Satellites detected only a short-lived surface signal during the extreme June 2023 North Sea marine heatwave. An autonomous ocean glider, however, found a stronger phytoplankton bloom hidden deeper in the water column that lasted more than a week. The result exposes a major blind spot in ocean monitoring: surface imagery cannot reveal biological changes concentrated below the surface.
What happened during the June 2023 heatwave
The event affected the northwestern North Sea, where researchers from the UK’s National Oceanography Centre (NOC), Bangor University and partner institutions tracked the water continuously with an autonomous ocean glider. Ocean-colour satellites observed a brief increase in near-surface phytoplankton, but the glider recorded a much larger and more persistent response beneath it.
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The difference was not a contradiction between instruments. Each observed a different part of the ocean. Satellites measure the light reflected from the upper ocean, while a glider travels through the water column and records conditions at multiple depths.
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What a subsurface chlorophyll maximum is
A subsurface chlorophyll maximum is a layer where phytoplankton concentrations peak beneath the surface. In a seasonally stratified sea, warmer, lighter water can remain above cooler, denser water. That layering affects how far light penetrates and how dissolved nutrients are distributed, allowing a productive layer to develop below the surface even when surface imagery shows little change.
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How large the response became
After a typical seasonal bloom, chlorophyll concentrations in this part of the North Sea are about 1–2 milligrams per cubic metre. During the heatwave, concentrations in the subsurface layer rose above 3 milligrams per cubic metre, with short-lived peaks exceeding 9 milligrams per cubic metre. The elevated readings persisted for more than a week and were higher than normal June observations for the region.
Chlorophyll is used as an indicator of phytoplankton biomass, so these measurements show that the heatwave coincided with a substantial biological response—not merely a warmer surface.
Why satellites missed most of the bloom
Ocean-colour satellites provide broad, frequent coverage, but their measurements are dominated by the upper optical layer. They do not profile the full water column. When the strongest chlorophyll increase occurs at the subsurface maximum, the signal can be diluted or hidden by the relatively quiet water above it.
In this case, the satellite record captured a short near-surface increase while the glider followed the deeper maximum as it intensified and remained elevated. Without measurements below the surface, much of the event would have appeared absent or modest in satellite products. The NOC describes this as a monitoring blind spot during marine heatwaves.
Satellites and autonomous gliders measure different things
| Measurement characteristic | Ocean-colour satellites | Autonomous ocean gliders |
|---|---|---|
| Vertical coverage | Surface-focused optical measurements | Profiles the water column at multiple depths |
| Temporal continuity | Repeated passes that can be interrupted by clouds and limited overpass timing | Continuous observations while the glider remains deployed |
| Spatial scale | Broad regional coverage | Localized coverage along the glider’s route |
| Subsurface chlorophyll maximum | Can miss a maximum below the surface | Can detect and track the maximum directly |
The instruments are therefore complementary rather than interchangeable. Satellites show where to look across a large area; gliders reveal how conditions and biology change with depth at a specific location.
What may have driven the underwater response
The researchers did not identify one definitive mechanism. The NOC says increased light availability and enhanced horizontal transport may each have contributed. Changes in the water column’s layering can alter both the light reaching phytoplankton and the supply of dissolved nutrients.
“These new results show that, when we think about the impact of marine heat waves on marine life, the main factor is not the rise in water temperature, it is the change in the layering of the water column,” said Tom Rippeth, professor of physical oceanography at Bangor University.
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Rippeth added that layering changes two essential ingredients for ocean life: “the availability of light and the availability of dissolved nutrients.” That interpretation is a qualification, not a final attribution. The observations establish when and where the bloom occurred, while the relative contribution of light, transport and nutrient changes requires further analysis.
How the glider revealed what a ship survey could miss
Traditional ship surveys collect highly detailed measurements, but they sample discrete locations and times. An autonomous glider can remain at sea during a rapidly changing event, repeatedly diving and surfacing while carrying sensors that measure optical and physical properties. Its vertical profiles made it possible to follow the subsurface chlorophyll maximum through the heatwave rather than relying on a single snapshot.
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Dr Charlotte Williams, the NOC physical oceanographer who led the study, said: “Autonomous ocean gliders allowed us to observe the marine heatwave as it happened and reveal biological changes that would have been difficult to capture using traditional ship surveys or satellites alone.”
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What this means for marine-heatwave monitoring
A surface-only view can understate the biological consequences of a marine heatwave. A satellite image that shows a weak or brief bloom does not prove that the water column is biologically quiet. Subsurface production can affect food availability for organisms at several depths and may change how carbon and nutrients move through a shelf-sea ecosystem, although this study did not quantify those wider ecological effects.
The practical lesson is to combine observing methods: use satellite coverage to identify regional patterns, then deploy profilers such as gliders and underwater optical sensors where vertical structure matters. Continuous depth-resolved records are especially valuable during heatwaves, when stratification can change quickly.
Where the findings were published
The study, “Chlorophyll extremes during a surface marine heatwave in a seasonally stratified shelf sea,” was published in 2026 in Communications Earth & Environment. Its DOI is 10.1038/s43247-026-04118-2.
The central finding is straightforward: the June 2023 North Sea heatwave produced a strong, sustained phytoplankton response below the surface that satellites largely failed to show. Understanding marine heatwaves therefore requires looking through the water column, not just at its surface.
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