Reef corals contain symbiotic, single celled algae (zooxanthellae) that provide over 90% of a coral’s energy budget. Coral bleaching is the loss of symbiotic zooxanthellae or the photosynthetic pigments from individual zooxanthellae, and results in an energy drain on the coral that can lead to reductions in growth, reproduction or even death.
Coral reefs in Hawaii provide critical services including commercial, recreational and subsistence fishing, create world-famous surfing and diving locations and are vital to Hawaiiís approximately $800 million a year marine tourism industry. Despite their economic significance, reefs are experiencing high levels of anthropogenic stress from ever-increasing population pressures.
The human-impacted reefs of the main Hawaiian Islands (MHI) are severely over-fished with significant differences in the density, size, and biomass of shallow reef fish assemblages compared to the pristine reefs of the northwestern Hawaiian Islands (NWHI). Sediments, nutrients and other pollutants from a variety of land-based activities also threaten reefs and consequently, Hawaii's reefs are starting to show significant decline. Several reefs on Maui have lost nearly 25% of their living coral between 1994 and 2006 primarily as a result of human influence. The most dramatic decline was at Honolua Bay, Maui where coral cover dropped from 42% to 9%.
Figure one shows Honolua reef compared to a healthy reef on Maui. In addition to managing local stressors, coral reef resource managers are also tasked with planning ahead to address the anticipated problems associated with global climate change, such as increases in coral bleaching and disease.
Coral Bleaching Reef corals contain symbiotic, single-celled algae (zooxanthellae) that provide over 90% of a coralís energy. Coral bleaching is defined as either the loss of symbiotic zooxanthellae or the loss of the photosynthetic pigments from individual zooxanthellae. This results in an energy drain on the coral that can lead to reductions in growth and reproduction and possibly death. Although coral bleaching can occur in response to stresses such as changes in salinity, light or irradiance, mass bleaching events are usually associated with increased sea surface temperatures (SST).
In 1997-98, mass bleaching occurred on reefs throughout the world due to increased sea surface temperatures associated with an El Nino event where an estimated 16% of the worldís coral reefs were lost (Wilkinson et al., 1998). Severe bleaching can result in the loss of live coral and a general decline in the integrity of coral reef ecosystems. It is predicted that the impacts of global climate change will result in more frequent and extensive bleaching episodes (Hoegh-Guldberg, 1999). Hawaii's coral reefs were not affected in the 1998 mass-bleaching event but coral bleaching has occurred in both the main and northwestern Hawaiian Islands on other occasions.
The first large-scale coral bleaching event in Hawaii occurred predominantly in Kaneohe Bay in 1996. In addition, a mass-bleaching event, also due to increased SST, was documented in the northwestern Hawaiian Islands (NWHI) in 2002 and 2004. Jokiel and Brown (2004) found that the sea surface temperatures in Hawaii have been steadily increasing over the past several decades and predict that if the warming trend continues, bleaching events will continue to occur in Hawaii with increasing frequency and severity.
Disease can be defined as any impairment of vital body functions, systems, or organs. The causal agent of a disease can be either biotic or abiotic. Biotic diseases are those in which the causal agent is a living organism such as bacteria, viruses or macro-parasites. Abiotic diseases result from environmental stressors, such as changes in physical conditions or exposure to toxic chemicals.
There has been a worldwide increase in the reports of diseases affecting marine organisms. In the Caribbean, mass mortalities among organisms in reef ecosystems have resulted in major shifts in community structure. For example, the mass mortality of sea urchins in the early 1980s throughout the Caribbean resulted in massive algal overgrowth and contributed to phase shifts from coral- to algae-dominated reefs (Hughes, 1994; Lessios, 1988). Disease has also been implicated in the dramatic decline of acroporids, one of the major frame-building corals in the Florida Keys, changing the structure and function of the coral reef ecosystem (Aronson and Precht, 2001; Patterson et al., 2002).
It is predicted that the changing environmental conditions associated with global climate change will result in future increases in diseases of marine organisms lending new urgency to understanding the epizootiology of marine diseases. In response to these threats to Hawaiiís reefs, the Climate Change and Marine Disease Local Action Strategy (CCMD LAS) was developed in 2006 with ongoing efforts aimed at implementing its goals and objectives. (have link to completed CCMD LAS document)
To understand and manage impacts to reef ecosystems from climate change and marine disease for increased resistance and resilience.
Greta Aeby, PhD
Hawai'i Institute of Marine Biology
Division of Aquatic Resources
Environmental Protection Agency (EPA)
Hawai‘i Institute of Marine Biology (HIMB)
Kewalo Marine Lab
NOAA Pacific Islands Fisheries Science Center (PIFSC)
NOAA Northwest Hawaiian Islands Coral Reef Ecosystem Reserve (NWHI CRER)
The Nature Conservancy
University of Hawai‘i
US Fish & Wildlife Service (USFWS)
US Geological Survey (USGS)