Inhibition of early life stages of corals by benthic algae is a critical bottleneck to the recovery and resilience of corals. Increasingly frequent and severe disturbances are causing largescale coral mortality, usually followed by colonisation and dominance by benthic algae. The capacity of corals to re-establish in such algal-dominated habitats will depend on the effects of the algae on growth and survivorship of juvenile corals. We experimentally evaluated the competition between juvenile corals Porites astreoides and algae and the effects of algae on the exposure of juvenile corals to damage by parrotfishes (family Scaridae) in a Colombian Caribbean reef. We also explored whether those effects were consistent among climatic seasons (upwelling and non-upwelling). Benthic algae had negative and positive effects on the juvenile corals. The removal of algal turfs and fleshy macroalgae enhanced coral growth. Unexpectedly, removal of algae from around the juvenile corals increased predation upon the corals by parrotfishes. When algae were removed, at least 50% of the corals were grazed by parrotfishes, but no bites were observed on corals with intact algae. Coral growth and parrotfish damage were not affected by season. However, damage by parrotfishes neither lessened survivorship nor resulted in negative growth for any of the grazed coral colonies. The beneficial effects of algae in protecting the juvenile corals from parrotfish damage seem to be outweighed by the negative effects of the algae on coral-algal competition. Understanding such complexities in the interactions between algae and early life stages of corals can provide insight into the roles of algae in coral reef resilience.
Sedimentation is frequently associated with coral reef degradation. However, there are few experimental studies evaluating the impacts of this process on the dynamics of algal recruitment on coral reefs. In a field experiment, we manipulated the levels of sedimentation and examined the impacts on the recruitment and growth of the brown macroalgae Dictyota spp. and Lobophora variegata, in two localities (Chengue and Granate) on the Tayrona National Natural Park, Colombian Caribbean. We found considerable variability in the responses of algae to sedimentation. This variability depended on the levels of sediments used, the type of algae employed and the population parameter considered (recruitment or growth). Sediment addition generally had a negative effect on the recruitment and early growth of the alga Dictyota spp. In contrast, sediment addition did not affect the recruitment of L. variegata, while sediment removal had a negative effect on recruitment. The locality with higher sedimentation rates (Granate) generally had lower recruitment and growth of both algae than that of the other locality with lower sedimentation levels (Chengue). Our study indicates that there is substantial variability and complexity in the responses of reef algae to the effects of sedimentation. The results also suggest that the process of reef degradation can modify early population dynamics of benthic algae, with important implications for understanding the ecology of reef degradation.
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