Increasing sea surface temperatures and persisting marine heatwaves lead to more frequent and severe coral bleaching events, but one natural phenomenon may enhance their resilience.
In the face of climate change and rising ocean temperatures, scientists actively seek ways to make coral reefs more resilient to these extreme conditions globally. A new study spanning two decades of research may indicate a series of reefs from the surface to 150 meters deep in the Gulf of Mexico are more resilient to warmer oceans as they are exposed to a wider range of temperatures brought on by a physical movement of seawater called “eddies.”
Eddies are swirls of ocean water rotating either clockwise or counterclockwise, and either pushing warm surface waters down (anticyclonic eddies) or bringing cool, salty deep water up (cyclonic eddies). Eddies are found everywhere in the ocean, some of them branching from major ocean currents, and they can carry masses of ocean waters over long distances.
To investigate these complex movements, scientists at NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML), the University of Miami’s Cooperative Institute for Marine and Atmospheric Sciences (CIMAS), NOAA’s Office of National Marine Sanctuaries (NMS) and National Environmental Satellite, Data, and Information Service (NESDIS) leveraged a collaborative dataset from the international Argo community to investigate an array of Argo floats deployed in the Gulf of Mexico from 2003 to 2022, measuring temperature and salinity from the surface to 2,000 meters deep with sea level altimetry for eddy signals.
“We use open-ocean observations from autonomous profiling Argo floats and satellite data to account for missing data on the shelf-edge coral reef environment, the eddies being the conduit,” explains Jen McWhorter, lead author on the study.
In the Gulf of Mexico, both warm and cold-core eddies are part of the Loop Current system. The Loop Current comes up from the Yucatan Peninsula, carrying warm and salty ocean waters from the Caribbean Sea, and exits through the Florida Straits, connecting ultimately to the Gulf Stream. When eddies separate from the Loop Current region, they bring significant changes in ocean temperature and salinity over a series of coral reefs and banks found along the continental shelf-edge known as the Flower Garden Banks National Marine Sanctuary (FGBNMS).
While reefs globally have experienced degradation, those at East Flower Garden Bank and West Flower Garden Bank have shown consistently high coral cover (over 50% of the bottom) over the last 30 years. The greater distance from shore, higher latitude and and greater depth of Flower Garden Banks compared to most tropical reefs may be reasons for its greater resilience to stressful conditions, but this study spanning two decades of research points to the role eddies may also play.
Tropical coral reefs can generally expect cooler temperatures in the winter and warmer temperatures in the summer, but in the face of climate change the average temperature across seasons is increasing. Eddies can either offset or exacerbate that rising temperature but ultimately, this study finds that the exposure to the large range of temperatures, frequently, is an added benefit.
Cyclonic eddies, bringing cool water upwards, may offer relief for these mesophotic corals during hot summer temperatures and marine heatwaves or heightened cooling in the winter. However, anticyclonic eddies push warm surface water downwards, keeping temperatures more constant through winter months – or hotter in the summer months and compounding an ongoing coral bleaching event.
Given the study finds the East and West Flower Garden Banks were exposed to eddies consistently over the two decades (~15 days/month), scientists at AOML believe the heightened resilience of these reefs may be attributed to the greater temperature range eddies bring beyond seasonal changes. In the context of a changing climate and warmer sea surface temperatures, this may seem a glimmer of hope for coral reefs, but it is projected climate change may lead to the slowing of the Loop Current, weakening the eddy activity in the region.
“These biophysical interactions (eddies) may provide insight to areas that can persist in the face of warming sea surface temperatures while offering species overlap with shallow coral reef environments. There is still so much we don’t know about mesophotic coral reef environments.”
– Jen McWhorter, Ph.D.
Previous studies led by scientists at AOML have illustrated how studying eddies can help scientists predict how oil from accidental oil spills might travel in the Gulf of Mexico and how larvae can be transported from Cuban coasts to Florida and the Gulf of Mexico. The influence of eddies on coral reefs within the Flower Garden Banks National Marine Sanctuary is therefore a testament to the complexity of physical oceanography, conveying how the movement of major ocean currents directly impacts essential ecosystems.
The growing need to understand how climate change will affect coral reefs and, in turn, coastal communities does not only require investigation of a specific area but extends to the major natural movements of water over great distances.
