The carbonate platform sediments are shoreward from the shelf-edge reef system. This is an area of hard carbonate pavement with thin carbonate sands and small patch reefs. The platform hardground has low coral growth under conditions of moderate to strong currents and variable turbidity. Sediments are present in small channels.
The Ponce Basin extends to about 3 km west of Ponce Harbor and grades northward into the nearshore zone. Water depths are 12 to 34 m. The water is generally turbid with Secchi disc readings of less than 5 m. Sediments in the Ponce Basin are poorly sorted terrigenous silts and clays with small amounts of carbonate mud and sand that have been carried in from surrounding areas and have accumulated in low-energy, deeper-water conditions. Similar sediments floor the Ponce Submarine Canyon. The northern part of the basin is dominated by dark to olive terrigenous muds. Beach (1975) measured 65 to 70 cm of mud lying over a calcareous sand, showing a change in the depositional environment. He measured sediment accumulation on an artificial reef near Ponce to get an accumulation rate of 4 mm per year for the terrigenous muds. Applying this rate to the accumulated mud blanket shows that it could have been deposited during the last 125 to 150 years. Land stripping for sugar cane, urbanization, and dredging for harbor facilities could account for increased erosion and the development of this mud accumulation.
These fine sediments are resuspended by wave action and ship traffic. The resulting underwater visibility can be less than 1 m. (pers. obs.) The resuspension develops a sediment plume that is transported by currents, carrying fine sediments over the reefs within and west of the basin. The nearshore province sediments are dark, fine-grained sand to fine-grained terrigenous muds. Moderate to strong longshore drift was observed.
Analysis of sediments collected showed that the samples from depths of the five meters were dominantly sands, due to wave action removal of the fine fraction. Deeper parts of the reef had more fine-grained terrigenous sediments. Bajo Tasmanian and Cardona reefs had large amounts of fine-grained terrigenous sediments and poor sediment sorting. Both areas showed finer grain size and more terrigenous sediments at 15 m, suggesting that wave energy removes some of the terrigenous fines at depths above 10 m. Cayo Ratones also showed an increase in fine terrigenous sediments with depth.
Secchi readings during October 2002 gave 10 to 11 m at all sites along the route of the sewer outfall pipeline between Cardona Ridge and the outer shelf ridge. Swell and wind waves created a strong bottom surge during the survey. These resuspended sediments obscur visibility.
The total cover of Montastraea cavernosa is not significantly different from values at La Parguera, but the reduction or absence of other coral species changes the relative abundance and reinforces the conclusion that Montastraea cavernosa is highly tolerant to a long-term sediment stress condition. No living corals were found below 12 m at Cayo Cardona.
The cover for the east and west transects at Cardona Ridge were similar, but lower than the results from 1993. This does not seem to represent a general loss of coral on the ridge, but variation in amount of coral cover on what is a hardground (defined as having less than 6% living coral on the surface).
The Outer Ridge results were very similar for the two October 2002 transects and for the comparison to the 1993 survey both in total coral cover and in cover by species. This ridge is also hardground with small clumps of coral and heavy cover by soft coral, gorgonians and sponge.
These hardgrounds have only a limited amount of coral cover, supplied by only a few species,
The average coral cover was 2.6 percent of the surface covered with living coral, and this was only seven species of coral. Eight other coral species were found during the surveys.
In general, all sites were characterized by the dominance of two invertebrate groups: gorgonians and sponges. The species composition of the gorgonians, notably the presence of species like Muricea, Eunicea and Pterogorgia is indicative of an environment with strong hydrodynamics and low topographic relief with potential strong sediment transport regimes. Sponges also showed growth pattern (e.g. more encrusting vs. erect) that was indicative of strong hydrodynamics.
All differential GPS positions were at the pipeline within the rubble zone. The leak site was referenced to the inspection chamber and leaks were seen north and south of this structure. At the leak site only the chamber stood above the rubble, but the pipeline was exposed at both of the other sites.
Changes were detected in the coral population and the H'c index of the Cardona Ridge site from 1993 to 2002. The Montastraea annularis and Porites porites species are not present in the transects measured during October, 2002. They also were not detected by visual inspection of the area, suggesting that they disappeared as part of the reef species composition at Cardona Ridge. The coral cover of Porites asteroides and Siderastrea siderea increased in disorder from 1993 to 2002 (higher H'c index). Species that are known to be tolerant of adverse environments (Diploria clivosa and Madracis decactis) were absent in 1992 but are present in 2002. The analysis of coral cover distribution suggests that the Cardona Ridge has been negatively affected since 1993.
Regardless of these changes in species composition and H'c index, the ANOVA analysis suggests that these changes are part of the normal variability of the reef system since the probability obtained for the variance test was reasonably high (p=.7206). In the case of the Outer Shelf Ridge, the changes in the coral cover do not seem to follow any trend. Coral cover disorder (H'c) decreased for three of the species present (Montastraea annularis, Agaricia agaricites, and Siderastrea siderea) and increased for two of them (Montastraea cavernosa and Porites asteroides). These changes in cover order/disorder of specific corals are not related to the corals species documented tolerance to adverse conditions. Two new species were present in the 2002 surveys that were not detected in the area during the 1993 surveys (Diploria strigosa and Diploria clivosa). Overall the statistical analyses do not indicate positive or negative trends in the coral species populations and the changes seem to be caused by natural random reef variations.
Results from the coral cover transects are very similar for the 1993 and 2002 surveys. Although the surveys were not conducted in the same locations (original surveys were in what is now the rubble zone), the transects in October 2002 are similar to the 1993 survey. Both the species present and the percent of cover are about the same at the Outer Ridge Site. At the Cardona Ridge, the species making up the cover are the same group, but total cover was 4% in 1993 and only 1.6% in 2002. This difference is probably caused by the patchy distribution of coral colonies on the hardground, and the clumping of the colonies that do occur. Only three significant tests (at 95% significance level) were obtained, one per site. These results suggest that most coral species do not change in a significant way between adjacent transects and that the differences in coral cover magnitude observed in the graphs are caused by natural variations of coral populations.
No Montastraea annularis were present in the 2002 survey, but several colonies were seen near the transect lines. The other principal components of the cover, Montastraea cavernosa and Agaricia agaricites were encountered in less percentage in the 2002 surveys. Colonies of the three species were in good condition, showing no decline due to increased adverse conditions.
Coral in the present environment and in that of 1993 show no apparent difference in condition. Personal observation is that conditions are about the same as described by Acevedo, et al. (1989). The remaining coral on the platform seem to have made an adjustment to the sediment influx.
The most striking feature in the rubble zone was the high rate of coral recruitment (plate 6). The most common coral recruits were Porites asteroides and Siderastrea siderea, but Diploria clivosa, Agaricia agaricites and Montastraea cavernosa were also present. The boulder area had a
heavy growth
of new soft coral and gorgonians.
Juvenile recruitment is considered to be an important aspect of continuing survival of reef environments. Recruitment at the three sites seemed to be normal as compared to the better reef areas in Puerto Rico. Coral recruitment (Porites asteroides, Siderastrea siderea, Diploria) is good in the hardground, also good in the rubble zone, despite low coral cover.
The Ponce reef survey in 2002 was done to determine whether a leak in the sewer outfall had adversely affected the coral cover and coral viability. No particular attention was given to the rubble zone during the study. We did take photographs and make observations of the colonization on the boulders emplaced over the pipeline.
There has been a visible higher rate of coral colonization on the rubble. Some of the colonies are more than 15 cm in diameter - growth in a period of five years.
The rubble stands one to two meters above the general level of 10 meters. It also has a rugged surface allowing better water movement across potential coral colonization areas and freedom from direct settling of resuspended fine sediments.
In a later study, we found that coral transplanted to concrete slabs with only 15 cm relief above the bottom fared better than surrounding coral because of the new surface provided.
Observation of the reefs at Ponce suggests that an adverse environment of high turbidity can be improved by providing surface relief of only one to two meters.