Differences in coral cover between high and low sediment influx areas in southwestern, Puerto Rico

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1 Differences in coral cover between high and low sediment influx areas in southwestern, Puerto Rico Juan L. Torres, Jack Morelock Abstract High sediment influx is one of the major causes of the decrease in coral cover on Puerto Rican reefs during the last three decades. To study the effects of sediment influx on coral cover around the southwest shelf of Puerto Rico, four stressed areas were chosen, along with two control areas with minimal terrigenous sediment influx. Coral cover was measured along 30 m transect lines at a 5-10 m depth using a photo-transect method. The results were correlated with sediment deposition rates. Sediment composition and grain sizes were obtained from sediment traps located at each area. Coral cover was significantly lower (p less than.001) at the stressed areas when compared to the controls. Species composition changed from primary framework reef-builders, especially Montastraea annularis, Colpophyllia natans, and Diploria sp. at the control areas, to high sediment-resistant species like Montastraea cavernosa and Siderastrea siderea at the stressed reefs. Also, species of the genus Agaricia that typically lives relatively deep (> 20 m), were commonly found living at transect depths in the stressed areas. This shows a compression in the reef species zonation from moderately shallow-water corals to deep plate-like species that are more adapted to low light availability, and other secondary species that can resist high turbidity conditions. No coral colonies were found below 12 m at the more stressed areas where only dead coral boulders were seen covered by a thick layer of mud representing what presumably are the remains of once healthy reefs. Introduction Massive reef-building corals provide the primary framework of the majority of the reefs, growing (under Gladfelter, et al., 1978; Hubbard and Scaturo, 1985; Goenaga, 1988; Van normal conditions) at a rate of approximately 10 mm/yr. Veghel and Bosscher, 1995 Massive corals, along with plate-like corals, branching corals, gorgonians, mollusks, sponges, and thousands of other marine invertebrates contribute to build the solid structure known as the coral reef. Reef accretion is in the order of only a few millimeters per year. An increase of turbidity in the water column from sediment influx into the inshore waters affects the symbiotic relationship between massive corals and photoautotrophic organisms called zooxantellae resulting in a reduction in the growth rates Goenaga, 1988; Torres, 1998 Loya, 1976; Morelock, et al., 1983; and coral cover. Acevedo, 1986; Acevedo, et al., 1989 In the worst of the cases, a loss of entire reefs occurred. The slow-growing corals, affected by the turbid waters can then become overgrown by rapidly growing algae and/or covered by terrigenous muds, changing the ecosystem into a dead hardground. Almost 50% of the Puerto Rican inshore reefs are already lost or badly damaged. Morelock, unpublished data This is mainly due to the development of most of our coastal areas with dramatically high increases in industries and urbanizations accompanied by higher sewage discharges and increased sediment-laden river runoff as a result of vegetation stripping.

The purpose of this study was to establish the effects of high sediment influx on the hard coral cover of four separate areas on the west and southwest shelves of Puerto Rico and comparing this to

2 The purpose of this study was to establish the effects of high sediment influx on the hard coral cover of four separate areas on the west and southwest shelves of Puerto Rico and comparing this to two control areas of limited sediment influence. The parameters included are species composition, total hard coral cover, and differences in reef zonation at the 5-10 m transect depths. Methods Study sites Figure 1 shows the distribution of all reefs studied. The sites will be referred to by their initials throughout the rest of the text. A more detailed description of all studied sites can be found in Torres A. Cayo Turrumote southeast, La Parguera (CTSE) Because of the low sediment input that enters this reef, Acevedo, et al., 1989 it was chosen as one control site. The Montastraea annularis complex (M. annularis, M. faveolata, and M. franksi Knowlton, et al., 1992 ) dominates coral cover. Colonies as large as 5 m high can be found in this area. This is a shelf reef located approximately 5 km off the southwest coast of Puerto Rico (17 o 55'80"N; 067 o 00'83"W). Water transparency is the best among the study areas. Secchi disk readings often reached the bottom at 16 m without disappearing. B. Cayo El Palo, Boquerón (CEP) El Palo Reef is a small shallow patch reef located approximately 1 km off the southeast coast of Boquerón Bay (18 o 00'25"N; 067 o 12'00W). Maximum depth only reaches 6 m and the bottom is covered with coarse sand dominated by Halimeda flakes. This site was chosen as a second control site because it can be compared to areas like Cayo Cardona in Ponce. Water is clear almost all year long since there are no river mouths near the area. Because of the shallowness of the site, no Secchi disk readings were done since bottom could be easily seen at all times.

3 C. Escollo Rodríguez, Mayagüez (CR) This is a shelf reef located about 2 km off the shore of Mayagüez Bay on the west coast of Puerto Rico (18 o 11'18"N; 067 o 11'75"W). The reef top reaches the surface during low tide. Water quality is minimal since the area is located close to the Guanajibo River mouth. The bay also receives terrigenous sediments from the mouth of the Yagüez River, which is located north of the reef. Secchi disk readings averaged 5 m during an earlier study. Goenaga, 1988 Turbidity has increase little since then. Due to the high entrance of terrigenous sediments in the area, coral cover has been compressed to the top 10 m of the reef. The bottom of the reef is at 10 m depth and covered by terrigenous muds. D. Corona La Laja, Guánica (CLL) This is a small reef located approximately 3 km off the entrance of Guánica Bay on the western part of the south coast of Puerto Rico (17 o 56'14"N; 066 o 53'86"W). The bottom reaches 17 m depth and is covered by fine-grained silty-sand and terrigenous muds. Top of the reef reaches 6 m depth and is covered by Porites porites colonies and the zoanthid Palythoa. Sediments from Guánica Bay are carried over the reef during high rainfall. Water quality varies with a Secchi disk average of 7 m. E. Cayo Caribe, Guayanilla (GUY) This reef is located at the edge of the Tallaboa Canyon about 2 km off the southwest coast of the Island (17 o 57'94"N; 066 o 44'18"W). Water quality is minimal. Secchi disk readings averaged 4.5 m. Sedimentation is believed to come from the Tallaboa Bay where some refineries were built onshore in the late 1970s and from Tallaboa and Guayanilla rivers. Coral cover has been reduced to about 6% since the refineries were built. Terrigenous muds cover the bottom of the reef. Cover has changed from one rich in hard corals to a hardground covered mainly by gorgonians with some massive coral colonies at the top of the reef in 10 m of water. F. Cayo Cardona, Ponce (CC) This reef is located west of the Ponce harbor entrance on the south coast of Puerto Rico (17 o 57'42"N; 066 o 38'25"W). It reaches the surface and extends down to about 20 m where the bottom is covered by terrigenous muds. Coral cover has been compressed to 8 m and is covered by a few scattered M. annularis colonies and some sediment resistant species (M. cavernosa, S. siderea, etc.). At the top, high wave energy removes the fine material that settles. Water quality is the worst of all sites with a Secchi disk average of 3.5 m. Coral cover measurements A photo-transect method, similar to the one described by Bohnsack 1979 and Weinberg 1981 with a 30 m line was used to obtain estimates of hard coral cover at each site. Depth sampled at each reef was slightly different CTSE, 10 m CEP, 5 m CR, 8.5 m CLL, 10 m GUY, 10 m CC, 5 m

4 This was primarily due to different reef conditions. Ten quadrats (100x70 cm) were laid at haphazardly selected points along the line. Each hard coral colony present under the quadrats was tagged and the quadrat photographed with a Nikonos V camera (28 mm lens). Each colony name was recorded on a plastic slate for aiding in the identification in the photos. All photos were scanned into digital format. Measurements of cover/species were made with the Sigma Scan Image program (Jandel Scientific). Total coral cover per site was obtained by adding the cover of all species from the quadrats. Cover of the three sibling species of the Montastraea annularis complex was added; thus the results represent the sum of the percentages found for the three sibling species. For statistical purposes, an index of dominance (dominant coral species/total cover/site) and species diversity was obtained by applying Simpson's Index, D = 1-Ep i 2 (1) were p i is the proportion of species i in the community. This index was preferred over the more commonly used Shannon-Weiner function because it is more sensitive to changes in the most common species instead of focusing on the rare species. Krebs, 1989 Also, a one-way ANOVA was used to test for differences in coral cover among sites. Reyes, 1980 The results were compared with those obtained by Goenaga 1988 and Acevedo, et al at similar sites in the same parts of Puerto Rico. Sediment Analysis Two sediment traps were located at each site for a one-year period. The trap contents were collected by SCUBA diving at monthly or bi-monthly periods. Plastic Nalgene 1 liter bottles with a wide mouth (63 µm diameter) were used as sediment traps. In places where current was minimal, this size proved to be the most efficient. Gardner, 1980a,b Trap contents were transferred to a beaker and allowed to settle for a day or two. Then, the water was removed by decantation and the samples were washed with distilled water for salt crystal removal and allowed to dry at ambient temperature until they were completely dry. The dry sample was then weighed and two sub samples were taken from each sample for grain size analysis. Another two sub samples were used for composition analysis. For this purpose, sub sample weight was noted and then each was treated with a sodium hypochlorite solution (50%) to dissolve all the organic components in the sub sample, then it was dried and weighed. It was treated with an HCL 10% solution to dissolve the calcium carbonate present, then dried and weighed. The remaining sediments in the sub sample were assumed to be terrigenous sediments. Sediment size analysis was performed following the procedures described by Folk Sand was separated from the mud fraction (finer than 4Φ) by wet sieving, dried, weighed and dry sieved with meshes of one Φ increments. The mud fraction was analyzed by the pipette method to 9 Φ, the division Folk, 1968 between silt and clay occurs. Results The nonparametric measure of heterogeneity used (Simpson's index of diversity) showed a low diversity of species and a high dominance of the Montastraea annularis complex at CTSE and CEP, while at the other sites, diversity was higher and the M. annularis complex shows a poor dominance over the other species (Table 1). This shows the great effect imposed by the influx of land-derived sediment on this species complex. In fact, some authors have classified M. annularis (and its sibling species) as highly Morelock, et al., 1983; Acevedo, et al., 1989; Torres, 1998; Torres, in press sensitive with respect to terrigenous sediment influx.

CTSE has been a reef dominated by the M. annularis complex during a few thousands years as demonstrated by Hubbard, et al. Folk, 1997 The results of this study show that the M.

5 CTSE has been a reef dominated by the M. annularis complex during a few thousands years as demonstrated by Hubbard, et al. Folk, 1997 The results of this study show that the M. annularis complex is 10.9% of the total coral cover (18.6%) present at transect depth (10 m) at CTSE. This means that along the transect, 59% of all the colonies found along the transect were from the M. annularis complex. Our results compare to those of Acevedo 1986 and Acevedo, et al who found a cover represented by this complex of 9.9% in a nearby area at the same reef. Other species that dominate at CTSE along the 10 m depth contour are: Diploria strigosa, Porites porites, and Colpophyllia natans. The highly sedimentresistant species Montastraea cavernosa and Siderastrea siderea appeared only scattered at CTSE and CEP. The colonies of the genus Agaricia that appear at CTSE were found mainly in between the Montastraea heads where relatively little light reaches. On the other hand, this genus was found at wellexposed places at the sediment-stressed sites.

6 The main loss in coral cover on the sediment-stressed sites was by the M. annularis complex (Figure 2). Cover of the dominant sediment-resistant species like Montastraea cavernosa and Siderastrea siderea was relatively higher at these sites than at CTSE and CEP, although their differences were not statistically different at the 0.05 level of confidence. Other secondary genera (i.e., Stephanocoenia, Mycetophyllia, and Meandrina) were only found scattered among sites, hence, their cover differences were not statistically significant. The transect studied at CR was characterized by the absence of M. annularis or any of its sibling species of any size. M. cavernosa and S. siderea dominated with a few colonies of Agaricia sp., Leptoseris cuculata and Stephanocoenia present. At CLL, M. annularis dominated, but to a lesser degree than at the control sites (6.5%) followed by Agaricia sp., C. natans, P. porites and Meandrina meandrites. The zoanthid genus Palythoa was only found at CLL at 10 m; at the other sites it was seen, but only at shallower depths not measured during this study. At GUY, C. natans and M. cavernosa dominated over M. annularis which only covered 0.98% of the transect area. At CC, all species covered less than 2% each with the M. annularis complex, P. astreoides, S. siderea, Agaricia sp., M. cavernosa being the few species present in the reef. Also, the hydrozoan Millepora alcicornis was present at transect depth (5 m). In summary, the total cover for CTSE was significantly higher than at the other sites including CEP. As a matter of fact, with the exception of CLL, all sediment-stressed sites did not show more than 7% total coral cover. A significant difference (p <.001) was found between stressed and control sites regarding the total resuspended sedimentation rates (mg/cm2/day). In fact, the difference between CTSE and CEP, and CR, CLL, GUY and CC is more than ten fold (Figure 3). No significant differences were found in grain sizes among sites; therefore, this factor was not taken into consideration. Nevertheless, it does not mean that this could be a very influencing factor at other sites. It is known that fine-grained sediments can adversely affect coral growth and cover. Morelock, et al., 1983 There was no difference in sand and mud percentages among sites (p =.681), with the mud fraction prevailing over the sand at all sites. Overall composition of resuspended sediments is shown in Figure 4. Total carbonates were significantly different between CTSE and the other sites, including CEP. CR showed the lowest carbonates percentages of only 17.2%; CLL showed less than 47%, and the other sites sediment-stressed sites had less than 40% carbonates overall. CTSE showed significantly less terrigenous sediments than the other sites, including

CEP and CLL. CR showed the highest percentage of land-derived sediments (78.4%), while more than 50% of the resuspended sediments at GUY and CC were terrigenous (53.8% and 57%, respectively).

7 CEP and CLL. CR showed the highest percentage of land-derived sediments (78.4%), while more than 50% of the resuspended sediments at GUY and CC were terrigenous (53.8% and 57%, respectively). From these, 28% in GUY and 37.6% in CC were terrigenous muds. The fact that the land-derived sediments are fine-grained in its greater majority makes them a detrimental combination for the corals remaining in the affected areas. No significant differences were found between CTSE and CEP and the sediment-stressed sites in sand composition. Instead, a great variability was found, for instance, in organic matter content in the sand fraction among sites. Thus, it could not be attributed nor discarded as a possible factor influencing coral cover at the studied areas. Discussion Reefs with high sediment loads may have lower diversity, percent cover, reduced growth rates, small Rogers, 1990 colony sizes, upward lift of the reef zonation, and a predominance of sediment-resistant species. The depths studied at all sites are within the massive coral zone described earlier in general terms for the Jamaican reefs by Goreau and Goreau, 1973 and for Puerto Rican reefs by Almy and Carrión-Torres, 1963 Goenaga and Cintrón, 1979 and by Acevedo The presence of this zone may have been the result of a Acevedo, 1986; Hubbard, change in slope as the insular shelf was flooded and colonized by massive coral species. et al., 1997 Under normal circumstances (i.e., clear waters, negligible entrance of terrigenous sediments), this zone is dominated by the Montastraea annularis complex, Colpophyllia natans, Diploria sp., Porites astreoides, Siderastrea siderea, and Montastraea cavernosa, in order of importance, as primary reef builders. The Montastraea annularis complex dominates the zone at CTSE partly due to its adaptation to the high light levels that prevail at these reefs and its aggressive ability against other species under these

8 conditions. The other species present at the reef occupy the few spaces remaining between the big heads of M. annularis or to the side of the bases of its colonies. CEP does not fit into this description since maximum depth reaches only 5 m. Nevertheless, mountainous colonies of M. annularis are commonly found here. Acropora palmata dominates only above 2 m. Below 2m, coral cover is dominated by the M. annularis complex, S. siderea, C. natans, D. strigosa, D. labyrinthiformis, and relatively big colonies of P. astreoides. The results found at CR compare to those of Goenaga Miller and Cruise 1995 found that M. annularis is affected by the low light availability and siltation pressure of the ephemeral high discharge events that occur near CR as the result of the sediment plume that arrives from the mouths of the Yagüez and Guanajibo rivers. At CLL, M. annularis, Agaricia sp., C. natans, and P. porites dominate. At GUY and CC, dead coral colonies prevailed over the healthy ones along the studied transect. At both reefs, M. cavernosa dominates over M. annularis (and its sibling species M. faveolata and M. franski). This represents a sign of the deterioration of the reefs from the high sedimentary conditions occurring. At CC, overall coral cover was greater than that found by Acevedo 1986 and Acevedo, et al The difference between our study and theirs corresponds to different transect locations at the same reef. The site described by them was located at the west edge of the Ponce submarine canyon, closer to the main resuspended sediment source in the Ponce Canyon. Notwithstanding, the results in species richness and cover by species compare. The high percentage of land-derived sediments is a critical factor affecting the coral cover and dominance of the M. annularis complex at all sediment-stressed sites. Indeed, a negative correlation was found between cover of the M. annularis complex and sedimentation rates (Figure 5). Local events at all the stressed sites may cause the turn-off described by Buddemeier and Hopley 1988 if they continue to deteriorate at the same rate as it had happened during the last decade.

9 Conclusions Coral cover has decreased in some Puerto Rican reefs over the past decade. A shift in the coral cover zonation was noted between the stressed sites and this correlates with the amount of resuspended sediments present in the water column. This is, a species change from poor to high sediment rejection capabilities occurred, with typically deep plate-like coral species found at much shallower depths in highly turbid areas. Cover of the main Caribbean reef-builders, the Montastraea annularis complex, was dramatically reduced at the stressed sites. As a consequence, this brought an increase in the species diversity not because of finding more species at the stressed sites, but because of the occurrence of more small colonies of secondary species present. Cover of secondary species was not significantly different among all studied sites, indicating the importance of these species as reef keepers in highly stressed areas. Click in this box to return to coral reef papers

Remote Sensing of Episodic Rainfall Events Affecting Coral Reefs in Southwestern Puerto Rico

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