Degraded coral reefs experience a transition from a dominant reef building community to one overrun by marine macroalgae. This shift affects fish recruitment, coral recruitment, competition, and predation and inevitably leads to an unstable ecosystem. Bahartan et al. studied the algae-coral interactions of the reefs on the coast of Aqaba, Jordan, and the coast of Eilat, Israel. They found that the proliferation of species of red algae as well as the most dominant species of turf-algae, Sphacelaria sp., was not associated with a sudden environmental disturbance, like a mass bleaching event. The red algae do not overcrowd the coral because they grow in spaces that have been vacated by other species. However, since they are fast growing, resistant to most environmental disturbances, and have an increased ability to absorb light in turbid water, the red algae continue to proliferate and grow in the shallow areas of the Eilat. The authors found that the reefs on the Eilat coast had a significantly higher percentage of algal cover in comparison to the reefs on the Aqaba coast. The algae harms coral recruitment and reduces the survival of crustose coralline algae, which contributes significantly to coral calcification and induces larval settlement of corals. The loss of coralline algae could also lead to a weakening of the reef structure and the ecosystem as a whole. Upon further examination, the authors concluded that the reefs on the Eilat coast were degraded, while those on the Aqaba coast were relatively healthy. Thus, the results revealed an algal takeover and a shift in the Eilat reef. Even though the mechanism of the algal takeover is unknown, the authors claim that examination of the coral-algae balance can provide a good indicator of early instabilities in the reef community.
Reef management is a good way to rehabilitate degraded coral reefs; since we cannot identify the early signs of degradation, it is difficult for us to predict such a decline in reef health. To help identify these early warning signs, Bahartan et al. (2010) studied the effects of macroalgae—Sphacelaria sp and red algae—on the relatively healthy corals of the Eilat, in the Red Sea. They found that the Sphacelaria sp. did not outcompete the coral reefs for space, as they expected, and are not the cause of coral mortality on the coast of Eilat. However, upon further examination of the Eilat reefs, they concluded that the transition of a coral-dominated reef to a turf-dominated one clearly indicates the degradation of a healthy reef. Thus, the turf-algae imbalance can provide an early indication of reef instability. .— Sachi Singh
Bahartan, K., Zibdah, M., Ahmed, Y., Israel, A., Brickner, I., Abelson, A. 2010. Macroalgae in the coral reefs of Eilat (Gulf of Aqaba, Red Sea) as a possible indicator of reef degradation. Marine Pollution Bulletin 60, 759–764.