Marine heatwaves (MHWs) are events characterized by prolonged warm coastal and ocean conditions with wide-ranging impacts on ecosystem health and associated industries. While research on MHWs has historically relied on surface-water datasets from satellite observations and buoy records, new research from the Batten School of Coastal & Marine Sciences & VIMS highlights the need to, quite literally, go deeper.
Published in the Journal of Geophysical Research: Oceans , the study originated as a class project led by Nathan Shunk, a third-year Ph.D. student focused on coastal physical oceanography at the Batten School & VIMS. Alongside his advisor, Assistant Professor Piero Mazzini, Shunk analyzed the horizontal extent and subsurface patterns of MHWs in the Chesapeake Bay from 1985 to 2023, producing what may be the most detailed analysis to date.
Shunk and Mazzini utilized computer models developed by Batten School & VIMS faculty members Pierre St-Laurent and Marjorie A. M. Friedrichs, among others, taking into consideration seasonal and depth variability. The research introduces a supplementary definition for "vertical marine heatwaves" as well as a visual classification scheme of full-column temperature conditions within a study area.
"As far as we know, this work provides the first characterization of subsurface heatwaves in an estuarine system using a high-resolution, 3-D model," said Shunk. "We can look at daily water temperature readings across more than 31,000 simulated locations and compare them to long-term averages, helping us understand where marine heatwaves occur, how they spread, and ways they might impact the Bay going forward."
New classification of heatwave patterns will support researchers & resources managers
Shunk's classification scheme illustrates the potential ways that MHWs manifest in water across both depth and time, revealing that tracking them through surface-water analysis alone does not convey the full picture of how ecosystems may be impacted further down the water column.
To create this classification scheme, Shunk explained: "We looked at how much of the water column was in a marine heatwave state, the start location, and whether it was simultaneously observed at the surface and the bottom. Our goal was then to ensure the classification scheme followed the simplest framework possible, grouping observed patterns into broad categories for practical research purposes."
The ability to identify these patterns will allow researchers to better understand how MHWs impact ecosystem health, informing advisory services provided to coastal resource managers and industry stakeholders responding to them.
MHWs have been associated with long-lasting consequences in estuarine ecosystems, including declines in fisheries as well as degradation of benthic habitats. They have also been found to contribute to multi-stressor environments, including co-occurrence with acidification, low light conditions and dead zones.
"This work is one of the necessary first steps in developing tools meant to support resource managers, watermen and other coastal practitioners in making informed environmental and economic decisions," explained Shunk. "The ultimate outcome of this work would be a tool that accurately predicts marine heatwaves with enough lead time for stakeholders to effectively safeguard our coastal and marine resources."
The Batten School & VIMS' Center of Excellence in Environmental Forecasting (CEEF) produces world-class forecasting tools, designed to be user-friendly and to inform daily decision-making across Virginia and beyond.
Full water column analysis reveals spatial and seasonal variability
The study found that MHWs near the surface were typically shorter, more frequent, and more intense, but affected a smaller fraction of the Bay's surface area at any given time. In contrast, heatwaves in deeper waters tended to occur less often and with lower intensity but lasted longer and were spread across a larger portion of the Bay's bottom habitat.
The scientists found that in the Chesapeake Bay's shallow waters, which are 30 feet deep or less and make up roughly 75% of the Bay, warm conditions at the surface often occurred simultaneously near the bottom. In deeper channels, however, surface and bottom waters were less connected, especially during spring and summer.
The study also found that marine heatwaves can develop differently across the Bay and throughout the year, highlighting their complex nature and the need for more robust monitoring that goes beyond surface water readings.
These findings suggest that surface temperatures alone do not always tell the full story of warming in coastal ecosystems and that further studies of their spatial and seasonal variability are required to fully understand, predict and manage their potential impacts on marine ecosystems.
"Surface observations from satellites and buoys can be representative of conditions throughout the water column in shallow environments such as shoals," explained Mazzini. "But they may miss significant warming events occurring at depth. Capturing the full structure of marine heatwaves therefore requires attention to subsurface conditions as well. To understand these events more completely, sustained monitoring of the full water column should be a priority."
Further research on MHWs will begin this year with Piero Mazzini joining Ming Sun, an assistant professor at the Batten School & VIMS, to explore the relationship between MHWs and oyster reefs in the Chesapeake Bay. The study is supported by W&M's Global Research Institute , whose Seed Funding awards enable researchers to generate preliminary data, test innovative methodologies and build the foundation for larger external grants.
