Study of Community Structure

This study of community structure of hermatypic coral reefs in terms of cover, species composition, dominance, and vertical as well as horizontal variations was done on Enrique Reef. Photo quadrats taken horizontally over a distance of about 20m at depths of 4, 6, 8, and 10 m were used. The coral community was measured at three sites on the reef: west end, middle, and east end, each about 460 m apart.

In addition to a vertical reef front zonation, significant horizontal changes were found which is an indication that extensive sampling is required to characterize the reef. The causes of changes are numerous and interactions so complex that it is difficult to explain the causes of the changes.

The reef slope was measured and compared to coral cover. The data indicate that slope changes can affect coral distribution, but definitive proof was not sufficient to make a firm conclusion.

Introduction

There are two common questions for an ecological investigation of marine communities: Enrique Reef was surveyed at four different depths in three separate sites. The percent of coral cover and number of colonies of selected species were analyzed to determine if there was a significant difference in their variability at each site at a specific depth, and if a significant difference existed between three sites on this reef.

The study determined the magnitude of species difference between sites and depths on the same reef and accounted for these differences. The causes of these differences were not part of the goal, but the slope of the reef surface at the different sites was measured and compared to the data.

Enrique Reef (17-57-15 N; 67-02-55 W) is approximately 0.7 km south of Isla Magueyes at La Parguera, Puerto Rico. The reef is located on a carbonate shelf that averages 18-20 m depth. It is aligned east-west direction with a length of 1.4 km and 0.4 km width. At the eastern end of the reef, the front is relatively broad and gently sloping to a depth of 6 m where the slope becomes steeper. The upper low sloping reef front is narrower to the west and the change to a steeper slope occurs at a shallower depth.

Distribution of scleractinean corals and other organisms in the coral reef has been related to a great variety of factors including depth, available light, substrate slope, sedimentation, wave energy, and biological interactions. It is difficult to find areas where these factors are alike, but on Enrique Reef these factors approximate equality. Despite this, zones and associations of coral reef species can be recognized from one reef to the other and also in different geographic areas. Regional variations exist, especially when there are qualitative differences in the composition of the hermatypic species.

Methods

Photo transects were used with two photos combined to make 1.4 m2 quadrats for each five quadrats of a transect. After the quadrats were run, the six dominant taxa for cover were selected for cover and colony numbers. When this was done only eight percent of the total species cover was deleted per transect and ten percent of the total number of colonies, indicating that if six taxa were selected, most of the dominant species on the reef were represented. The six taxa from each quadrat were then pooled and twelve dominant taxa obtained. Ten of the twelve were the same for both cover and colonies, suggesting that the dominant species in cover are also the dominant in number of colonies.

The two transects at the same depth in the same site were so similar that they were pooled to form one 14 m2 transect, and the same approaches were used to compare transects at the same depths among the three sites. - confusing, clarify

Results

There were few differences in species cover and number of colonies between transects 1 and 2 at the different depths. In most cases, there were almost no differences in species cover and when there were differences they were very small percentages. The largest differences between transects were caused by differences in Montastraea annularis between one transect and the other. IAt 10 m, differences between transects diminished in magnitude.

The results showed that coral species populations do not change between the contiguous transects taken at each depth in the three sites because significant tests on variability were very low and there is a probability that some of the small differences could be caused by multiple testing error due to the large quantity of tests.

The results show large differences in Acropora palmata cover and colony numbers between the east site and the other two, with the east site being considerably higher. This was probably a function of preference for shallow water and slightly higher wave energies. At the west site, Montastarea annularis was dominant in colony numbers to 6 m and in cover to 8m, but Agaricia agaricites dominated below 8m in colony number and cover. At the middle site, Montastraea annularis cover dominated from 4 to 8m, but Agaricia agaricites dominated again at 10m with a strong showing of Montastraea cavernosa also. Montastraea annularis cover was dominant on the three sites from 4 to 8m averaging about 7.5% cover. At 10m the Montastraea annularis cover diminished and the other species increased in cover.

Depth preference by Montastrea cavernosa is seen in that its presence and cover was very low at depths above 6m, but more abundant deeper than 8m. Montastraea cavernosa cover was also lower at the west site at almost all depths.

The statistical analyses clearly indicate that differences between sites at specific depths were present. There was definitely a gradual change from one extreme of the reef in respect to the studied coral species populations. The presence of these changes was confirmed in all tests made in this study (one-way ANOVA).

The platform was broad in the east and became gradually narrower to the west. The platform can be divided into two slope areas; a shallow zone with a gentle slope and a deeper zone with a steeper slope. The slope of the shallow and deeper areas was almost the same at the three sites, but the change in slope occurred at a different depth for each site, with the depth of the slope break deepening to the east. Corals at the 4 and 6m depths at the west and middle site are on the same slope, but at the east site is on a different slope. ANOVA results indicate that there is a difference at 4m for coral colonies and for coral cover indicating that species colony variability between sites was very large at that depth, but cover variability was not.

There is no proof from the data that relates coral changes in colony number or coral cover to the changes in slope. But significant tests at 8 and 10m were produced by differences (cover and colonies) in all three sites without any specific pattern and this is not the case for 4 and 6m depths were all significant tests were caused by changes on the middle and east sites with most of them in the areas where the slope is breaking. The results seem to support the possibility of slope changes as an important factor causing differences in coral reef populations.

Conclusions

Coral species cover and colony number were found to change horizontally when distances between sites were more than 460m. More striking differences were found comparing the east and west sites which were 920m apart. Nine out of twelve species studied were found to change significantly in cover and colony number when the sites were compared (table 1). Some of the most dominant species in both parameters present considerable changes from one site to the other and in a number of cases, the middle site values were intermediate, indicating gradual changes as distance increased. Neighbor transects at any one site at the same depth had no horizontal differences that were significant. Variability seems to be affected when abrupt slope changes appeared. There were also coral populations of some species that remained almost the same over the entire 1.4 km sampled distance. These species seem to be less sensitive to the changes that made the other species populations differ. --Were these the same species that showed sediment resistant in Acevedo et al. 1989

coral cover number of colonies
4m 6m 8m 10m 4m 6m 8m 10m
Montastraea annularis #
Montastrea cavernosa # # # #
Agaricia agaricites # # #
Porites astreoides # # # # # #
Porites porites #
Siderastrea siderea # # # # #
Colpophyllia natans
Diploria strigosa
Diploria clivosa
Diploria labyrithiformis #
Mycetophyllia sps #
Acropora palmata # 3
# - significant probability of difference between sites

Higher percent of cover by Acropora palmata at the east and lower cover to the west by several species suggest that conditions were more favorable at the east site.

Different species varied at different rates with distance. Sample size and method of survey will affect results. Significant horizontal variation was found in 33 percent of the statistical tests for coral colony number and 20 percent on the cover between sites. The data show that more than one location should be sampled on a reef to describe community structure and zonation because there may be horizontal variation present as well as vertical.