ZSL scientists have shown the devastating effects of back-to-back heatwaves in BIOT. Coral cover was reduced by 60% in 2016 and by a further 30% in 2017. Nonetheless, this also indicates that some coral species have a higher resilience to rising temperatures thus offering some hope for the future.
Although representing les than 1% of our marine habitats coral reefs harbour a quarter of marine life. There are strong economic, conservation and scientific reasons for us to safeguard these reefs in a changing world.
In 2015 an eight weeklong heatwave struck BIOT, causing the seawater of the area to be unusually hot for a prolonged period. Researchers compared surveys of the reef before and after the heatwave in order to map the changes and harm it caused to the reefs. Evidence from 2015’s observations shows a 60% reduction in coral cover in the MPA. This included a dramatic 86% decline in the branching Acropora species that was previously a dominant reef building coral.
A year later, and before any recovery was possible, BIOT was subject to another bleaching event caused by a four-month long heatwave. Using data from Peros Banhos atoll an estimated 68% of the remaining corals had bleached. This suggest that around 70% of the reef area covered by corals was lost between 2015/2017.
Although more severe than the first heatwave, the second did less damage than the previous one. The first one killed corals most vulnerable to heat stress, leaving the more resilient colonies to try and survive the second. The fact that a few species have a higher heat tolerance is a glimmer of hope. According to Dr. Catherine Head the increasing severity and occurrence of heatwaves will inhibit shallow reefs ability to regenerate from bleaching events. It previously took 10 years for coral reefs to regenerate in BIOT, there is a high probability that the more vulnerable coral species will completely die off in the near future if these bleaching events become more frequent.
Preliminary reports from 2019 suggest a grim future for BIOT’s reefs as another heatwave was recorded and observations made of the start of yet another bleaching event of unknown impact. Research is key to understanding how corals adapt to rising sea surface temperatures and how we can positively impact their recovery.