Deep-water corals also bleaching badly

Even the deep-water corals of Australia’s Great Barrier Reef are not safe from the damaging effects of rising ocean temperatures, a new study finds.

Living at depths of between 30 and 150 metres below the surface, these “mesophotic” reefs were thought to be protected from severe bleaching events, like those that devastated the reef’s shallow-water corals in 2016 and 2017.

Now, an international team of marine scientists has discovered that the 2016 heat spike caused coral death up to 40 metres down.

“It was a shock to see that the impacts extended to these dimly-lit reefs,” said Pedro Frade of Portugal’s University of Algarve, lead author of the study published in Nature Communications.

Using remotely operated vehicles (ROVs) to place sensors up to 100 metres deep, the researchers recorded water temperatures before and after the bleaching occurred. They also scuba-dived to check the reefs for signs of damage once the bleaching was underway.

By May 2016, water temperatures at the mesophotic reefs were a whopping 1.4 degrees Celsius higher than the monthly average for the previous three years. Major bleaching and mortality were reported for almost a quarter of these deep reefs.

The surveys also confirmed previous reports of damage to nearly half of shallow-water corals.

Remarkably, reefs at 40 metres deep experienced the same heat exposure as those at 10 metres – a testament to the strong effects of the late-season warming.

Local oceanographic conditions play a major role in regulating water temperature at mesophotic reefs, the researchers say. Deep-water currents bring cold water up the continental shelf, cooling them.

However, “when this upwelling stopped towards the end of summer, temperatures rose to record-high levels even at depth,” explains co-author Pim Bongaerts of the California Academy of Sciences in the US.

Temperatures at the deep-water reefs did not drop to 28 degrees Celsius until mid-April, nearly two months later than normal.

Despite the damage to the deep water reefs, the severity of coral bleaching was much greater in shallower ones — a sign that depth still does confer some protection from thermal extremes.

For coral species that are more susceptible to bleaching, the authors predict a shift in abundance and distribution as water temperatures continue to rise.

“There will be winners and losers as a result of long-term impacts of mass bleaching events on coral reefs, and in the Great Barrier Reef in particular,” they state.