Site: Mo'orea Coral Reef LTER
The offshore fore reef on the north shore of Moorea a year prior to the start of a large perturbation (an outbreak of crown-of-thorns seastars, was followed immediately by Cyclone Oli) that killed almost all of the coral on the fore reef around the island. The live coral shown in this photograph covers about half of the bottom. Time series data for several previous disturbance events on Moorea indicate that coral on the fore reef returned to its pre-disturbance levels within about a decade and that macroalgae were not able to overgrow the reef following these sudden losses of coral. MCR investigators found that the increased benthic production following the widespread loss of coral fueled a rapid increase in biomass of herbivorous fishes, primarily parrotfishes, that kept macroalgae in check.
Russell J. Schmitt

Tropical reefs around the world are being disturbed more and more often by events that greatly reduce the amount of living coral on reefs. In some cases the disturbed reef returns to its previous state of high coral cover; however, in other instances, the reef becomes dominated by persistent stands of macroalgae that greatly slow or prevent the re-establishment of coral. Macroalgae can overgrow a tropical reef when herbivores fail to keep them in check, so MCR scientists were particularly interested in determining what influences whether and how herbivores respond to the rapid, large-scale loss of coral.

The island of Moorea in French Polynesia is an ideal natural laboratory to explore the processes that affect the capacity of tropical reefs to return to coral dominance following disturbance events. One reason for this is that much of the usually high cover of live coral on the outer (fore) reef around the island has been killed by large disturbances on several occasions over the past four decades. Times series data show that, following each of these major disturbance events, the cover of coral on the fore reef returned to its high, pre-disturbance level within a decade, in large part because the disturbed fore reefs of Moorea never became overgrown by macroalgae.

A recent large disturbance at Moorea gave MCR investigators the opportunity to understand why macroalgae has not flourished following the sudden loss of coral on the fore reef. Macroalgae were kept in check because the numbers and body sizes of herbivorous fishes, primarily parrotfish, increased tremendously very shortly after the disturbance. The positive response of herbivorous fishes to increased benthic primary productivity associated with coral loss offshore was driven largely by parrotfishes that initially retreat to nearshore nursery habitat within the lagoon before moving to offshore reefs by the time they reach the age of one year. The nearshore nursery habitat for parrotfish is far more resistant to large-scale disturbances that kill coral on the fore reef, but is relatively more sensitive to localized perturbations associated with local human activities.

These results underscore the importance of connectivity between the lagoon and offshore reefs for preventing the establishment of macroalgae following disturbances to the fore reef. They also indicate that protecting nearshore nursery habitat of herbivorous fishes is critical for enhancing reef resilience. Current management strategies focus heavily on governing fishing pressure on herbivores. MCR’s research indicates the vital need for more ecosystem-based management strategies that move beyond fishing to include consideration of essential nursery habitat and critical connectivities.

Graph for
Temporal trends in the cover of live coral (blue diamonds) and macroalgae (green squares) on the ocean floor, as well as biomass of herbivorous fishes that consume algae (red dots), on two major habitats of Moorea: the offshore fore reef (panel a) and the nearshore fringing reef (panel b) just prior to and following perturbations that began in 2007. Shown are the mean ± 95% confidence limits calculated from time series data collected at six locations distributed around the island. The decline in coral on the fore reef was caused by a brief population outbreak of the predatory crown-of-thorns seastar that began in 2007 and ended in 2010; Moorea also was hit by high waves from Cyclone Oli in February 2010. Both of these perturbations harmed coral on the fore reef (panel a), but had little effect on corals in the lagoon (panel b), some of which serve as the nursery habitat for young parrotfishes. Parrotfishes move to the fore reef as they grow, where they keep the growth of algae in check.
modified from Adam et al. in review