On a quiet July morning in 2018, sensors deep beneath the brackish waters of the Chesapeake Bay recorded something invisible to the eye but seismic in consequence: a pulse of warmth creeping along the seafloor, undetected by satellite, yet spreading steadily through the estuary’s lower depths. This was no ordinary marine heat wave—it was a vertical marine heat wave, a phenomenon only now being fully understood thanks to a groundbreaking study led by Nathan Shunk, a third-year Ph.D. student at the Batten School of Coastal & Marine Sciences and the Virginia Institute of Marine Science (VIMS). From 1985 to 2023, Shunk and his advisor, Assistant Professor Piero Mazzini, analyzed over three decades of 3D temperature data across 31,000 simulated locations in the Bay, revealing that marine heat waves are not just surface events—they unfold in complex, layered patterns that demand a new way of thinking about coastal resilience.
For years, marine heat waves were monitored almost exclusively through satellite imagery and surface buoys, leaving the underwater world largely unobserved. But the Chesapeake, like many estuaries, is a stratified system where temperature, salinity, and oxygen levels vary dramatically with depth. By applying high-resolution computer models developed by VIMS researchers Pierre St-Laurent and Marjorie A. M. Friedrichs, Shunk and Mazzini were able to map heat waves in three dimensions, tracking not just when and where they occurred, but how they moved through the water column. They found that surface heat waves tend to be shorter and more intense, affecting smaller areas, while deep-water events last longer and spread across broader swaths of the Bay’s bottom—habitats critical to oysters, crabs, and fish.
Their work introduces a new classification system that categorizes vertical marine heat waves by where they start—surface, bottom, middle, or synchronously—and how they evolve over time. Of all events studied, 40% began at the surface, 25% at the bottom, 15% in the middle, and 20% synchronously. This distinction matters: bottom-initiated heat waves, though less intense, can persist for weeks, exacerbating low-oxygen “dead zones” and stressing benthic ecosystems already burdened by pollution and overfishing. The study also revealed that 75% of the Bay is nine meters deep or less, meaning surface warming often spreads rapidly through the full water column, especially in summer months.
The implications extend beyond science. Marine heat waves have been linked to fishery declines, habitat degradation, and the co-occurrence of stressors like acidification and poor water clarity. With this new framework, resource managers can anticipate not just the arrival of warm waters, but their depth, duration, and potential impact on species and industries. “As far as we know, this work provides the first characterization of subsurface heat waves in an estuarine system using a high-resolution, 3D model,” Shunk said. The ultimate goal? A predictive tool that gives watermen, oyster farmers, and coastal planners enough lead time to adapt.
In a warming world, seeing beneath the surface may be our best way to safeguard what lies below.