Coral coverage around some of the most popular tourist islands on the Great Barrier Reef has dropped by almost half in the last 18 years, according to a new study.

Scientists said they were shocked after analysing data from monitoring dives between 1999 and 2017 at 100 different locations across the Whitsunday Islands, Magnetic Island, Keppel Islands and Palm Islands.

The study looked at the coverage of hard corals the rock-like structures that are the foundations for building reefs and found they were being hit by multiple impacts, including heat stress causing bleaching, cyclones, flood plumes and poor water quality.

Daniela Ceccarelli, of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, said: The loss [of hard corals] was between 40 and 50% at each island group. We were pretty shocked actually.

She said that inshore reefs like the ones studied were more susceptible to impacts from sediments and nutrients running off the land, especially reefs that were less exposed to waves that could wash pollution away more quickly.

Published in the Ecological Society of Americas Ecological Applications journal, the study found persistent shifts from coral to macroalgal dominance on some reefs in the central and southern parts of the system.

The study aimed to tease apart the relative importance of different stressors on inshore island reefs, including the impacts of heat stress, flood plumes and exposure to cyclones.

Co-author David Williamson, also from James Cook University, said: The impacts of individual disturbance events were patchy. Some reefs avoided the worst effects but the cumulative impacts of multiple, frequent events reduced coral cover and diversity.

On reefs around the Whitsunday Islands, one of the most popular destinations in the Great Barrier Reef marine park, coral cover had been relatively stable until 2016, after coral bleaching and then Cyclone Debbie saw a sharp drop in hard corals.

Ceccarelli said that even though the findings were shocking, there were still many stunning areas for tourists and divers to visit around the islands, including the Whitsundays where some individual reefs were in excellent condition.

The reef is not dead, but it might be on intensive care

She said some disturbances on reefs were natural and important for maintaining diversity, but she added: We are seeing increased frequency of these disturbance events and we worry [reefs] wont get enough time to recover between them.

She said that faster-growing corals could recover from major disturbances after about seven to 10 years, but for a whole coral community that included slower-growing corals, recovery times were between 15 and 20 years.

Scientists are concerned that major coral bleaching and heat stress events caused by ocean heating will hit the Great Barrier Reef at intervals too short to let corals recover.

The reef, the worlds biggest coral reef system, was hit by major bleaching events in 2016 and 2017, with impacts concentrated in the northern parts of the reef.

The study also questioned a previous conclusion that corals on inshore reefs were more tolerant of higher temperatures.

The study concluded that although rapidly escalating climate change impacts are the largest threat to coral reefs of the Great Barrier Reef it was also important that localised impacts, including runoff from the land, was proactively managed.

Its definitely not too late to save the reef, she said. Its not dead, but it might be on intensive care.

In a major five-yearly report published in August, the Great Barrier Reef Marine Park Authority downgraded the reefs long-term outlook to very poor for the first time since 2009 when the report was first published.

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Coral cover around popular Great Barrier Reef islands has almost halved - The Guardian