While being dazzled by the corals, manta rays, and schools of fish at Palmyra, it is easy to forget that many coral reefs around the world are struggling. Numerous interacting factors have already led to widespread coral decline. Of particular concern is that all of these current impacts can reduce the resilience of the corals, thus making the reefs more vulnerable to the inevitable effects of climate change that lie ahead.
by Gareth J. Williams, member of the Benthic Team
Coral reefs are declining worldwide due to pollution, intense overfishing, and the global effects of climate change. Two other culprits—coral bleaching and disease—are also taking a toll on coral health. When corals die, the decreased coral cover on the reef has knock-on effects that can impact, and often change, reef-associated communities such as the fish.
The term coral bleaching refers to the loss of color from a coral colony, giving the colors a ghastly pale appearance. The normal vibrant coral colors are due to the pigments of the coral’s resident algae, the zooxanthellae. Bleaching results when the breakdown of this symbiotic association leads to either the loss of photosynthetic pigments by the zooxanthellae or the loss of zooxanthellae from their coral hosts. Either way, the resulting decline in photosynthesis can mean death for the corals. Bleaching occurs in response to environmental stress, predominantly to elevated water temperatures. Much recent research has focused on understanding how local human impacts interact with global climatic stressors to influence the extent and location of coral bleaching (the bleaching pattern) on coral reefs.
In late 2009, the strengthening of the El Niño Southern Oscillation led to sea surface temperature increases throughout parts of the eastern and central equatorial Pacific. In our current work in the Line Islands in the equatorial central Pacific, we are studying how the impacts of temperature anomalies like this one vary depending on the degree of local human disturbance to the reefs. The Line Islands, with their gradient of human influence ranging from almost no disturbance to heavy impacts, provide a natural experiment for us. Our observations here will allow us to start disentangling the likely complex interactions among the multitude of stressors that combine to determine the bleaching patterns on coral reefs.
Given climate change, we can be certain that more bleaching events lie ahead, and likely they will be more frequent and more severe. In light of this, the future of coral reefs will depend on their ability to recover from a major bleaching event, i.e., their resilience. Is there any way we can increase their resilience? The bleaching event that hit the remote Phoenix Islands in 2002 suggests yes. Observations there demonstrated that reefs free of all local human impacts are better able to recover from bleaching. Thus an important aspect of our work is to provide insight into how various stressors resulting from human activities act to decrease reef resilience to coral bleaching.
In contrast to coral bleaching, the other culprit—coral disease—manifests in diverse ways. Usually when people think of a disease, they think of an infectious disease, one caused by a specific microbe that is capable of causing the disease when introduced into a healthy host. Corals display evidence of having such infectious diseases. Some of their tissue-loss diseases are thought to be caused by bacterial pathogens, but identifying the specific microbes involved has been possible in only a few instances. As we know from our own experience, not all diseases are infectious—witness human cancers. Corals, too, can suffer from tumor-like growths. No convincing evidence exists that suggests these growth anomalies are infectious, but their causes are to date unknown. We do see that specific coral diseases show strikingly different responses to different environmental conditions. Some occur on heavily degraded reefs and appear to be associated with decreased water quality and increased temperature stress; others occur on healthier reefs with higher coral cover, thus suggesting that a key factor may be the greater abundance of potential coral hosts. Our work throughout the Line Islands allows us to investigate some fundamental questions about coral disease ecology, questions such as how the occurrence and prevalence of various diseases are affected by various environmental factors.
Coral bleaching and coral disease often go hand in hand, as bleaching can weaken the corals and make them more vulnerable to disease. Here at Palmyra Atoll, the late 2009 bleaching event was followed closely by an outbreak of coral disease . To understand the relationship between these two threats—bleaching and disease—and their combined impact on reef health in the remote Pacific is a research priority of ours. Given the increased occurrence of both bleaching and disease in recent years, this is a top priority.