Monitoring Change in Coral Colonies by Changing Physical Environments

Jack Morelock and Wilson Ramirez

Grant No. : Evaluation of the effects of sedimentation and nutrients on coral and the long term monitoring of recovery after environmental mediation.

Abstract

To determine the effect of degrading or improving water quality on coral reefs, we have set up an experiment as part of a broader investigation of coral in which we are determining the effects of sediment and nutrient stress on living coral cover. We selected sites on the shelf off Mayagüez, Puerto Rico where four sites of coral reefs are impacted by terrigenous sediment and/or nutrient influx, and one clean water site. To measure recovery of individual coral removed from stress conditions and the effect of stress conditions on healthy corals, we transplanted corals from stressed environments into the clean water environment. These transplants were onto concrete slabs to avoid local bottom effects. Coral from the clean area were also transplanted as controls. We transplanted coral from the clean environment into stressed conditions at the other sites to observe the effects on these coral. Alizerin red was used to mark the coral at the time of transplant. Twelve species of coral were transplanted from clear water to stressed habitats and four of these species were transplanted from stressed environments to clean water. Photographs were taken bi-monthly to observe changes. At the end of the study, growth rates were measured. The study ran for two years, showing the following effects:

Coral Reef Surveys

Coral reef surveys were made at 10 m at the following sites to determine the basic habitat environment (Figure 1): From Punta Jiguero to Punta Guanajibo the shelf is narrow (Figure 2) . turbid and deposition of fine grained sediments on the coral reefs is high (greater than 10 mg/cm2/day). The sediment facies mapped by Morelock, et al. 1983 shows a large area of fine-grained terrigenous sediments in both bays (Figure 3) . Additional sediments are added from discharge of the three local rivers and during periods of high rainfall, the bays are very turbid. Sediment resuspension is a common occurrence.

The coral surveys for both Bahía de Añasco and the carbonate platform west of Punta Guanajibo were published in Morelock et al. 2001

At 10 m, the reefs of Bahía de Añasco have less than 5 percent total coral cover except for Manchas Interiores (13%) compared to 16 percent at Escollo Negro (Figure 4) . The cover by species shows low diversity at the Añasco and a much higher diversity shown for the cover by species at Escollo Negro (Figure 5).

Coral Transplants

Four species of coral (Montastraea annularis, Montastraea cavernosa, Porites asteroides, Siderastrea siderea) were collected at Escollo Negro and dyed with Alizerin Red to mark surface at time of transplant. They were transplanted to Manchas North, Manchas Interiores 8, Algarrobo and to control slabs at Escollo Negro. We also collected colonies of: Porites porites, Madracis decactis, Agaricia agarcia, Stephanocoena intersepta, Diploria strigosa, Colpophyllia natans, Dendrogyra cylindrus and Meandrina meandrites. These were transplanted to concrete slabs at the sediment and nutrient distressed sites.

The same four species were collected from the three stressed sites and transferred to concrete slabs at Escollo Negro. We then had two conditions:

section describing coral responses

Sediment Influx

discussion of sediment traps. rate of sediment influx, composition of sediments and grain size.

Nutrient Levels

results of analysis of sediments from the reefs

Coral Growth Rates

coral growth rates from photographs and from x-ray analysis. discussion
also include concept:

high resistance of Siderastrea siderea and Porites astreoides to sedimentation as well as their slow extension rates even in sites where the entrance of terrigenous sediments is negligible. Also, even though colonies of the massive reef-builder Montastraea annularis can keep their slow growing rate in sediment-stressed reefs, their cover is greatly reduced. Cortés and Risk 1985 found a definite relationship between sedimentation rates and linear extension rates for M. annularis (Figure).

The relationship of the amount of terrigenous sediments collected in the sediment traps and the coral cover by species and the extension rate of the three species do produce interesting conclusions. Montastraea annularis cover is drastically reduced in areas with high terrigenous sediment influx. Cover by Porites asteroides and Siderastrea siderea remains unchanged indicating that these coral are resistant to (not affected by) stress from an influx of terrigenous sediments. The Montastraea annularis colonies that remain alive at stressed sites and Porites asteroides and Siderastrea siderea continue to have an extension rate relatively unaffected by the sediment stress. Sediment stress can reduce the coral cover by certain species (including Montastraea annularis) but does not have an appreciable effect on growth rate (as indicated by lineal extension rates). This suggests that the actual colonies living at sediment-stressed reefs can survive for some period of time, yet, this may not be the case for longer exposures to high sediment influx as suggested in earlier findings during a long-term time-series analysis. Torres, 1998, 2001 Most probably their recruitment rates are more affected since available substratum may be covered by fine terrigenous muds and turf algae, which impedes the settlement and later development of some coral species.

Discussion and Conclusions

(Figure )