Physics of coral as an indicator of reef health

Stanford scientists recently addressed this imbalance by demonstrating that measuring the physics of just a small portion of reef with a single instrument can reveal insights about the health of an entire reef system. The findings point to low-cost methods for scaling up monitoring efforts of these enigmatic living structures, which are at risk of devastation in a changing climate. The results appeared in the Journal of Geophysical Research: Oceans Dec. 14.

"This approach is like building a weather station for coral reefs," said lead study author Mathilde Lindhart, a PhD student in civil and environmental engineering. "If we have a couple of weather stations around, we can then determine the weather everywhere on the reef."

Limited resources

For decades, marine scientists have often relied on a single instrument to calculate the flow around reefs because the measurements must be made with limited time and costly tools that can only be anchored in certain locations. As a result, they have had to assume that one measurement is representative of flow over the entire reef. This new work confirms that assumption is correct, bringing renewed credibility to previously collected data.

"Replication is the foundation of our ability to trust science," said senior study author Rob Dunbar, a professor of Earth system science in Stanford's School of Earth, Energy & Environmental Sciences (Stanford Earth). "Our results are building a solid foundation for other studies of coral reef physics."

The study authors tested a suite of current meters, which send out sound waves that scatter off the currents and suspended particles, including sediment and plankton, then return with a shift in frequency that translates into flow velocities. They measured the fluid dynamics at different resolutions, with ranges from about 3 to 40 feet, depending on the instrument.

"Marine biologists that do research on specific fish or corals or other organisms need to measure the flow," said study co-author Alexy Khrizman, a PhD student in Earth system science. "It's very important to know that the choice of the instrument is not going to affect the research. It's also important that we get the flow and turbulence work correct, otherwise our calculations of production and calcification will not be correct."

Serendipitous science

The researchers conducted field work in different locations within the Salomon Atoll in the Chagos Archipelago in the Indian Ocean, south of the Maldives. They were collecting data about a reef off Île Anglaise as part of a larger initiative to study the British Indian Ocean Territory Marine Protected Area when they realized they were prepared to test the assumption that one instrument would provide enough information to understand the flow of the entire reef.

"We were sort of testing our toolbox," Lindhart said. "We had all these instruments in the water already and were actually looking for something else -- it's rare that you have the opportunity to measure the same thing, but in different ways."

The researchers used the data they collected to construct a three-dimensional model of the reef and its flow, bringing new clarity to the life of these underwater cities.

"This is the first three-dimensional construct that tells us how the roughness and its variability from place to place impacts water flow over the reef," Dunbar said. "There's a direct correlation between the roughness of the coral reef and the biodiversity of the reef."

Fundamental insights

Through their research, the study authors aim to answer foundational questions about how these incredibly complex structures interact with incoming energy.

"There are so many ways to study reefs, what we sometimes call the currency by which you're going to see what's going on. For most people, it's fish or the corals themselves," Dunbar said. "What's really new is that our currency is different -- this paper is about using the physics of moving water as currency."

They also hope the findings will be useful to conservation managers. Coral reefs are like "super-efficient cement factories," according to Dunbar, producing architectures and buildings that are self-healing. Although they comprise less than 1 percent of the surface area of the ocean, reefs are home to about 25 percent of all marine life.

"In order to make any kind of projection about climate change, we need to know how they are working right now," Lindhart said. "The beautiful thing about physics is that it's the same everywhere -- once we've established some principles, you can take them and use them somewhere else."

Dunbar is the W.M. Keck Professor at Stanford Earth and a senior fellow at the Stanford Woods Institute for the Environment. Study co-authors include Stephen Monismith, the Obayashi Professor in the School of Engineering, and David Mucciarone, lab manager in the Department of Earth System Science.

This research was supported by the Bertarelli Program in Marine Science, a Stanford Graduate Fellowship, the Gerald J. Lieberman Fellowship and National Science Foundation grant OCE-1948189.