Coral Bleaching at Maro Reef, September 2004
Written By Dan Suthers. Edited by Greta Aeby and Jean Kenyon. Photographs by Greta Aeby. Graphs provided by Kyle Hogrefe.
The color of coral comes largely from algae that lives inside the coral in a mutually beneficial relationship call "symbiosis." "Bleaching" refers to loss of color when the symbiotic algae leaves the coral (the coral turns white). A more detailed explanation of bleaching is provded below. See also the 2002 article by Mark Heckman and .
At Maro Reef, divers on our expedition have mentioned that they are beginning to see coral bleaching. Photographs taken in the same region of Maro Reef in 2003 and 2004 by Dr. Greta Aeby are shown to the right. You may click on the thumbnail images to obtain somewhat larger images. (Cost of satellite transmission limits feasibility of posting the full resolution images.) We are posting these images for coral specialists, who are invited to send their comments via the "Ask About It!" facility.
There are different kinds of coral animals, some of which build reefs and some of which do not. Reef building corals have a special relationship with a form of single-celled algae called "zooxanthellae." This kind of relationship is called "symbiosis" to indicate that the organisms involved help each other live. Specifically, the algae produces sugar through photosynthesis, and the coral takes a large percentage of this sugar to be used as food energy. As much as 90% of the food required by corals can be produced by zooxanthellae. (The coral obtains the other 10% of its food by actively feeding on microscopic animals in the water called "zooplankton.") The algae, in turn, may benefit by being provided with a "home" in the coral (protected yet accessible to light). Also, corals may filter ultraviolet light that is harmful to the algae. The details of this symbiotic relationship are still being investigated, but this provides the basic idea.
Significance of Coral Bleaching
What is the significance of bleaching for the coral? Considering the facts just reviewed, an appropriate analogy might be "starvation diet." The coral loses 90% of its energy income, which it must try to compensate for through feeding. The coral is not dead: it can recover depending on the extent and duration of the bleaching event. Extended bleaching can lead to death of the coral.
Why would a coral lose its zooxanthellae? Some causes that have been established include unusually high or low temperatures, unusually high or low salinity, excess light, sedimentation, or high levels of nutrients or toxins. In general, coral bleaching is a response to environmental stresses (including, but not limited to, global warming), and un-reversed it can lead to coral mortality.
It is normal for any population to have some level of disease. For example, if you survey the people in your community, some of them will have the common cold. We become especially concerned only when the prevalence of the disease is higher than expected for a healthy population. This argument applies equally to coral reef ecosystems. It is not unusual to observe occasional coral bleaching in any reef such as the NWHI. We take notice when the prevalence is higher than expected.
Observations in the NWHI
No widespread coral bleaching was observed in the NWHI in expeditions conducted in 2000 and 2001. During NOWRAMP 2002, the first evidence of mass coral bleaching in the NWHI was recorded as far south as Maro Reef, and was greatest in Pearl and Hermes Atoll, Midway, and Kure Atoll (see this press release, in PDF). Initially this appeared to be a temporary phenomenon: no significant bleaching was found in 2003, and we were encouraged at the outset of the present expedition when very little bleaching was observed at French Frigate Shoals and Gardner Pinnacles. However, at Maro Reef, coral biologists Jean Kenyon and Greta Aeby documented high levels of bleaching in about 50% of Montipora (rice corals) surveyed. Minor bleaching was observed in other corals, including Porites (finger, lobe and plate corals) and Pocillopora (cauliflower corals). The Hi`ialakai will be at Pearl and Hermes Atoll for five days of coral reef surveys beginning Sept. 27, 2004, where we expect to follow up anecdotal accounts of widespread bleaching reported by NOAA Fisheries.
In an earlier article, I mentioned that the Mooring Team (physical oceanographers) found that surface temperature recorders (STRs) deployed at Maro Reef for a year are showing summer 2004 peak temperatures to be about one degree centigrade higher than temperatures recorded by the same sensor last summer. Graphs of two of their sensors are shown to the right (click for larger image). We look forward to their analysis of data from other sensors throughout the NWHI.
What is the significance of this finding? Clearly, one genus of coral (Montipora) is being impacted by some environmental stressor. The coral is not dead yet, but continuation of the responsible stressor could lead to mortality, especially within this genus.
A major motivation for ecological assessments during the NWHI expeditions is to obtain a detailed environmental and biological profile of a healthy reef ecosystem. Such a profile can be used as a baseline to monitor the NWHI for problems in the future, and as a standard against which to compare clearly impacted reefs (e.g., the Main Hawaiian Islands). Although bleaching is an unfortunate event, we are fortunate to have begun our monitoring efforts in the few years before this event began. Continued monitoring is critical to understand the impact of climatic and other trends on ocean ecosystems.
 Gulko (1998)
See also these related NOWRAMP 2002 feature stories:
Coral Bleaching by Mark Heckman
Talk About It!
"Inside Scoop" on coral reef health from the 2004 expedition: EXPANDED!
Asked by Siri from Hawaii Pacific University on Nov 5, 2004.
Hello, my name is Siri and I am a student at Hawaii Pacific University taking a class in environmental communications. We have each picked environmental topics in Hawaii to write about which will be posted on a website of our creation, and I have chosen the topic of coral bleaching in the outer atolls and reefs of Hawaii. I have been following the expedition of your two ships and was wondering if you can provide me with any additional information on the health of the reefs at this point in time. I know the expedition just got back and I was just seeing if I could get the "inside scoop". What are some predictions on the reef's health? Do you think that global warming is responsible for coral bleaching? What new discoveries or data did you find? Is there something that we as everyday citizens can do to help? I appreciate you taking the time to look at my questions, and I hope that the expedition provided some insight as to how our underwater world is doing. Thank you for your time. Sincerely, Siri Masterson
Answered by Greta Aeby, Hawaii DLNR and Ron Hoeke, CRED/NOAA on Dec 15, 2004.
[We received two excellent replies to this question, which are posted together below. -- Dan Suthers]
From Greta Aeby, Hawaii DLNR Division of Aquatic Resources
> I was just seeing if I could get the "inside scoop".
In 2004 we found moderate levels of bleaching that followed the same patterns observed in 2002, i.e., shallow backreefs had more bleaching and there was differential susceptibility among species with Montipora tending to bleach first.
> What are some predictions on the reef's health?
The reefs are in good shape and should remain that way as they are remote and free from human impact.
> Do you think that global warming is responsible for coral bleaching?
That is certainly a possibility and the prediction of scientists is that sea surface temperatures will continue to rise. This will lead to increased stress on the corals but what scientists don't know is how adaptable the corals and/or their zooxanthellae are and how they may or may not be able to adjust to increases in temperature through time. What we hope to do is find out all we can about the different coral species and their sensitivities toward bleaching in hopes of being able to predict and thus possible mitigate any problems.
[Editor's note: see my additional comments at the end.]
> What new discoveries or data did you find?
Everytime scientists go to the NWHI something more is learned about what organisms are up there and how the ecosystem functions.
[Editors note: I am working on an article on new discoveries from the 2004 trip, including possible new species. I am finding that it may take a long time, even years, to verify the discovery of new species. Scientists are reluctant to announce a discovery until they are sure. But did you see our article on the Pearl and Hermes, under Features? -- D.S.]
> Is there something that we as everyday citizens can do to help?
Yes, please be aware that these oceans are all connected and many coral reefs in other areas are severly degraded. As such, coral reefs such as ours in Hawaii that are in better condition must be protected. In the main Hawaiian Islands overfishing is a huge problem. A healthy coral reef is in a delicate balance among all its organisms. For example, overfishing reduces the number of alage eating fish--algae and corals are competitors for space on the reef--less fish means more algae which results in algae gaining the competitive edge against coral. Land-based pollution is also a problem. Since we are an island anything sprayed on land will eventually end up in the nearshore environment where corals are, including pesticides, fertilizers, etc. Most people take our reefs for granted and do not realize the level of protection they need if they are going to survive through time. Pass the word.
-- Greta Aeby
[Editor's Note: At the recent NWHI Scientific Symposium, data were reported that showed a striking corresondence between bleaching and increased sea surface temperature. Bleaching associated with increased shallow water temperatures were especially seen in the northern atolls. Although the scientific evidence for global warming is now very strong, we should keep in mind that this is a regional and local phenomenon and local sea surface temperature can increase due to a number of reasons not limited to global warming. One talk I attended noted that shallow water surface temperatures increase in areas that receive less water mixing due to the shape of the reef (confined atolls mix less with the open ocean) and due to lighter winds during this time in the region of the northern atolls. There appears to be links to the El Nino cycle and also to the Pacific Decadal Oscillation (a variation in large scale ocean currents). The exact relationships between bleaching and these events, and between these events and global warming, are not known and require further study. Ecosystem science is complex, which is why it is so interesting!
Subsequently I asked Ron Hoeke (one of the NWHI Symposium speakers) to comment; his article is below. -- D.S.]
Ron Hoeke, Coral Reef Ecosystem Division, NOAA Fisheries
I’ll try and answer Siri’s questions in the following paragraphs. Her questions were: What are some predictions on the reef's health? Do you think that global warming is responsible for coral bleaching? What new discoveries or data did you find? Is there something that we as everyday citizens can do to help?
Over the last 10 years or so, scientists have learned a number of things about coral reef bleaching and climate change. Corals bleach when they get stressed and expel their symbiont algae, zooxanthellae. Massive reef bleaching events, when significant numbers of corals in entire reefs bleach, often lead to major coral mortality and decreased coral cover. Generally, these events happen during unusually high sea surface water temperatures (SST), although other factors, such as UV light penetration have been shown to play a major role too. During the 1997/1998 El Nino, many places, such as Australia’s Great Barrier Reef, experienced massive coral bleaching events. This was something of a wakeup call to many researchers. In 2002, during much milder El Nino, the Great Barrier Reef bleached again, this time more severely, with up to 90% coral mortality in some areas. A number of studies strongly suggest that the overall number and severity of mass coral bleaching has increased greatly in the last few years, and that this is driven, at least in part, by warming due to human-induced climate change. A seminal paper, covering everything from the physiology of coral bleaching to predictions of future SST rise and associated coral bleaching events is by Ove Hoegh-Guldberg, an Australian researcher; it was published in 1999. Several studies have followed, and almost all show the world’s coral reefs to be threatened, both from climate change related coral bleaching and from other human stressors, like local pollution and fishing.
Due to their northern location (for coral reefs) and remoteness, some people thought that the Hawaiian Islands, particularly the Northwestern Hawaiian Islands, would be among the last areas to be effected by climate change related coral bleaching, because of their fairly northern location and remoteness. The first recorded major bleaching event in the Hawaiian Islands was documented by University of Hawaii researcher Paul Jokiel and others in Kaneohe Bay in 1996. NOAA led Pacific Reef Assessment and Monitoring Program (RAMP) expeditions to the Northwestern Hawaiian Islands documented the first recorded major bleaching events in the northern end of the chain in 2002 and 2004. Extended reconstructions (about 50 years) of SST in the region show that the water got much warmer during the summers of both 2002 and 2004 than during any other summer in the last 50 years. It appears that even the most remote and northward of the Hawaiian Islands, the atolls of Pearl and Hermes, Midway, and Kure, are at great risk of future bleaching. In fact, recent evidence suggests that these atolls are at the greatest risk of future bleaching events (associated with high water temperatures) of all of the Hawaiian Islands.
Despite all this evidence that corals are being threatened by climate change, several important things must be remembered. Climate change is very complex, with different areas changing in different ways at different rates. Scientists have been recording coral reef health for only about the last 50 years, and the number of observations has increased sharply in the last 10 years. It is possible that the evidence for warming and increased coral bleaching is part of some longer cycles; cycles we do not yet understand because we haven’t been looking at them for long enough. We also don’t know how well corals, or the coral reef ecosystem as a whole, can adapt to these changes. Some corals species are more susceptible to bleaching than others and the same species have different susceptibilities in different areas. Global warming and increased coral bleaching are not a given, but the evidence is piling up. How bad it could be is still a mystery. This is why more study is so important.
The best thing individuals can do to help coral reefs is to spread awareness of the measurable impact humans have on coral reefs, both at the local and the global level; the importance of reducing greenhouse gases; and to strongly support scientific research of coral reefs and climate change.
Suggested further reading:
Hoegh-Guldberg, O., 1999. “Climate change, coral bleaching and the future of the world's coral reefs.“ Marine and Freshwater Research, Vol. 50 No. 8 Pages 839 – 866
Jokiel, P.L., Brown,E.K., 2004. “Global warming, regional trends and inshore environmental conditions influence coral bleaching in Hawaii.” Global Change Biology, Vol. 10 Pages 1627-1641
Hughes, T. P., Baird, A. H. , Bellwood, D. R., Card, M., Connolly, S. R., Folke, C., Grosberg, R., Hoegh-Guldberg, O., Jackson, J. B. C., Kleypas, J., Lough, J. M., Marshall, P., Nyström, M., Palumbi, S. R., Pandolfi, J. M., Rosen, B., Roughgarden , J., 2003, “Climate Change, Human Impacts, and the Resilience of Coral Reefs.” Science, 15 August 2003; 301: pg. 929-933.