Study warns of deadly future marine heat waves in East Coast estuaries

A first-of-its-kind study led by William & Mary's Batten School & VIMS is predicting estuaries along the East Coast of the U.S. will experience marine heat wave conditions for up to a third of the year by the end of the century. With estuaries serving as important nursery habitats for nearly 75% of all fish species and supporting more than 54 million jobs, this could have devastating consequences for marine ecosystems as well as the fisheries and communities that depend on them.

The study, published in Scientific Reports, used long-term monitoring data from the National Oceanic and Atmospheric Administration's (NOAA) National Estuarine Research Reserve (NERR) program to examine conditions in 20 estuaries across the U.S. over the past two decades. The results showed rising frequencies of marine heat waves in East Coast estuaries that—if continued as modeled—could have disastrous ecosystem consequences.

"The Chesapeake Bay, for example, currently experiences marine heat wave conditions approximately 6% of the year (22 days per year), and that is already placing stress on the ecosystem. Our study shows that estuaries across the East Coast could experience these conditions for more than 100 days of the year by 2100," said Batten School & VIMS Assistant Professor Piero Mazzini, co-author and academic advisor to lead author Ricardo Nardi. "This research should serve as a warning to policymakers and environmental managers who are charged with protecting these important ecosystems."

The news was better for West Coast estuaries, which did not show significant warming trends and may serve as important future refuges for many species. The researchers hypothesized that this was due to persistent wind-driven regional upwelling in the Pacific Ocean, which brings deep, cold water up to the surface.

Most research on marine heat waves focuses on larger areas of open ocean, where satellites are able to provide long-term thermal data, or on individual estuaries.

"Our study is the first to paint a picture of the effect of climatic variability influencing marine heat waves in estuaries across the nation, and it would not have been possible without the long-term data provided by NOAA's NERR monitoring system," said Nardi, who pursued the research as part of his master's thesis at W&M's Batten School of Coastal & Marine Sciences.

The study also showed connections between large-scale climate patterns, such as El Niño and the Pacific Decadal Oscillation (PDO), in modulating marine heat waves, especially in West Coast estuaries where positive phases can more than double the occurrences. While estuaries are typically considered to be interdependent, the researchers found strong relationships between estuaries within similar geographic regions. This finding points to atmospheric heat exchange as the dominant driver of the heat waves.

This study builds on past research by the Batten School & VIMS, which was one of the first to document estuarine heat waves in the Chesapeake Bay. As he advances his master's thesis, Nardi plans to study the extent to which estuaries are connected to open-ocean processes.

"We need to carefully quantify all of the factors influencing heat within these systems, including connections between the estuaries, their tributaries and coastal ocean conditions," said Nardi. "These are critical ecosystems and future conservation efforts will depend on our understanding of the factors influencing them."