Even in the deep, corals are episodically exposed to thermal stress at intervals different than those corals near the surface.
While scientists routinely use sea-surface temperature data collected by satellites to predict the temperature-driven stress on reef communities, a new research shows that surface measurements alone may not accurately predict the full extent of thermal stress on deeper corals.
A new study describes a novel approach for predicting warm temperature-induced stress on corals from the sea surface through a deeper expanse ranging from 30-150 meters (100-500 feet) known as the mesophotic zone.
Corals at this depth are thought of by some in the science community as being safer from ocean warming than their shallow-water counterparts. But according to the Scripps Oceanography team, even in the deep, corals are episodically exposed to thermal stress at intervals different than those corals near the surface.
The researchers utilized nearly two decades of data sets, including sea level, sea-surface temperature, and temperature observations that ranged between the surface and deep into the mesophotic zone, to develop a forecast tool for the vertical extent of how corals will be stressed by temperature. This research was conducted at three reef locations around the island nation of Palau, located in the tropical Pacific Ocean.
PhD candidate Travis Schramek, lead author of the study said, "We're now adding the dimension of depth into the problem where before we were only skimming the surface of what temperature stress meant for corals. We see that the heat-induced stress penetrates all the way into the mesophotic zone during larger bleaching events."
In addition to looking at sea-surface temperature data collected by global satellites, the scientists used a network of reef temperature recorders maintained by CRRF divers in key locations across Palau down to depths of 90 meters (295 feet).
For nearly 20 years, a small team has conducted weekly dives at locations across Palau as part of a long-term temperature-monitoring program. The observations showed that deeper zones are showing bleaching coincident with the higher temperatures, right along with shallow reefs.
The researchers said that they hope these results will instigate more temperature stress event surveys to better understand the mesophotic zone in Palau and other tropical regions.
By coupling sea level and sea temperature data sets, Schramek found that the height of the ocean surface is a strong indicator of how water temperatures are changing tens of meters below. He and the team then further used 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 to depths that included the deeper mesophotic, typically thought to be a refuge from thermal stress.
"A surprising outcome of the study is that the oceanic conditions along the dramatic reef walls that are the boundaries of Palau are very representative of the broader Western Pacific," said Eric Terrill, an oceanographer.
He added, "As a result, we had a surprising amount of success in predicting the vertical structure of the temperature fields that the coral communities would be exposed to, even during El Niño conditions."
Schramek said, "Now that we've observed this ecosystem in a unique way, we can start to better assess how corals in the mesophotic zone are stressed. If we can better understand how they're stressed, then we can better understand how to protect them."
The team's new insights on how to predict temperature stress on deep corals may contribute to a better understanding of the entire reef system as a whole, which could inform conservation and policy efforts to protect this valuable and diverse ecosystem.
The findings appeared in the Journal of Geophysical Research Letters.