Spanning more than 2,300 kilometers through the Pacific Ocean, the Great Barrier Reef is famous for its beautiful coral and diverse ecosystem.
More recently, the reef has become an example of the devastating power of climate change. From 2015 to 2016, ocean water off the northeast coast of Australia became dangerously warm for the coral. The heat stress made corals eject the symbiotic algae that live alongside the hard coral skeleton. When the algae leave, the remaining coral becomes a stark white color in a process known as bleaching.
Bleached coral is not dead; it can recover. However, without the algae, the ecosystem is much more vulnerable and more likely to die off in the near future. The 2015–2016 event resulted in the bleaching of more than half of all the coral in the Great Barrier Reef. Now, in a new study, McMahon et al. quantify the ecosystem-level impact of the event at Lizard Island, one of the most devastated sites in the Great Barrier Reef.
To analyze how the coral bleaching is affecting the Great Barrier Reef, the scientists quantified net ecosystem productivity and calcification (NEC) and compared the data to similar studies conducted before the 2015–2016 event began. NEC measures the flow of calcium carbonate through the environment, but it is difficult to measure directly. The researchers instead measured changes in seawater chemistry around the reef, including the total alkalinity and dissolved inorganic carbon. These data reveal whether more calcium carbonate is being removed from the water and added to the corals—an indication of growth—or more is being added to the water, indicating the reef’s skeleton is breaking down.
The authors calculated that the reef was taking up 44% less calcium carbonate compared to the prebleaching studies.
Although the authors say their results indicate a definite decrease in NEC as a result of the bleaching, they also caution that subtle differences in methodology and analysis can cause large changes when calculating NEC. The main problem, they say, is that the calculation is reliant on measuring the reef water residence time—how long water takes to flow through the reef system—which is extremely difficult to do.
Still, the study provides a rare and valuable window into the cost of coral bleaching events at the ecosystem scale. As global ocean temperatures continue to rise and mass bleaching events continue to skyrocket in number, these insights will become more relevant and far-reaching than ever. (Journal of Geophysical Research: Oceans, https://doi.org/10.1029/2018JC014698, 2019)
—David Shultz, Freelance Writer