Rising Ocean Temperature Leads to Coral Disease Outbreaks
7 Jun, 2007 04:11 pm
Disease is one of the primary factors causing the global loss of reef building corals. Results from a paper recently published in PLoS Biology indicate that anomalously high ocean temperature increased the severity of the coral disease white syndrome on the Great Barrier Reef. Disease outbreaks only occurred on reefs with high coral cover after especially warm years. The disease was largely absent on cooler, low cover reefs.
Infectious diseases are thought to be one of the primary causes of mass coral mortality. Many reef ecologists suspect that anomalously high ocean temperatures are one of the factors responsible for increasingly common and severe coral disease outbreaks. This idea is supported by local observations (e.g., some coral diseases become more prevalent in the summer) but until now has never been tested at large spatial scales over relatively long periods.
We tested the temperature–disease hypothesis by examining the relationship between ocean temperature and the frequency of white syndrome, an emerging coral disease on Australia’s Great Barrier Reef. The design of the investigation was very similar to a human health study, where subjects are monitored over time to investigate links between things like diet and heart disease. We tracked the fate of 48 coral reefs across 1500 km of Australia’s great barrier reef and asked whether reefs that got especially warm were more likely to experience disease outbreaks. Disease frequency was monitored by the Australian Institute of Marine Science and ocean temperature was recorded with satellites by scientists at the National Oceanic and Atmospheric Administration. After assembling an international team of scientists and combining the two databases, we worked with epidemiologists to apply statistical techniques borrowed from human epidemiology to analyze the relationship between temperature, reef health, and coral disease outbreaks.
We found that anomalously high ocean temperatures were strongly associated with white syndrome frequency. We also found that coral cover, the percentage of the bottom covered by living coral–which is a key measure of reef health–was also positively related to disease outbreaks. Warm water and healthy coral populations can be a lethal combination leading to epidemics and mass coral die offs.
The temperature increases required to trigger a white syndrome outbreaks were relatively modest as most disease outbreaks occurred on reefs where the temperature was only 1-2 °C warmer than usual. The severity of marine diseases could increase with temperature for several reasons. For example, some pathogens prefer the warmer temperatures, enabling them to reproduce more quickly and in some cases causing them to become more virulent. Additionally, many host animals such as corals and tropical fish can become stressed by abnormally high temperatures. This can wreak havoc with their immune system, making them more susceptible to opportunistic parasites and pathogens.
The fact the only the healthiest reefs experienced white syndrome outbreaks was one of the most surprising results of the study. Many reef scientists have assumed that reefs with very high coral cover were innately more resilient to disturbances like disease outbreaks and that disease only became prevalent on highly degraded reefs. Several mechanisms could underlie the finding that coral cover was positively related to disease frequency. For example, high coral cover could facilitate disease transmission among coral colonies when they are closely spaced. We also suspect that wounds that result from corals fighting in such conditions could have increased infection rates and allowed the pathogen to move more quickly through the population.
The number, prevalence, and impacts of diseases of corals and many other types of marine animals have been increasing over the last 20-30 years. Our results suggest that ocean temperature could be one of the main culprits, and if global warming is not reversed we will likely see more instances of disease wiping out other ecologically and economically important species.
When they are healthy, coral reefs provide human societies with massive economic benefits through fisheries, tourism and invaluable services like buffering from storms. We clearly need do a far better job of developing technologies and implementing smart policies to reduce greenhouse gas emissions. In the meantime we can continue mitigating the effects of other coral stressors like terrestrial runoff and destructive fishing practices.
Bruno, J.F., et al. 2007. Thermal stress and coral cover as drivers of coral disease outbreaks. PLoS Biology 5(6):e124 doi:10.1371/journal.pbio.0050124
Selig, E.R., et al. 2006. Analyzing the relationship between ocean temperature anomalies and coral disease outbreaks at broad spatial scales. In; J.T. Phinney, O. Hoegh-Guldberg, J. Kleypas, W. Skirving, and A. Strong (eds.). Coral reefs and climate change: science and management. American Geophysical Union, Washington, DC, Pages 111-128