by Natalie Ireland
Coral reefs are regularly disturbed by natural phenomena such as bleaching, storms, and outbreaks of predators, such as the corallivorous sea star Ancanthaster planci. Corrallivores are animals that eat coral polyps. Coral reef ecosystems are resilient, and are often able to recover from large-scale disturbances quickly. However, anthropogenic stressors such as overfishing, nutrient enrichment, and sedimentation can prevent coral reefs from recovering. Nutrient enrichment, caused by terrestrial runoff, creates the perfect environment for benthic algae to grow on disturbed and broken coral reefs. Overfishing, working in tandem with nutrient enrichment, causes an overgrowth of algae if there are not enough fish to graze it, and the successive degradation of the reef. Sedimentation is also another side effect of terrestrial runoff. Sedimentation buries corals, which blocks light from reaching them and potentially stops coral recovery. However, sedimentation, when not paired with any other stressor, can also stop the growth of algae by burying surfaces for algae to grow on. Gil et. al. (2016) set out to test the interactive effects that overfishing, sedimentation, and nutrient enrichment have on coral reefs in French Polynesia. They hypothesized that these anthropogenic disturbances, when working interactively, will negatively impact corals, while promoting algal cover.
To test their hypotheses, the authors used two different corals, Acropora pulchra, a thin-branching coral, and Porites rus, a bulbous coral to test the effects of these anthropogenic stressors on corals with different morphologies and ecological functions. This study found that nutrient enrichment and overfishing together resulted in the largest increase in algal turf, while sedimentation, paired with either nutrient enrichment or overfishing, decreased algal turf growth; macroalgae biomass growth was significant when overfishing occurred, but insignificant when nutrient enrichment or sedimentation occurred. The coral species A. pulchra was unable to be tested for survival because during the experiment, 100% of the tested corals were eaten by corallivores. The coral species P. rus was tested for percent survival in each anthropogenic stressor scenario. Sedimentation and overfishing working together did not hinder the coral from recovering. This scenario had the most live coral cover by the end of the experiment, while the other scenarios also had a lot of live coral cover, but proved to be statistically insignificant. This study was important because it showed that different coral morphologies act differently when under the same anthropogenic factors.
This study showed that while nutrient enrichment and overfishing can cause both algal turf and macroalgae growth in disturbed coral areas, it also showed that high amounts of sedimentation can prevent algal growth in most coral reefs. Coastal ecosystem managers can use this information to better understand and control benthic algal blooms. Controlling terrestrial activities that create runoff and controlling overfishing could be the answer to suppressing algal blooms that are preventing coral from recovering after an initial natural disaster.
Gil, M.A., Goldenberg, S.U., Bach, A.L., Mills, SC., Claudet, J., 2016. Interactive Effects of Three Pervasive Marine Stressors in a Post-Disturbance Coral Reef. Springer 35, 1281-1293.