seagrasses, and back reefs) to mesophotic depths. The management implication is that protection of fishes that depend on both shallow and mesophotic coral ecosystems should extend beyond shallow waters and include mesophotic depths.
- Refuges for large overfished species – Commercially-important snappers and groupers are more abundant at depths greater than 50 m. At one time, these fishes were common in shallower waters, suggesting that mesophotic coral ecosystems may serve as a refuge for heavily exploited species. Spawning stocks within mesophotic depths greater than 50 m may help replenish overfished shallow resources.
- Vulnerable to anthropogenic disturbances – High sedimentation rates and reduced light penetration, as observed near Ponce, can reduce the depth to which mesophotic coral ecosystems can extend, and when severe can smother mesophotic benthic habitats. Protection of mesophotic coral ecosystems from local threats is warranted.