The XL Catlin Seaview team has wrapped up our expedition to Hawaii, where we completed surveys of 20 sites amongst the islands of Maui, Lanai, Molokai, and Molokini. We came looking for the phenomenon of coral bleaching, and we saw lots of it. On nearly every dive we imaged a seascape of colonies glowing white, standing out starkly from the darker ocean bottom of volcanic rock or ancient limestone.
The familiar browns, reds, and greens of a healthy living reef were drained out, lost as the symbiotic photosynthetic algae that live within the coral tissues were expelled. This loss of algal symbionts leaves the corals stressed and potentially starved, as these little cells provide much of the metabolic sustenance that their coral hosts need to thrive. Bleaching thus does not lead directly to coral death, but it is a very stressful event for the coral, and is associated with significantly increased mortality for colonies over a time frame of weeks to months. The corals can recover their color and vigor, but the rate of recovery and mortality following this event is what will structure what these systems look like for years to come. It is this eventual community response to this event that we are investigating.
The pattern of bleaching was very varied, with notable differences between coral species, and from one transect to another, and even among corals of the same species living right next to one another. This complexity makes for extremely variable and sometimes confounding data, and so we have gathered on this expedition over 15,000 images covering more than 30 kilometers of the reef, with the very size of this set of images is the key to being able to distill out an understanding of how bleaching has affected these islands.
High water temperatures are the main underlying cause of coral bleaching, and the central Pacific is experiencing some of the highest temperatures in modern records. This increase is not much, only a few tenths of a degree, but it is the cause of significant disruption to the finely balanced coral ecosystem. These events are forecast to become more common in the coming years, as global greenhouse gasses continue to climb, and more heat is retained in the mass of the world’s oceans.
Given the thousands of stark white coral colonies we have seen on this expedition, this bleaching is a clear indicator that we are at a tipping point in terms of our impact on the coral ecosystem, which we rely on for fisheries production, diving and snorkeling recreation, and coastal protection. As marine biologists, we record these bleaching events in the hope that our science will guide the conservation of coral reefs in the future. However, we must also see this as a warning light of even larger potential environmental change, affecting the largest and most vital systems of the earth, controlling oxygen production, weather, fires, and agricultural production. As world environmental leaders gather this month for the United Nations Climate Change Conference (COP21), our imagery of these white corals will hopefully provide a voice for the ocean in this discussion of our shared environmental future.