Extreme weather occurrences are the result of human activity, and they pose a hazard to all ecosystems. The occurrence of mass bleaching on coral reefs is triggered by marine heat waves caused mostly by these activities. A coral reef is an undersea ecosystem made up of hundreds of polyps that secrete calcium carbonate. They are heterotrophic and stationary in nature. Corals and zooxanthellae show a mutualistic interaction in most cases. The disruption of the symbiosis between corals and zooxanthellae causes mass bleaching, causing reefs to turn pale and become physiologically and nutritionally impaired. A short period of abnormally high temperatures in a sea or ocean is known as a marine heat wave.
Great Barrier Reef (GBR), spanning 2300 kms in length was affected majorly due to the record breaking marine heat wave caused in 2016. GBR is divided into three segments- North, Central and South. 2016 marine heat wave majorly affected the North GBR and a bit of Central GBR. It was observed via satellites that different corals die at different rates of heat exposure. Intensity of coral-die off is measured in Degree Heating Weeks (DHW), unit being °C weeks.
Between March and November 2016, the North GBR, which covers 700 kilometres, lost 50.3 percent of its coral reefs, while the South GBR only lost 30%. Many coral reefs died in March because they were unable to reclaim their symbionts. When the heatwave intensity was 4-8°C weeks for the first few months, the median loss was 15.6 percent, rising to 27 percent as the intensity increased. With a 4°C week intensity, total coral reef loss was 44 percent, rising to 66 percent and 80 percent when the intensity reached 8°C weeks and more than 9°C weeks, respectively. While the reefs that experienced less than 25% of bleaching recovered in 8 months.
The response of coral assemblages on reefs exposed to a broad range of heat stress, ranging from 0 to 10°C-weeks, was strikingly nonlinear, establishing a winner versus loser relationship between them. According to a non-metric multi-dimensional scaling (nMDS) analysis, post-bleaching mortality disproportionately changed the assemblage structure and functional diversity of corals on reefs that suffered significant levels of bleaching. On these extensively bleached reefs, the abundances of all types of corals declined to varied degrees. Fast-growing, three-dimensional species such as tabular and staghorn Acropora, Seriatopora hystrix, and Stylophora pistillata, which dominate many shallow Indo-Pacific reefs, all dropped by more than 75%.
According to satellite-derived DHW data, during the 2016 bleaching episode, 28.6% of the 3,863 reefs that make up the Great Barrier Reef were exposed to thermal exposures of more than 6°C-weeks, and 20.7 percent (800 reefs) were exposed to more than 8°C-weeks. The abrupt, regional-scale shift in coral assemblages has also radically reduced the abundance and diversity of species traits that facilitate key ecological functions. In most cases, reefs have transitioned away from fast-growing, branching, and tabular species, which are essential suppliers of three-dimensional habitat, and toward a depauperate assemblage dominated by taxa with simpler morphological traits and slower growth rates. Substantial mortality occurred on the Great Barrier Reef in 2016 well below 6 °C-weeks, beginning instead at 3–4 °C-weeks, and with typical losses exceeding 50% at 4–5 °C-weeks. Identified threshold for breakdown of assemblage structure was approximately 6 °C-weeks.Because many of the surviving coral colonies continue to die slowly even after their algae symbionts have recovered due to tissue loss, the chances of a full recovery to pre-bleaching coral assemblages are slim. Also, larval recruitment to replace deceased corals and subsequent colony expansion will take at least a decade.
As the severity of global warming continues to rise, the 2016 marine heat wave has begun the first step of that transition on the northern, most pristine part of the Great Barrier Reef, irreversibly transforming it. This necessitates emphasising the need for risk assessments of ecosystem collapse, particularly if global climate action fails to keep warming to 1.5–2 °C above pre-industrial levels.