Coral reef recovery and resilience on patch reefs in Kāneʻohe Bay, Oʻahu

Abstract

The resilience of coral reef ecosystems has been studied and discussed in the literature for over four decades: however, discussion of resilience in coral reef communities has been limited due to the small number of long-term studies that span decades. Increasing frequent and severe disturbances such as major weather events, fresh water kills, and increase in nutrients and sedimentation from surface runoff are causing increased stress to the Kāne`ohe Bay marine ecosystem and negatively impacting the coral reef community. Two events released corals from competition for substrate and initiated the recovery of coral cover on Kāne`ohe Bay patch reefs; the diversion of sewage and associated nutrients outside of the Bay by 1978 and the virtual disappearance of the native macroalga Dictyosphaeria cavernosa (Forsskål) Børgesen in the spring of 2006. The resilience, or the recovery of, Kāne`ohe Bay reefs may depend on the ability of remaining corals to vegetatively grow and outcompete macroalgae that once dominated the Bay and on their larvae to successfully settle and recruit on substratum. At the same time, the composition of the macroalgal and herbivore communities that occupy the reefs are critical to the outcome for the reefs. In the 45 year period from the mid-1960's to 2011, mean coral cover has increased at annual rates of 1 to 4% in the intervals following release from competition, comparable to the published rates of recovery observed at other sites following acute disturbances. Five years after the mass disappearance of D. cavernosa, the two dominant corals in the Bay showed the maximum gains in percent cover. These two species represented the reef-building corals that have been present in the Bay historically, Porites compressa Dana and Montipora capitata Dana. Although the diversity of the other former coral species in the Bay have decreased over time, the resilience of these remaining species on reef flats and reef slopes throughout the Bay may be key in the future as impacts from global climate change increase.