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Scientists find corals under pressure in deeper waters too



A coral reef research foundation (CRRF) scientific diver climbs a reef wall in Palau at a depth of 90 meters (295 feet). The diver was on a mission to photograph mesophotic coral ecosystems and regain temperature gauges deployed the previous year. Credit: Patrick Colin, CRRF

Coral reefs around the world are threatened by the warming of the oceans, which play an important role in coral bleaching. Scientists routinely use sea-surface temperature data collected by satellites to predict the temperature-driven stress on reef communities, but recent research shows that surface measurements can not accurately predict the exact extent of deep coral thermal stress.

A new study, led by researchers at the Scripps Institution of Oceanography at the University of California at San Diego and the Coral Reef Research Foundation (CRRF) at Palau, describes a novel approach to predicting warm temperature-induced stress on corals from the sea surface, a deeper extent known from 30-1

50 meters (100-500 feet) as mesophotic zone.

Corals at this depth are considered by some in the scientific community to be safer from the warming of the ocean than their shallow-water counterparts. But the Scripps Oceanography team found that corals are also exposed to thermal stresses at depth in depth, at intervals that are different from those near the surface.

The researchers used nearly two decades of data sets – including sea level and sea surface temperature and temperature observations that ranged between the surface and deep into the mesophototic zone – to develop a vertical expansion prediction tool of how corals are stressed by temperature. This research was conducted at three reef sites around the island nation of Palau in the tropical Pacific.

This novel approach to measuring and predicting temperature stress on coral reefs is described in a new study published in the Journal on August 27 Geophysical Research Letters .

"We are now inserting the dimension of depth into the problem where previously we just skimmed the surface of the coral temperature stress boundary," said Scripps Ph.D. Candidate Travis Schramek, lead author of the study. "We see that heat-induced stress penetrates into the mesophoton zone on major bleaching events."


In addition to sea surface temperature data collected from global satellites, scientists used a network of reef temperature recorders maintained by CRRF divers at key locations in Palau to a depth of 90 meters

CRRF has this range of temperature gauges Retained in Palau, which extend since 1999 from the surface to the mesophotonic reefs. Only a small number of divers in the world have the training, skills and equipment to safely and routinely dive into the mesophotic zone. One of these rare scientific divers is Pat Colin, director of CRRF and co-author of the study.

For nearly 20 years, Colin and a small team have performed weekly dives at locations throughout Palau as part of long-term temperature. Monitoring program. The studies evaluating bleaching in the mesophotic zone are severely limited, so the Scripps team, which included the scientists Mark Merrifield and Eric Terrill, found Colin's observation data very valuable.

The observations showed that deeper zones associated with bleaching appear at higher temperatures, as do shallow reefs.

"Our understanding of the ocean will really continue to be determined by observations, the models are really informative, but the way we ground them is by observing the Earth system," said Schramek. "Observations like what Pat has collected "demonstrate the power to fly and deploy instruments and to observe the Earth in a unique way."

The researchers hope that these results will allow more temperature stress events to better understand the mesophilic zone Palau and other tropical regions Also, the daily gauges from the Malakal Harbor Station in Palau from 1970 to 2017. These data, collected by the Sea Level Center of the University of Hawaii, allowed the team to study regional sea-level recordings in the region.

The blue water of the lagoon falls quickly into the deep n ocean in the southern extension of palau. Credit: Travis Schramek, Scripps Institution of Oceanography, UC San Diego

Combining sea-level and sea-temperature data, Schramek found that sea-level elevation is a strong indicator of how water temperatures change by tens of meters. He and the team then continued to use this data to predict the temperatures experienced by coral reefs living near the surface as well as those living in deeper waters. Schramek developed an algorithm to apply the accepted coral stress algorithms at depths containing the deeper mesophot, which is typically considered to be a refuge from thermal stress.

"A surprising finding of the study is that the oceanic conditions along the dramatic reef walls, the boundaries of Palau are very representative of the wider western Pacific," said Scripps Oceanographer Terrill. "As a result, we had an astonishing amount of success in predicting the vertical structure of the temperature fields to which the coral communities would be exposed, even during El Niño conditions."

He added that the results show promise in applying the method to the other islands in this Pacific region, which does not benefit from the long-term timekeeping that Palau has as a result of the CRRF dedicated measure campaign.

The team's new findings on how to predict temperature stress on deep coral contribute to a better understanding of the entire reef system as a whole, which could impact conservation and policy efforts to protect this precious and diverse ecosystem.

"After observing this ecosystem in a unique way, we can begin to better assess how corals are stressed in the mesophoton zone," said Schramek. "If we can better understand how stressed they are, we can better understand how we can protect them."


Further information:
The bleaching of coral reefs is reduced where the daily temperature fluctuations are large

Further information:
Geophysical Research Letters (2018). agupubs.onlinelibrary.wiley.co … 10.1029 / 2018GL078782

References in Journal:
Geophysical research letters

provided by:
University of California – San Diego


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