The fragile ecosystems created by coral reefs are tenuous. Corals are sensitive to small changes in temperature, salinity, and sunlight. These strict requirements are hard to meet in the modern world, with polluted run-off, global warming, and other human impacts. As a result, coral bleaching and silted reefs are increasingly common sights.
But enough of the bad news. It turns out that there are some positive aspects to the story. A study by the Wildlife Conservation Society suggests that corals living in changeable areas are more resilient. Some reefs experience seasonal temperature fluctuations, and this may enable their continued survival under the warming effects of climate change. However, corals with more stable temperatures are rapidly stressed, causing the irreversible ejection of symbiotic algae.
Coral reefs are limestone deposits created by tiny animals related to sea anemones and jellyfish. Hard corals include the reef-building species. These animals live in colonies and capture food using tiny tentacles. Additionally, hard corals are photosynthetic – despite being animals. This process happens thanks to zooxanthellae that live inside coral polyps. These photosynthetic algae have a mutual relationship with corals, where polyps provide structural safety and nutrients, while the algae return nutrients back. The surfeit of food allows the coral polyps to grow and build reefs.
Once a reef structure has been produced over thousands of years, it becomes the home for many species, including fishes, corals, and a diversity of other invertebrates. Reefs are unique environments. They act as storehouse of biological diversity. In addition, people value them as places to visit and experience, as well as being fishery resources. Reefs also physically buffer neighbouring shorelines and estuaries.
Most reefs are found in tropical waters. There are also some reefs in warm areas like southern Japan. For optimal growth of hard corals, water temperature should be 23-29 degrees Celsius – although they can survive a slightly greater range of temperatures.
In the East African study, reefs near islands had more temperature variation, compared to those reefs flushed directly by ocean currents. These hardier sites might survive some warming to act as sources for seeding new reefs. But it will require conservation and awareness, particularly as human activities are one of the major threats to reefs. In particular, fringing reefs around islands are subject to land-based pollution, from agriculture to deforestation and dredging. Additionally, destructive fishing techniques destroy reefs. This new information about the survival of island “shadow” reefs is another tool for conservation efforts.
