The effects of sediment stress on coral diversity and abundance near Guanica Bay, Puerto Rico
Coral cover comparisons among Guanica and La Parguera reefs, southwestern Puerto Rico
Two theses investigated the Guanica environment:
del Valle, Miguel A. 1981. "Ecological aspects on the occurrence and distribution of benthic marine algae at three sites in Bahia Guanica, Puerto Rico." M.S. thesis, University of Puerto Rico RUM.
Torres, Juan L. 1998. "Effects of sediment influx on the linear extension rates of Montastraea annularis in southwest Puerto Rico." M.S. thesis, University of Puerto Rico RUM.
This area was also mapped and published by the USGS as described in the parguera map section. The facies map for Guanica uses the same units as the Parguera map.
Coral reef transects were run at four sites. Coral cover was greater than at Guayanilla, but less than at Parguera.
Two student projects related to coral reefs at Guanica are included.
Fringing, barrier, and submerged shelf edge coral reefs are found on the east, south, and west coasts of Puerto Rico. Thirty-five Scleractinian species of coral have been recorded from reefs at La Parguera, on the southwestern coast, the area of highest coral diversity. Loya 1976 Coral reefs are absent from the north coast due to a high rate of rainfall and a large amount of river sediment. This area is also subject to high winds and wave action, which has a negative impact on coral reef development and growth. Maximum reef development is found on the south coast due to small amounts of river sediment influx along with low amounts of rainfall, and less wave action. The insular shelf of southern Puerto Rico is less than three kilometers long and is cut by the Guanica, Punta Ventana, Guayanilla, Tallaboa, Cuchara, and Ponce submarine canyon tributaries. Morelock et al. 1979
Modern terrigenous sediments are found in the canyons while carbonate sediments are dominant on the outer shelf. Heavy ship traffic and flooding from storms allows discharge from these canyons to increase and in turn effects the amount of fine sediments being suspended in the water column. In high concentrations, these sediments can have negative impacts on nearby reef systems. The effect of sedimentation on reef systems is varied, ranging from a decrease in the abundance of species and diversity as a result of a shift in species from those which generally dominate Caribbean reefs to ones more adapted to tolerate high sediment loads. Lasker 1980
High sedimentation rates can also cause death due to direct deposition of sediment on the coral species, which smothers the coral, or indirectly by cutting down on the available space for larval settlement. Morelock et.al. 1979 Previous studies quantifying the effects of sediment on coral reef systems have been carried out on the west coast of Puerto Rico near Mayaguez, Loya 1976 on the South coast near the Ponce submarine canyon system, and the Guayanilla submarine canyon system. Morelock et al. 1979, 1989
Similar studies have been carried out in St. Croix near the Salt River Submarine Canyon and in the turbid waters off the East coast of Costa Rica. This study examines three coral reefs located successively further from a source of terrigenous sediment in front of the Guanica submarine canyon. The reefs surveyed are known as g3-buoy, Canyon North, and Cana Gorda respectively. Due to the low amount of boat traffic passing into Guanica Bay, the submarine canyon system is flushed out to the nearby reef tract during storms. In addition to the examination of sediment effects on reef systems with distance from a sediment source, this study also examines the changes in coral cover and diversity over a ten year time period on two of the reef sites, g3-buoy, and Canyon North. This could be useful for documenting whether the amount of sedimentation has increased in the Bay over time, which may be an indication of higher erosion caused from deforestation and other man made changes. Cortes et al. 1985
Statistics were compiled for percent cover by species, total coral cover, and the number of species present as well as a measure of diversity using the Shannon Weaver Diversity Index.
Most studies have found a decrease in percent cover and a decline in number of species as well as species diversity with an increase in terriginous material. Morelock et al. 1979, Loya 1976, Cortes et al. 1985, Hubbard 1986, Lasker 1980 This study shows a decrease in total coral cover with an increase in distance from the sediment source as well as inconsistent numbers of total species present. This inconsistency may be due to the low amount of flooding from storms in the Guanica Bay as well as the low amount of boat traffic found in the Bay area. Their may actually be more boat traffic at the Cana Gorda site than near the entrance to the bay which may attribute to heavier wave action and therefore more sedimentation. A similar situation was found in the Ponce submarine Canyon study. Morelock et al. 1989 The highest percentage of a coral species on the site closest to the sediment source, g3-buoy, was Montastrea cavernosa which was expected in a situation which involves heavy sedimentation as this species is better at removing sediment than other more abundant Caribbean coral species. The highest percentage of coral found on the Canyon North site was Montastrea annularis which was not expected as this species is generally more abundant in areas not effected by sedimentation as they tend to not be very sediment tolerant. The highest percentage of coral found on the site furthest away from the sediment source, Cana Gorda, was Porites astreoides which is another coral known to be resistant toward sedimentation. The percentages of all coral species can be found in Table I.
It has been found in areas of high sediment Montastrea cavernosa, Agaricia agaricites, Porites astreoides, and Siderastrea siderea are the most resistant species with a general shift from massive to platy coral as was found in Ponce, Guayanilla, and Costa Rica. Morelock et al. 1979, 1989, Cortes et al. 1985 This is consistent to the findings of this study.
Diversity Indexes were all relatively high with g3-buoy having a total diversity index of 1.59, the Canyon North having a very high diversity index of 2.33, and Cana Gorda having a diversity index of 1.63. The other parameter of this study is to compare the total amount of cover, number of species present, and diversity for the g3-buoy reef site and the Canyon North reef site over a ten year time period between 1988 and 1999.
The results of this study are more consistent with what is expected. The Canyon North showed a marked decrease in total coral cover from 1988 to 1999 from %14.21 to %8.46 respectively. A significant decrease in the percentage of Porites astreoides, Siderastrea siderea, Meandrina meandrites, Colpophyllia natans, and Diplora strigosa were found.
The number of species found in each area remained the same. The diversity index was found to increase from 1.74 in 1988 to 2.33 in 1999 which is not consistent with what would be expected. At the g3-buoy, the total coral coverage is found to have stayed the same over the ten year time period with a drop in species abundance from ten species in 1988 to seven in 1999 with the absence of Diplora strigosa and Colpophyllia natans all together. There is also a significant increase in the percentage of Montastrea cavernosa from 1988 to 1999, which may indicate a switch in ecology to a more sediment tolerant species. The diversity index drops as is expected over the ten-year period. The changes in total cover, amount and diversity between 1988 and 1999 are shown in Table II.
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The diversity of coral reefs and associated fauna varies along depth and habitat gradients, with the richest zones for coral diversity at or just below the zone of strongest wave action. Hughes 1989, Jackson 1991 In very shallow fore reef and lagoonal environments, single species often dominate large areas, while in the deep reef slopes the plating species dominate. The combination of high growth rates and disturbance from storms acts to enhance diversity in the zones affected by wave action. Corals in lagoons, back reef environments and deep reef zones may be severely limited by the low flow characteristic of those habitats, Sebens and Done, 1993 thus limiting the number of species that can persist under such conditions. Habitat alteration that increase sedimentation or reduce flow (breakwaters, coastal modification, eutrophication) can thus have negative effects on coral growth and diversity.
Over the past two decades, most of the "health" of many corals worldwide has deteriorated, as measured by indicators such as the diversity and abundance of reef-building corals (Scleractinians). Rogers, 1985; Grigg and Dollar 1990 Reefs are directly affected by human activities, in particular from overfishing, Roberts 1993; Hughes 1994 sedimentation, eutrophication and pollution. Brown 1987; Sebens 1994
Puerto Rico has coral reefs on the east, west and south coasts, being more common the fringing, barrier shelf and submerged shelf edge reefs. The southwestern area, at la Parguera, has a broad landward indentation fringed with mangroves, mud fiats and shallow saline lagoons. The insular shelf is 8 to 10 km wide and reefs on the shelf are aligned east-west dividing the shelf into distinct reef dominated sedimentary provinces. Morelock et at. 1977 There have been several works that aim to describe the relationships between sedimentation and coral reefs modifications in Puerto Rico. Acevedo et at. 1989; Morelock et al. 1983
This study is aimed to describe and compare the total coral cover, species dominance and diversity (based on coral cover) of reef corals at two different reef areas (at the same depth, 10 m) located at the southern coast of Puerto Rico, such as the Guanica and La Parguera reefs.
Fieldwork at Guanica reefs (being similar for that of the other years and locations) involved the use of quadrats (gridded-frame) that were deployed all along several selected transect areas located at the following sites: Cana Gorda, Canyon, and Buoy. For each of these photoquadrats a line was laid down on the reef bottom (transect), corresponding to the fore reef area and parallel to the depth contour of 10 m, in order to settle a set of quadrats (frames). These quadrats were placed on randomly selected points marked on the guide-line (the number of quadrats varied among the other locations). Each reef coral colony within each of the eight quadrats was identified, as much as possible to species level, properly tagged and its name recorded in a slate. After proceeding to identify corals, then the photographs were taken for each quadrat by using a Nikonos V (28 nun lens and 105 strobe; Kodak ASA 200 color film) from a vertical distance of 1.2 m (giving a photo area 24 inches2).
After the film was developed and prints were made, an outlining of living coral is marked in the photograph and coral identification is written in the photo for each colony that was tagged in field. Then, the photographs were scanned. For those data sets from other years, a set of previously scanned photographs were used and attempts to identify reef corals from that photographs were made as much as possible. After having the photograph files ready, they were imported to another software program in order to do the measurements on coral covers. Then, by using the Canvas 5.0 software program the coral cover for species was calculated. For doing this, each scanned photograph was analyzed by outlining each coral colony and identifying to species according to the tag number on it given during the field survey. When a coral lacked tag number, then the identification was made according to its appearance and morphological characteristics (as much as possible). All calculated data sets on coral cover were then imported to Excel program in order to determine total cover and coral cover percentage per location (Guanica, Parguera). Statistics test used was t test from the SAS program software. The level of significance was ct = 0.05. Total percentage coral cover per species found for the four sites of each reef areas (either La Parguera or Guanica) was calculated by dividing the sum of each coral cover species by the total coral cover obtained for the reef area.
Species richness was obtained for each site and reef area by using a modified form of the Shannon-Weaver diversity index, using number of species and coral cover data determined from the photoquadrats H'c = -SUM(Pi * In Pi); where Pi = Ci/C = cover of corals belonging to the ith species/total cover. The evenness index (J') was obtained for H'c using H'max = ln(# species) by the equation: J' = H'/H'max.
A data pooling was necessary to make the comparisons between four sites for each reef area. For La Parguera sites, Enrique west and Enrique east were pooled into one Enrique site. For Guanica reefs, Canyon data from 1988 and 1999 were pooled into one Canyon site because there was no significant difference between them (t-test); similarly for Cana Gorda and Bean in order to have one Cana Gorda site and also for Coronas 96 and Coronas 97. So, the four sites for each reef area (Parguera, Guanica) are as follows: Turrumote, Enrique, Laurel and Media Luna for La Parguera and Carla Gorda, Canyon, Buoy and Corona for Guanica respectively.
Reef coral Montastrea annularis is the most dominant species for the two reef areas, with a total coral cover of 37.1 in2 (31.8 %) in La Parguera reefs and 37.7 (31.5 %) in Guanica reefs . The six most dominant reef corals per cover (in2) according to reef area after M. annularis are as follows: Agaricia spp. with 25.6 (21.95 %), Montastrea cavernosa with 19.1 (16.38 %), Diploria spp with 10.4 (8.61%), Diploria labyrinthiformes with 6.3 (5.40 %), Porites astreoides with 5.8 (4.97 %) and Diploria strigosa with 3.4 (2.91%), all for La Parguera reefs. While Agaricia spp. with 15.52 (12.72 %), Porites astreoides 13.47 (11.26 %), Colpophyllia natans with 12.38 (10.35 %)Montastrea cavernosa with 8.94 (7.47 %), Siderastrea siderea with 7.82 (6.54 %) and Porites porites with 7.51 (6.28 %), all for Guanica reefs.
Reef coral M. annularis has the highest coral cover in Turrumote (15.5 in2) and in Canyon (20.87 in2) (Fig. 3). M. cavernosa has highest coral cover in Media Luna (8.5) and in Buoy (6.17). Agaricia spp. is highest in Enrique (21.2) and in Corona (7.7). P. astreoides is highest in Media Luna (2.1) and in Cana Gorda (5.24) (Fig. 3). C. natans is highest in Corona (5.49), but not so high in Media Luna (0.7). S. siderea is high in Cana Gorda (3.61), but not so high in Laurel (0.9). Total coral cover per site is highest for Enrique (46.7; 40.5 %) and Canyon (46.46; 38.87 %), while the lowest is for Laurel (20.5; 17.58 %) and Buoy (16.18;13.53 %). There is not a significant difference in coral cover between the seven more dominant reef corals when compared among reef areas (Parguera-Guanica, t-test). By pooling data for comparing total coral cover per reef area (four sites per reef area, the eight sites included) there is not a significant difference (t-test, Least significance difference LSD= 21.49).
Regarding the modified diversity index based on coral cover (Table 3 and 4), the highest diversity value is for Cana Gorda site (H'c = 2.147; total coral cover (tcc) = 25.45 in2; 21.29 %) from the Guanica reefs, with an evenness value (J') of 0.864. The lowest diversity value is for Turrumote site (H'c = 0.947; J' = 0.411; tcc = 21.7 in2; 21.7 %) from La Parguera reefs. The highest diversity value for La Parguera is in Media Luna (H'c = 1.773; J' = 0.626; tcc = 27.7 in2; 23.75 %). The lowest diversity value for Guanica is in Buoy site (H'c = 1.604; J' = 0.824 tcc = 16.18 in2; 13.53 %). The total diversity value for each reef area (pooling the eight sites) is H'c =1.990 (J'= 0.654) and H'c = 2.198 (J' = 0.746) for La Parguera and Guanica respectively. There is not a significant difference (t-test, LSD = 0.547) between these diversity indexes when pooled in order to be compared between reef areas (La Parguera and Guanica).
All the slightly differences involved in coral cover and diversity are definitely due to the number of species involved in those sites related to their different .types of growth and interspecific relationships. Scleractinian corals may exhibit several growth forms and all the specific forms they display can be often attributed to the surrounding conditions of the environment in which they grow, Barnes 1973 their spatial distributions Dana 1976 and their species specific interactions. Lang, 1973 In addition, it has been found that coral reefs face a great problem due to the excess in terrigenous sediment influx, which contributes to shape coral reef structure and position in reference to the coast. An inverse correlation between natural sedimentation and coral growth rates has been described. Dodge et al. 1974
The reefs at Parguera are considered to be free of terrigenous sediment influx, Morelock et al. 1977 so it is possible that coral cover differences between the four sites analyzed here in Parguera are due to influences other than terrigenous influx. On the other hand, the reefs at Guanica are probably to be more influenced by terrigenous sediments due to they are located relatively nearby the mouth of the Guanica embayment. This is probably explained by the fact that the lowest total coral cover found is precisely at the Bouy site, which is located just in front of that embayment. There is evidence, from a nearby place, that high levels of terrigenous sedimentation tend to affect the reef growth in the surrounding environment; such a place is in front of the Guayanilla Bay. Morelock et al. 1977
Total coral cover for Parguera is slightly similar to that of Guanica, but this latter being surprisingly higher than the former. This result could be explaining, at some point, that the probable terrigenous sediment influx on Guanica reefs is not so high as to affect total coral cover on the sites analyzed. Nevertheless, although there is not a significant different between total cover of the sites, this result has to be taken with caution, since data pooling (data from different years) could be masking the real condition of the sites under analysis. On the other hand, there is not more accurate data as to unequivocally conclude what are the reasons for this result.
The highest total coral cover found in Enrique is mainly accounted for by the presence of the reef coral Agaricia spp.; while the total coral cover in Canyon is accounted for by Montastrea annularis. This latter reef coral species is the most dominant in coral cover of the two reef areas, having a slightly more high coral cover in Parguera than in Guanica. M. annularis and Agaricia spp. are the two species that were found to dominate the slope zone at 22 to 24 m for Parguera. Morelock et al. 1979; Acevedo et al. 1989 However, for this work at the 10 m depth zone not only for Parguera, but also for Guanica those reef corals are dominant. That M. annularis has the highest coral cover for this work could be probably due to some taxonomic bias. It has been found that M. annularis is considered to be a species complex comprised by at least three sibling species (M. annularis in itself, with M. faviolata and M. franksi, Knowlton et al. 1992; Weil and Knowlton 1994 which are differentiated by the type of growth mainly. They are relatively similar to an unfamiliar eye, so it is easy to include the three species just in one, M. annularis. Nonetheless, for file Canyon site, at Guanica, at least M. franksi could be identified.
It has been found that reef coral species have resistance to sediment stress up to a certain critical level. Rogers 1979 So, on this regard, Acevedo et al. 1989 found at Ponce reefs that M. annularis and Agaricia agaricites decreased significantly with increased sediment conditions, having the main loss in coral cover for M. annularis. In this work, there is not a significant difference in coral cover between sites. This could probably be explaining that the terrigenous sediment influx is not so high for the Guanica area as to influence coral cover of M. annularis. However, it is necessary to have more accurate data as to conclude something about this fact.
Regarding M. cavernosa, it is found that its cover is highest at the Media Luna site (Parguera), being its total coral cover higher at La Parguera than Guanica. Morelock et al. 1979 found that the last surviving colonies in areas of high sediment in southwestern Puerto Rico were M. cavernosa, Siderastrea siderea, Agaricia agaricites and Porites astreoides. However, in this work, the coral cover of M. cavernosa at Guanica is not so high as that of La Parguera. This latter reef coral is considered to be one of the most sediment-resistant of the scleractinians. Lasker 1980 P. astreoides and Siderastrea siderea have higher coral cover at Guanica than that of La Parguera. These corals were also found to be sediment-resistant at certain level, because they were found as to be last surviving in high sedimentation areas. Coral covers of Diploria species (D. clivosa, D. strigosa and D. labyrinthiformes) are very low not only at La Parguera, but also at Guanica.
The method of photoquadrat for analyzing coral cover proved to be one well suited for the purposes aimed here. This method is considered to be one of the most recommended by some authors who aim to work with reef coral cover measurements. When well applied this procedure can collect a long-lasting and re-checking photographic material. Shortcomings in taking data from photquadrats involve: small or inconspicuous corals can not be taken into account, shadowing colonies by octocorals (gorgonians and plexaurids), pictures out of the frame due to marine current effect, photo definition, mislabeling tags and canopy effect from other stony corals.
Shortcomings in determining coral cover from photographs (using the Canvas software program) are: coral cover measurements from the photographs don't take into account perspective, so, branching and massive corals are problematic as to being measured with precision.
All the aforementioned coral cover results have to be taken with certain caution. This is due to the fact that most of the data analyzed came from previously taken photographs in past years. From this photographic material several corals had to be identified as much as possible without fieldwork support, so in some cases coral identification was not so easy to properly be determined because of either photo definition. So, there were some coral colonies that could not be identified. Nevertheless, that situation was not of great concern for this analysis. However, It would be recommended for next research to take into account a very precise reef coral identification in the field for avoiding discarding of non-identified.
The comparison among the areas of Guanica and La Parguera is considered to be a preliminary one. On this regard, it would be strongly recommended to have a time-series monitoring database exactly for the same sites at different years (at least 2 consecutive years) in order to have more elements to do an exhaustive analysis on coral cover comparisons in relation to sedimentation influxes among these locales. Nevertheless, the present analysis gives some idea about the current situation on coral cover of two relatively small coral reef areas.
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