Why do some bays on St. John have thriving coral reefs while others look like a wasteland? Why does sargassum seaweed pile up on certain beaches? What might happen to nearby marine life if you build a pier in a particular spot? How long might it take for fuel from a shipwreck to contaminate a nearby reef? These are some of the questions that a recently published study led by Weifeng “Gordon” Zhang, Ph.D. of Woods Hole Oceanographic Institution, can help answer.

Zhang is a member of Woods Hole’s Reef Solutions Initiative, which has been conducting research in the Virgin Islands for more than a dozen years. Each member focuses on at least one aspect of coral reef research; together, they form an interdisciplinary team with the shared goal of rebuilding coral reefs around the world. Zhang’s specialty is coral reef hydrodynamics — how water flows, and how its movement affects coral reefs. “He deploys instruments at strategic locations to measure water motions and hydrodynamic conditions over coral reefs,” said Amy Apprill, Ph.D., who heads up the Reef Solutions team. “He also develops high-resolution hydrodynamic computer models to quantify underwater weather, study larval transport in reefs, and guide efficient reef experiments and restoration activities.” Zhang’s most recent studies, which were published in Frontiers, present a very specific computer model of the movement of the waters surrounding St. John. Source readers, beware: the paper is very technical; it’s written for scientists who can use the data for guiding decisions on coral reef restoration as well as others involved in conservation, planning and management activities. (Zhang also has data for St. Thomas, although it is not as refined as the data from St. John at this point.) Zhang sees his computer model as akin to providing “underwater weather forecasting,” and it is a hugely complex task. It requires observations by those with local knowledge, data gathered from high-tech instruments, and supercomputers to process all the information. Zhang began to develop the software for this study about three years ago after deploying underwater sensors to measure water flow, temperature, and tide height over time. Every three to six months, team members would collect the sensors, download the data, incorporate it into the model and test it for accuracy.

Some of the data comes from a buoy operated by NOAA off the south side of St. John. (Source readers interested in real-time conditions for swimming and boating will find this site useful.) For each location, the model takes into account air temperature, humidity, wind speed and direction, water temperature, and salinity. There are numerous other factors that alter the flow of water in any particular spot, including the depth and contours of the bottom, the topography (shape) of the surrounding land forms, orientation (whether the location faces into or away from prevailing winds) and seasonal changes. “The ocean is ‘forced’ by tides, weather, and large-scale ocean flows that change constantly,” said Zhang. During certain months, plumes of water from the Orinoco and Amazon rivers in South America make their way north toward the Virgin Islands, bringing water that is less salty (and therefore less dense). The way these layers of different densities interact can affect the flow of water and the overall health of coral reefs around St. John.

Zhang’s work has been particularly useful to Reef Solutions team member Aran Mooney, Ph.D., who measures how newly spawned coral larvae are attracted to the sounds of healthy coral reefs.

“Corals are mobile only in their larval stage. That’s when they float through the ocean, seeking a good place to settle where they will metamorphize into the stationary animals familiar to divers and snorkelers. They use a wide range of chemical and biophysical cues to locate favorable habitats,” wrote Source writer Bernetia Akin. “The study showed that larvae were far more likely to settle on a reef when they heard sounds that indicated it was healthy, such as noises indicating fish were present in significant numbers,” Akin continued. Zhang’s model has helped Mooney determine whether the larvae from coral spawned at a particular reef can drift to sites where they can settle and thrive. “Some coral larvae are only on the surface at the beginning, then they gradually sink and begin to swim a bit,” said Zhang. The density of the water, the wind-driven currents, the tides and the seasons will all affect where they eventually settle,” he added. The model could also be used to study the spread of coral diseases, such as Stony Coral Tissue Loss Disease, determine sites with minimal coastal impacts and to aid in the selection of locations where micronutrient supplements might be most useful to enhance coral growth and immunity. All of this work depends on observation by local experts, and Woods Hole Oceanographic Institution has developed a partnership with the University of the Virgin Islands to conduct research as well as the Virgin Islands National Park. Zhang is continuing his research in other locations, including the Solomon Islands, Little Cayman, Dominica, Hawaii, and the Marshall Islands; each study will take two or three years. “We want to build highly accurate hydrodynamic models for reefs around the world,” he said.

The information he gathers can help guide decisions for conservation and development. Zhang said when Reef Solutions scientists held a public meeting on St. John in April, listeners were curious about applying the model to the controversial Summers End Marina project in Coral Bay, which has been pending since 2014. Zhang said he would need more detailed information for his model to determine the impact of the marina. “With some investment, we could work on that,” he said, then added, “The model can only say what the impact on flows in the surrounding area would be, not whether we should build it or not.” The model could also help planners decide how they might deploy booms or other mitigation devices to direct sargassum away from intake valves for desalination plants, a problem that has been ongoing in the Virgin Islands. But Zhang points out that where the sargassum is diverted to is a decision left up to politicians and planners. The Reef Solutions team’s main purpose is to help protect coral reefs. “If you have limited resources, what do you focus on? You can’t protect it all,” Zhang said. “Coral reefs around the world are under threat, and their health affects the livelihoods of countless local communities.” With hundreds of thousands of tropical islands, the question becomes how to scale up Zhang’s project. “I want to speed this up. I have some ideas involving machine learning,” he said. “I need some time and financial support to concentrate on it and make headway.” NOTE: For more information on the Reef Solutions team, click here. By holding down the cursor on team members’ photos, you can see a summary of their work.