Effects of Benthic Macroalgae on Coral Recruitment

Increased sea-surface temperatures cause large scale bleaching events that lead to coral mortality, which is usually followed by dominance and recolonization by benthic algae. The physical structure and the chemical composition of the algae can have far-reaching effects on the level of coral recruitment, and can either enhance or inhibit coral recovery. Diaz-Pulido et al. (2010) studied the effects of eleven species of benthic macroalgae on the swimming activity and larval settlement of Platygyra daedalea in the Great Barrier Reef. They discovered that the macroalgae significantly affected the larval settlement rates and swimming activity of the coral. The upright fleshy macroalgae significantly reduced the larval settlement of the coral, while the algal turfs and crustose coralline algae enhanced the larval settlement. Thus, the results indicate that some macroalgae can increase larval settlement rates and consequently, enhance coral recovery and resilience. .— Sachi Singh
Diaz-Pulido, G. Harii, S. McCook, L. J. Hoegh-Guldberg, O. 2010. The impact of benthic algae on the settlement of a reef-building coral. Coral Reefs 29, 20308.

The nature and composition of the algal community is critical in the process of coral reef recovery. Previous studies have shown that different macroalgae have different effects on the rates of coral settlement but these studies have been limited in their approach. Diaz-Pulido et al. studied the effects of eleven species of macroalgae on the settlement of spawning coral Platygyra daedalea, which is found commonly in the Great Barrier Reef and the Indo-Pacific. The authors used discs of Porolithon onkodes as settling agents for the larvae and prepared treatments with different macroalgae in order to compare the differences in larval activity and settlement rates They used plastic algal mimics to study the effects of the physical structure of the macroalgae on the rates of coral settlement. A few days after the treatments, the number of larvae settled on the Porolithon onkodes discs and the number of larvae swimming at the surface of the water were counted. The authors found that 3% of the coral larvae settled in the plastic mimic treatment and 30% of the coral larvae settled in the Porolithon onkodes treatment; these results indicate that calcareous and upright fleshy algae inhibit the settlement of coral larvae while algal turfs and crustose coralline algae enhance the larval settlement. The authors suggested that the larvae might sense the topographical changes in the surface and might try to avoid the upright physical structure of the fleshy macroalgae. Since the Porolithon onkodes has a high deposit of magnesium calcite, it calcifies solidly and provides a good framework for the coral larvae to settle on. Thus, while the upright fleshy algae inhibit Platygyra daedalea settlement, the coralline algae and algal turfs increase the larval settlement and can be used to replenish degraded coral habitats and promote coral recovery. 

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