Rincón is home to unique Elkhorn coral (Acropora palmata) thickets. Elkhorn coral is a candidate for the Endangered Species Act.
Steps and Tres Palmas reefs are some of the best-developed fringing coral reefs found off the west coast of Puerto Rico. The coastline at Rincón is fringed by a narrow sandy beach, with beach rock at the waters edge. Tres Palmas and Steps Reefs are two hardground areas, separated by a channel 50-150 m wide. The reefs start immediately seaward of the beach rock and slope from 0.5m to 8-10 m depth. The reef extends out for less than 200 m before terminating in a shallow sand flat (8-10 m depth). In shallow water (0.5-3 m depth) the reef is dominated by Acropora palmata (Elkhorn)with isolated brain, star and mustard hill corals. Elkhorn colonies form a dense stand that begins about 5 m offshore and extends seaward 20-30 m. The densest areas of Elkhorn growth are near Steps and Tres Palmas, and colonies also occur at a lower density from just north of the marina to the dome. The deeper portion of the reefs (from 2-8 m) is dominated by Diploria strigosa, but many other massive and branching corals, sea fans, soft corals and other invertebrates also occur here.
A second reef begins from 250-400 m offshore. This reef is completely submerged, and slopes gradually seaward to about 70 feet. It is shallowest at the landward edge (0.5-2m ) where the reef is colonized by isolated A. palmata colonies, and massive and plating corals dispersed over the remainder of the hardground areas. There is relatively high cover (25-40%) in moderate depths (15-20 m) and several large massive boulder corals and plating corals.
Importance of Acropora palmata (Elkhorn coral):
- Storm damage: Elkhorn coral thickets reduce incoming wave energy, offering critical protection to coastlines. Loss of this species may negatively affect shorelines with mangrove and grass bed habitats that rely on calm water provided by these effective reef barriers. Fringing reefs with Elkhorn thickets, like those found in Rincón, are also particularly important to coastal communities and the beach as they form a buffer that protects shorelines from erosion during storms. The loss of Elkhorn thickets results in higher wave action reaching coastal environments, and this can lead to erosion and loss of nearshore grassbeds and mangroves. In Rincón, the Elkhorn thickets front a narrow sandy beach. There is high wave action during winter. This is associated with offshore transport of sand, which accumulates among the corals on fringing reefs and in the surrounding area. Without the presence of a large stand of Elkhorn coral, it is likely that much more sand will be carried offshore during periods of high wave action, and the beaches may eventually disappear.
- Fisheries habitat: The high structural complexity produced by the interdigitated branches of A. palmata colonies provide essential fish habitat. Studies from Florida and the Virgin islands have shown that a higher number of lobsters, snappers, grunts, parrotfish and other large reef fish occur in areas with live stands of Elkhorn coral. In many locations Elkhorn populations have died, but erect skeletons (standing in place) may remain for 10-20 years. Dead colonies continue to provide high relief habitat utilized by a number of organisms. The skeletons are rapidly overgrown with algae and benthic invertebrates, and fish communities become dominated by schools of herbivorous fish like surgeonfish due to increased biomass of algae. Over time, however, the skeletons eventually collapse, eliminating high-relief topography and habitat for predatory fish and motile invertebrates.
- Reef growth: Coral reefs were formerly dominated (prior to 1980s) by three species of coral – Elkhorn coral, staghorn coral (Acropora cervicornis) and star coral (Montastrea annularis complex). A. palmata formed characteristic thickets in the shallowest, exposed areas, on fringing reefs and the outer portions of offshore reefs. These often extended along the coastline or the crest of the reef for several kilometers. A. cervicornis also forms thickets, but it occurs in intermediate depths (5-25 m) on the fore reef in areas with moderate to low amounts of wave action, and shallow calm back reef environments. M. annularis is a complex of three species of massive corals that occurs throughout most reef environments (it is uncommon in areas dominated by Elkhorn coral).M. annularis grows very slowly, and colonies may live for hundreds of years forming immense structures several meters tall.
Tolerance to terrestrial impacts: Elkhorn coral is an environmentally sensitive species that requires clear, high saline, well circulated water with moderate temperatures (25-29 C). A. palmata is intolerant of prolonged periods of high sedimentation; this species lacks a well developed ciliary mucus system found in sediment-tolerant species like Porites astreoides andMontastraea cavernosa. It can only tolerate short periods of increased water turbidity if the site is exposed regularly to moderate to high levels of wave action. Rogers (1983) found that even low doses of sediment accumulate on the flattened branch surfaces, resulting in rapid tissue necrosis; in addition, injuries regenerate more slowly at elevated sedimentation levels (Meesters and Bak, 1995). Rincón’s reefs are affected by poor water quality conditions during the rainy season in summer due to run-off, but murky conditions generally persist for short periods and water clarity improving after a few days. In winter high wave action prevents accumulation of sediment on branches. Clearing of the land adjacent to Steps reef would cause a significant increase in run-off, which is likely to have a significant impact on nearshore Elkhorn coral populations.
Potential impacts associated with a loss of Elkhorn coral populations in Rincón:
The disappearance of these coral thickets may ultimately affect the diversity and abundance of reef organisms, the rate of carbonate deposition and reef growth, and the skeletal contribution to coral cayes and boulder ramparts (Hernandez-Avila et al., 1977; Gladfelter et al., 1978; Williams, pers. comm.).
- Reduced Diversity. In addition to the loss of one of the most important reef builders in the Caribbean, many organisms that rely on A. palmatafor habitat, feeding areas, and refuge will disappear.
- Tourism. Steps reef is a very popular site for snorkeling, due to the shallow water and close proximity to land. Steps is one of the few reefs in Puerto Rico accessible immediately off the shore.
- Beach erosion. Loss of Elkhorn coral would result in stronger waves reaching the shoreline, which will subsequently cause substantial increase in erosion of sand. Increased erosion of sediments will ultimately affect other benthic reefs invertebrates found slightly deeper than Elkhorn coral and also those found on the outer reefs. In addition, increased erosion is likely to result in decreased water clarity which will affect the amount of light reaching photosynthetic reef organisms.
Reference: Andrew Bruckner, Ph.D, letter to Puerto Rico Department of Natural Resources. December 6, 2001.