Coral reefs around the globe are being threatened by warming ocean temperatures, which is a major driver of coral bleaching. Scientists have frequently used sea–surface temperature data collected by satellites to predict the temperature-driven stress on reef communities, but a new research reports that surface measurements alone might not be enough for accurately predicting the full extent of thermal stress on deeper corals.
The study, led by scientists from the Scripps Institution of Oceanography at the University of California San Diego and the Coral Reef Research Foundation (CRRF) in Palau describes a new approach for estimating stress induced by warm temperature on corals from the sea surface through a deeper expanse ranging from 30-150 meters (100-500 feet) known as the mesophotic zone.
Corals in the mesophotic zone are thought to be as being comparatively safer from ocean warming than other corals in the shallow-water levels. But this study by Scripps Oceanography team shows that even deep water corals are sometimes exposed to thermal stress at intervals which are different than those of corals near the surface.
For the study, researchers worked with 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 that could give the vertical extent of how corals will be stressed by temperature. This research was carried out at three reef locations around the island nation of Palau, located in the tropical Pacific Ocean. The study was published recently journal Geophysical Research Letters.
“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,” said Travis Schramek, a Scripps PhD candidate and also the lead author of the study. “We see that the heat-induced stress penetrates all the way into the mesophotic zone during larger bleaching events.”
For about 20 years, Colin and a small team have carried out weekly dives at locations across Palau as part of a long-term temperature-monitoring program. Since the surveys to assess bleaching in mesophotic zone are extremely limited, the Scripps team, which also included scientists Mark Merrifield and Eric Terrill, found that Colin’s observational data sets are exceptionally valuable. The observations have shown that deeper zones are demonstrating bleaching coincident with the higher temperatures, along with the shallow reefs.
“Our understanding of the ocean is really going to continue to be driven by observations. The models are really informative, but the way that we ground them is through observing the Earth system,” said Schramek. “Having observations like what Pat has collected shows the power of actually going and deploying tools and observing Earth in a unique way.”