Illuminating Effects of Mesoscale Eddies on Coral Reefs

This summer marks year two of a three-year ASU BIOS study designed to expand understanding of how ocean eddies might be affecting coral reefs, as well as what role eddies may have played in reef accretion and overall functioning in the past.

“Most of the knowledge we have so far about eddies is from the open ocean. But we don’t really understand what happens when they hit the reef,” said marine benthic ecologist Yvonne Sawall, principal investigator of the U.S. National Science Foundation-funded project. “Do we see clear signals of eddies on the reef and, if so, how do they impact coral or coral reefs in general?”

Emanating from ocean currents, eddies are temporary, traveling spheres of swirling water that are instrumental in transporting and mixing nutrients in ocean basins. Their swirling motion can bring deeper, cooler water to the surface or push warmer surface water down. In the first case, the upwelling of nutrients typically found in colder, deeper waters, has been found to increase biological production of surface waters in the open ocean.

“Reefs are naturally exposed to environmental fluctuations and some of this might be caused by eddies, but we don’t really know because no one has really measured it so far,” Sawall said. Reefs that are exposed to strong environmental fluctuations may have a higher capacity to deal with extreme events, such as heat waves, which are predicted to become more frequent in the near future.

Now in the midst of its second year of field work, the coral reef eddy project is investigating the contemporary and historic effects of eddies on coral metabolism and growth. In addition to Sawall, the research team includes biogeochemical oceanographer Damian Grundle and research technician Roderick Bakker. Employing a prototype set of in-situ incubation chambers (dubbed “the Bio Resort”), field work in Bermuda is pursuing the study’s first two objectives: 1) assessing the impact of eddies on physical and biogeochemical reef water properties and 2) assessing the effect of eddy-driven changes in reef water properties on coral calcification.

The study’s third goal, assessing eddy impacts over the past several decades by examining oceanic time-series data and coral skeleton proxy records, is led by Nathalie Goodkin, an oceanographer at the American Museum of Natural History (AMNH) in New York. Goodkin is also implementing the project’s overarching goal of transferring skills and knowledge to the STEM (science, technology, engineering and math) pipeline by preparing materials to educate secondary school teachers on eddy-reef interactions, teaching high school students in eddy identification and exposing undergraduate students to research opportunities.

Linking the study’s various aspects, Goodkin visited ASU BIOS twice last year and once this year, and the Bermuda-based team traveled to New York, where Sawall and Grundle each presented seminars at the AMNH.

Noting that Bakker extracts data from a National Oceanic and Atmospheric Administration (NOAA) website that tracks the location of eddies based on sea surface height anomalies, Sawall said the team didn’t (unfortunately) encounter a single eddy on the reef during the first year of field work, which began in July 2021. There was a hurricane in September 2021, however, which provided another, unanticipated opportunity of investigating the effect of an (extreme) mixing event on the reef water biogeochemistry and coral performance.

While sample and data analyses are still ongoing, substantial changes of the reef water biogeochemistry are evident in particular in temperature (decrease), but also in, for example, pH (increase) and oxygen (increase). Also coral photosynthesis rates slightly increased after the hurricane, meaning that “we have an indication that the productivity increased after the hurricane, which is actually counter-intuitive because the temperature really dropped,” Sawall said. She added that the summer of 2021 was unusually warm and excessive heat could have stressed the corals. The working theory, Sawall said, is that cooler surface waters and a nutrient boost ushered in by the hurricane relieved that stress and improved conditions for photosynthesis of corals, as well as of the reef as a whole.

Outside the hurricane, the data, so far, provides a solid basis for understanding the diurnal, day-to-day and month-to-month variably under non-eddy conditions. Sawall and Grundle are now waiting for an eddy to arrive, which is monitored by simulations with sea surface height anomaly data from the NOAA website.

“We’re keeping an eye out for this,” Sawall said, adding that there tends to be a high occurrence of eddies in the Sargasso Sea.

Meanwhile, there’s been considerable progress on the study’s third objective.

“We’re pretty close to finalizing a model that will allow us to go back in historical data sets of water properties such as temperature and salinity and identify when eddies occurred,” said Sawall. “Then, we will apply this model to into the skeleton of the coral core and extract chemical proxies back through time to reconstruct temperature and salinity and identify eddy histories.”

“The next step is to see how eddies affected coral calcification (increase or decrease) in the skeletons and modern time,” said Sawall.

Scientists often analyze proxy data – preserved physical characteristics of corals and other organisms – as substitutes for direct measurements to flesh out knowledge of past environmental conditions.

Sawall said Goodkin has been analyzing core samples from the study site, enabling the team to compare what’s revealed by coral core proxy data with actual ocean measurements taken over the years at Hydrostation 'S', the world's longest-running time-series for physical and chemical oceanographic data, and the Bermuda Atlantic Time-series Study, another long-term time-series study examining biogeochemical cycles in the Sargasso Sea.

“The idea is really: can we use corals as an archive for eddy activity? That’s one of the big questions of this project. We may be able to relate that to climate change, or not. But it’s definitely an interesting aspect to consider,” Sawall said.

In terms of STEM, the team is collaborating with BIOS Director of Education and Community Engagement Kaitlin Noyes to create DataBytes. For this, they have already created a schematic of the “Bio Resort” that illustrates how the in-situ incubation chamber works.

In addition, Goodkin said this fall a Columbia University Department of Earth and Environmental Sciences PhD candidate will work with several high school students at AMNH to run yearlong experiments examining physical processes of the upper ocean.

“The students will be exposed to the complete scientific process from hypothesis testing to data presentation,” Goodkin said.

With a third year of field work planned for May to September 2024, all phases of the project are due to wrap up next year.