Researchers from William & Mary’s Virginia Institute of Marine Science (VIMS) and the Batten School of Coastal & Marine Sciences analyzed long- term temperature records from 20 U.S. estuaries and found that standard definitions of marine heatwaves (MHWs) miss a large amount of cumulative heat exposure from the warmer periods that surround them.

The study, published in Communications Earth & Environment, examined more than 2,580 MHWs recorded over roughly two decades. MHWs are typically defined by temperature thresholds with clear start and end dates.

The researchers observed that these events are often embedded in longer periods of elevated water temperatures lasting weeks to months. Lead author Ricardo Utzig Nardi noted that the warm- water anomalies on either side mean focusing only on the formal MHW window does not accurately represent real- world conditions experienced by marine life.

Key findings include:

• Pre- and post- MHW warm phases often contribute as much or more cumulative heat exposure as the heatwave itself.
• Conventional assessments underestimate total heat exposure by more than 150% on average.
• About two- thirds of the events were classified as “individual” MHWs embedded within roughly 60 days of elevated temperatures beyond the heatwave window.
• The remaining one- third were “compound” events associated with prolonged warming of about 90 days before and after the MHW; these generated more than three times the cumulative heat exposure of the MHW phase alone.

This matters because estuaries are critical nursery habitats and support major fisheries and coastal economies. The extra sustained heat can increase physiological stress on organisms (such as seagrasses, shellfish, and fish), alter ecosystems, and raise risks beyond what isolated heatwave metrics suggest. The work provides a framework for treating MHWs as part of broader warming episodes to better assess ecological impacts.

The research builds on earlier VIMS work on estuarine MHWs (including projections that East Coast systems could experience heatwave conditions for a substantial fraction of the year later this century) and uses high- frequency in situ data from networks such as NOAA’s National Estuarine Research Reserve System. Related studies from the same group have also examined co-occurrence of estuarine and coastal MHWs and subsurface (“vertical”) structure of heatwaves in places like Chesapeake Bay.

In short, the “hidden source of heat” is the extended warm anomalies surrounding the formally defined heatwaves, which substantially amplify total thermal stress on marine ecosystems.

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Estuarine marine heatwaves (MHWs), prolonged periods of anomalously warm water in coastal embayments, bays, and river mouths, pose significant and often underestimated threats to highly productive but vulnerable ecosystems. Estuaries serve as nurseries for ~75% of U.S. commercial fish and shellfish species and support tens of millions of jobs, making impacts cascading and economically consequential.

The recent VIMS/Batten School study (Nardi et al., 2026 in Communications Earth & Environment) is especially relevant: by analyzing >2,580 MHWs across 20 U.S. estuaries, it showed that pre- and post-event warm anomalies (often lasting weeks to months) boost total cumulative heat exposure by ~153% on average compared with the formal MHW alone. This means organisms experience far more sustained thermal stress than standard metrics capture, amplifying risks.

MHWs drive direct physiological stress, mortality, habitat loss, and cascading community changes. Key effects include:

Foundation species and habitats: Seagrasses (e.g., eelgrass Zostera marina in Chesapeake Bay and other temperate systems) suffer die- offs when temperatures exceed thresholds (often ~25- 30°C or +1- 5°C above normal summer peaks). This leads to habitat compression, reduced carbon storage, sediment destabilization, and loss of nursery functions. Macroalgae and other vegetation can show long-lasting biomass declines. In some cases, heat-tolerant or invasive species expand.

Benthic and shellfish communities: Mass mortalities of oysters, clams, mussels, and other invertebrates occur, especially during low tides coinciding with heat or in shallow areas. Thermal stress reduces growth, reproduction, and survival; combined with low dissolved oxygen (DO), it intensifies effects. Bioturbation and nutrient cycling by benthic organisms can shift.

Fish and mobile species: Fish kills are documented. Habitat compression (warm surface and hypoxic deeper waters) squeezes preferred zones. Blue crab migration and spawning patterns in systems like Chesapeake Bay can be disrupted. Larval stages are particularly vulnerable. Range shifts favor warm-adapted or subtropical species while stressing cold- adapted ones.

Water quality and multi- stressors: Warming reduces oxygen solubility and accelerates microbial respiration, expanding hypoxic/anoxic zones (observed in Chesapeake Bay studies spanning decades of data). This compounds eutrophication common in many estuaries. Harmful algal blooms can intensify; pH may drop. Subsurface warming often persists beyond surface MHWs, especially in stratified seasons.

Food webs and biodiversity: Cascading effects include shifts in plankton communities, altered energy flow, reduced biodiversity, and changes in predator- prey dynamics. Recovery can take years; some systems show persistent regime shifts.

East Coast systems (including Chesapeake Bay, the largest U.S. estuary) show clear increases in MHW frequency and cumulative intensity driven by long- term warming. Projections indicate East Coast estuaries could experience MHW conditions for ~1/3 of the year by 2100 if trends continue, far beyond the current ~6% (≈22 days/year) in places like Chesapeake Bay. West Coast estuaries generally lack strong trends and may act as thermal refugia, partly due to upwelling.

In Chesapeake Bay specifically:

• MHWs average ~2 events/year lasting ~11 days (intensities ~3°C, peaks higher).
• Subsurface effects include expanded hypoxia and warming that can lag or persist relative to the surface.
• Documented links to seagrass losses, fishery species declines, and potential habitat squeeze for crabs and fish.

Global reviews confirm similar patterns: foundation- species mortality (seagrasses, etc.), fisheries disruptions, and ecosystem-service losses occur across temperate and subtropical estuaries and coasts.

Socioeconomic Consequences

Fisheries and aquaculture: Declines or temporary collapses in shellfish and finfish landings; closures; reduced recruitment lasting years. Economic losses from MHWs globally already reach hundreds of millions to billions annually in affected regions.

Ecosystem services: Reduced carbon sequestration, water quality regulation, coastal protection, recreation, and cultural values.

Communities: Threats to food security, livelihoods in coastal areas, and tourism.

Why the “Hidden Heat” Matters

Standard MHW definitions (temperature exceeding a threshold for a set duration) miss the extended warm anomalies. The ~153% higher cumulative intensity means chronic sublethal stress accumulates more than previously quantified.

This can push organisms past physiological tipping points, delay recovery, and worsen multi- stressor interactions (e.g., heat and low DO). The new framework from the 2026 study aims to better quantify total exposure for ecological assessments, lab experiments, and management.

MHWs are projected to intensify with ongoing climate change. East Coast U.S. estuaries face particularly acute risks.

Adaptation strategies include enhanced monitoring (full water- column temperature and DO), early- warning systems, habitat restoration (e.g., seagrass), fisheries management adjustments (timing/closures), and reducing local stressors (nutrients) to build resilience. Large- scale emission reductions remain essential for long- term mitigation.

Related research from the same VIMS group has examined co- occurrence of estuarine vs. coastal MHWs, vertical structure in Chesapeake Bay, and future exposure trends, providing a growing toolkit for understanding and responding to these events.

In summary, estuarine heatwaves threaten the foundation of coastal ecosystems and economies through direct mortality, habitat loss, water- quality degradation, and food- web disruption. Accounting for the full thermal history (including surrounding anomalies) reveals substantially greater risk than previously assessed.

Estuarine marine heatwave (MHW) early warning systems are emerging but remain less mature than open-ocean counterparts. They rely on high- frequency in situ monitoring, regional ocean models, atmospheric forecasts, and (increasingly) machine learning.

The goal is to provide lead times of days to months so managers, fishers, aqua culturists, and conservation groups can take proactive steps, such as adjusting harvests, relocating stock, enhancing monitoring, or protecting sensitive habitats.

Chesapeake Bay is the most advanced example:

• VIMS Chesapeake Bay Environmental Forecast System (CBEFS): Operational MHW forecasts began in 2025 (surface in March; bottom waters in May). It uses two definitions:
  • Standard MHW: Forecast water temperature exceeds the 90th- percentile climatology for ≥5 consecutive days.
  • Alternate high- temperature alert: Surface temperatures forecast above 82.5°F (≈28°C) for ≥5 days.
• Forecasts require the affected area to exceed ~1,000 acres to focus on significant events. Maps show current/forecast locations, and time- series plots show the Bay area under MHW conditions.
• Supporting research includes probabilistic 35-day SST and extreme- temperature forecasts that show value for 1- 2 weeks of lead time, especially when protection costs are low relative to potential losses (e.g., for catch- and- release mortality or Vibrio risks in oysters).
• NOAA Chesapeake Bay Office and partners (including Scientific and Technical Advisory Committee recommendations) are developing a public Marine Heatwave Alert System linked to fisheries impacts, habitat preferences, and other parameters (DO, salinity). This includes analyses of definitions, buoy/satellite data, and communication products.

National Estuarine Research Reserve System (NERRS): The System- Wide Monitoring Program (SWMP) provides high- frequency (15- min) temperature and water- quality data from dozens of stations across ~30 reserves. These long- term records (used in Nardi et al. studies of 20 and estuaries) form the observational backbone for detecting MHWs and validating models. Real- time data are publicly available and support trend analysis and potential forecasting.

Other regional examples include short -term (e.g., 3-day) multi- parameter forecasts in systems like Puget Sound (temperature, oxygen, pH, etc.), which can flag heat- related risks.

The recent VIMS work on underestimation of heat exposure, vertical structure, and East vs. West Coast trends underscores the need for systems that capture full thermal histories and multi- stressor risks.

Expanded monitoring, coupled models, and stakeholder- designed thresholds are priorities for improving these systems.

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Persistent warm water anomalies before and after marine heatwaves amplify heat exposure and associated risks

Marine heatwaves (MHWs) threaten marine ecosystems, but standard definitions capture only the core event and miss pre- and post- event warm anomalies. This leads to systematic underestimation of the total heat exposure organisms actually experience.

The study analyzed over 2,580 MHW events from 54 stations across 20 U.S. estuaries. Main findings:

• Pre- and post- event warm phases are largely independent of the core MHW metrics (intensity, duration, etc.).
• These surrounding phases often last longer than the formal MHW itself.
• Although less intense than the core event, when added to the MHW they produce a substantial increase in total heat exposure, approximately 153% higher cumulative intensity (°C·days) compared with the MHW alone.
• This extra exposure cannot be reliably inferred from standard MHW metrics alone.

The authors present a new framework for quantifying these pre- and post -event anomalies. It is designed to be broadly applicable to both coastal and open- ocean systems and is intended to improve understanding of MHWs, assessment of ecological impacts, design of laboratory experiments, and ecosystem management.

Additional notes from the page

• Data relied on long- term monitoring from the National Estuarine Research Reserve System (NERRS).
• The article is open access; a PDF is available via the Nature site.
• This is the peer- reviewed version corresponding to the news coverage about the “hidden source of heat” surrounding ocean/estuarine heatwaves.

Published: Communications Earth & Environment (2026)  Open access (CC BY-NC-ND 4.0)

DOI: 10.1038/s43247-026-03739-x

Authors: Ricardo U. Nardi, 
Piero L. F. Mazzini, 
Ryan K. Walter & 
Jian Shen 

Abstract

Marine heatwaves are a major threat to marine ecosystems globally, yet standard marine heatwave definitions, despite providing consistent metrics, do not capture pre- and post-event warm anomalies, leading to systematic underestimation of total heat exposure experienced by marine ecosystems. Here, we analyze over 2580 marine heatwave events from 54 stations across 20 US estuaries to provide a comprehensive characterization of pre- and post-marine heatwaves thermal anomalies. We show that pre- and post-event phases are largely independent of core marine heatwave metrics and often persist longer than the marine heatwave itself. Although these phases are less intense than the marine heatwave event, when combined with the marine heatwave, they account for a substantial increase in total heat exposure—resulting in an approximately 153% higher cumulative intensity (°C·days) compared to the marine heatwave alone—that cannot be reliably inferred from standard metrics. We present a new framework to quantify pre- and post-event anomalies that can be broadly applied across coastal and open-ocean systems. This framework is critical for understanding marine heatwaves, assessing ecological impacts, guiding laboratory experiments, and informing ecosystem management.