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A June 2023 marine heatwave in the northwestern North Sea produced a substantial phytoplankton bloom below the surface—one that ocean-colour satellites mostly failed to detect. An autonomous ocean glider measuring the water column continuously found chlorophyll concentrations above 3 milligrams per cubic metre (mg/m³), with brief peaks above 9 mg/m³ that lasted for more than a week.
The finding, reported in a 2026 Communications Earth & Environment paper, shows why surface imagery alone can underestimate biological responses to marine heatwaves. The strongest signal formed in a subsurface chlorophyll maximum, where seasonal layering created favorable combinations of light and dissolved nutrients.
What happened during the 2023 North Sea heatwave
In June 2023, an extreme marine heatwave affected the northwestern North Sea. Researchers from the UK’s National Oceanography Centre (NOC), Bangor University and partner institutions tracked conditions with an autonomous ocean glider that repeatedly profiled the water from near the surface downward.
Satellite ocean-colour products showed only a short-lived increase in phytoplankton near the surface. The glider revealed a different picture: a stronger response deeper in the water column that remained elevated for more than a week and exceeded normal June observations for the region.
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The study was published in 2026 as “Chlorophyll extremes during a surface marine heatwave in a seasonally stratified shelf sea” in Communications Earth & Environment (DOI: 10.1038/s43247-026-04118-2).
How large was the hidden bloom?
The bloom developed in the subsurface chlorophyll maximum (SCM), a layer where phytoplankton concentrations naturally become higher than in the water immediately above or below. In a seasonally stratified sea, lighter surface water can sit over denser water, separating organisms and nutrients into layers while still allowing enough light to reach a productive depth.
After a typical seasonal bloom, chlorophyll concentrations in the region were about 1–2 mg/m³. During the heatwave, the glider measured concentrations above 3 mg/m³, with short-lived peaks exceeding 9 mg/m³. Those values were not a single surface snapshot: the elevated readings persisted for more than a week as the glider moved through the water column.
Chlorophyll is used as an indicator of phytoplankton biomass because it is the light-harvesting pigment in these organisms. It is not a direct count of cells or a complete measure of ecosystem health, but a marked increase indicates a major change in the amount of photosynthetic material present.
Why satellites missed much of the bloom
Ocean-colour satellites are excellent at mapping broad patterns across the sea surface. Their sensors infer chlorophyll from light that leaves the uppermost water, providing frequent, wide-area coverage. That strength is also their limitation: the signal is dominated by near-surface conditions and does not reveal the full vertical profile.
In this event, the largest chlorophyll increase was below the layer that satellites could observe clearly. The surface signal was brief, while the deeper maximum was both stronger and longer-lasting. A satellite image could therefore suggest a modest or rapidly ended bloom even while substantial phytoplankton remained beneath it.
The NOC describes this as a monitoring blind spot during marine heatwaves. Water can become optically clear at the surface while a dense chlorophyll layer sits deeper, or surface conditions can change before the subsurface response develops. Without measurements below the surface, those stages are easy to miss.
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An SCM is a depth zone—often tens of metres below the surface, depending on local conditions—where chlorophyll concentration reaches a maximum. It is common in stratified seas. Phytoplankton need light for photosynthesis and nutrients such as nitrogen and phosphorus for growth; the depth at which those resources overlap can be more productive than the immediate surface.
Seasonal stratification changes both the position of that overlap and the rate at which water mixes. A shallow, brightly lit surface layer may be nutrient-poor after an earlier bloom, while a deeper layer can receive enough light and retain access to dissolved nutrients. The North Sea observations show that a heatwave can alter this balance without producing a dramatic surface-colour signal.
What may have caused the response?
The researchers did not identify one definitive mechanism. The NOC says increased light availability and/or enhanced horizontal transport may have contributed. Both possibilities are consistent with a bloom that developed below the surface rather than appearing as a simple warm-water surface event.
Tom Rippeth, professor of physical oceanography at Bangor University, emphasized that temperature is not necessarily the controlling variable by itself. In his words, “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.” He added that this change affects “two of the key ingredients sustaining life in the ocean: the availability of light and the availability of dissolved nutrients.”
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Satellite sensing and glider observations compared
| Capability | Ocean-colour satellite | Autonomous ocean glider |
|---|---|---|
| Vertical coverage | Primarily the optically observable surface layer | Profiles the water column from near the surface to the glider’s operating depth |
| Temporal continuity | Repeated passes, limited by orbit, clouds and daylight | Continuous or near-continuous measurements while deployed |
| Spatial scale | Broad regional and basin-scale maps | Localized tracks and profiles along a mission route |
| Subsurface chlorophyll maximum | Can miss a maximum that is weakly expressed at the surface | Can measure the depth, intensity and evolution of the maximum directly |
| Best use | Finding large-area surface patterns and placing events in regional context | Resolving vertical structure and changes as an event unfolds |
Neither instrument replaces the other. Satellites provide the geographic breadth needed to identify where unusual surface conditions occur. Gliders supply the vertical detail and persistence needed to determine whether the main biological response is above, below or between those surface signals.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why continuous water-column measurements matter
Traditional ship surveys can collect detailed profiles, but they are intermittent and expensive. A bloom that develops and fades between scheduled cruises may never be sampled at its peak. The glider in this study followed the event as it happened, repeatedly recording chlorophyll and the surrounding physical structure.
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.”
That capability is particularly important for heatwaves, which can evolve over days rather than months. A continuous profile can show whether a chlorophyll maximum is intensifying, sinking, spreading horizontally or disappearing, while also documenting the temperature and layering conditions associated with the change.
What the result means for marine-heatwave monitoring
The North Sea case does not mean satellite-based bloom estimates are useless. It demonstrates that a surface-only estimate can be incomplete. Ecological assessments that rely solely on satellite colour may understate the amount, duration or depth of phytoplankton activity during a stratified heatwave.
A more reliable observing strategy combines the two scales:
- Use satellites to scan large areas, identify unusual surface conditions and place local measurements in a regional map.
- Deploy gliders where heatwave conditions or satellite anomalies warrant detailed follow-up, especially in seasonally stratified shelf seas.
- Measure the physical structure alongside chlorophyll, including depth-dependent temperature, light and nutrient conditions, so that a biological change can be interpreted rather than merely detected.
- Repeat profiles through the event instead of treating one cruise or one image as a complete account of the bloom.
The broader lesson is about visibility. A bloom can be ecologically significant without turning the surface visibly green. Detecting that hidden response requires instruments that look through the water column as well as sensors that look across it.
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