Marine heatwaves, defined as periods when sea temperatures exceed a seasonally variable threshold for at least five consecutive days, have been growing in both frequency and intensity around the world as the climate warms. The most severe events, categorized on a scale that runs from I to IV, represent temperature extremes so far outside the historical norm that they have virtually no precedent in the local record. The June 2023 event studied by the team reached Category IV status, the most extreme classification, making it a natural laboratory for understanding what happens to the base of the marine food web when the ocean overheats.

The setting was a seasonally stratified shelf sea, an environment common across the temperate latitudes where warming surface waters sit atop colder, denser deep water, separated by a sharp transition known as the thermocline. This layered structure fundamentally changes how nutrients, light, and plankton interact. In spring, strong mixing delivers nutrients to the sunlit surface, fueling the famous spring bloom. By early summer, stratification locks those nutrients away from the surface, and phytoplankton become concentrated in a subsurface chlorophyll maximum, a hidden layer of elevated algal biomass that thrives near the thermocline where light still penetrates and nutrients remain accessible. Satellite ocean color sensors, which measure the light scattered by chlorophyll at the very surface, are effectively blind to this layer.

The research team, led by Charlotte A. J. Williams of the National Oceanography Centre in Liverpool, deployed an autonomous ocean glider to profile the water column repeatedly through the heatwave. Gliders are torpedo-shaped robots that alter their buoyancy to dive and rise in sawtooth patterns, collecting high-resolution measurements of temperature, salinity, and chlorophyll fluorescence as they go. This vertical dimension of observation proved essential. Surface chlorophyll, the quantity that satellites would detect, rose rapidly from less than 1 milligram per cubic meter to more than 5 milligrams per cubic meter, briefly pushing concentrations beyond the climatological range defined by two standard deviations above and below the long-term mean by as much as 40 percent. Yet this dramatic surface excursion was short-lived, a fleeting pulse rather than a lasting shift.

The subsurface story was entirely different. Within the subsurface chlorophyll maximum, concentrations that would normally sit at typical post-bloom values of 1 to 2 milligrams per cubic meter climbed to more than 3 milligrams per cubic meter, and during episodic peaks they exceeded 9 milligrams per cubic meter, a several-fold intensification of the hidden algal layer. Unlike the surface spike, this subsurface enhancement persisted, sustaining elevated biomass over the course of the event. When integrated over the entire water column, the total chlorophyll inventory reached 140 milligrams per square meter, a depth-integrated measure that captures the full standing stock of phytoplankton and revealed that the heatwave had substantially increased the total amount of algal life the sea could support, not merely shuffled it around vertically.

Understanding why the heatwave produced these extremes requires grappling with the interplay of physical and biological processes. The authors found that the bottom mixed layer, the cold water hugging the seafloor, warmed at an accelerated rate during the event, and that horizontal temperature gradients across the region strengthened. These signals are consistent with a possible influence of horizontal transport processes, meaning that the heatwave may have been associated with lateral movement of water masses, potentially advecting warm water and possibly associated nutrient or biomass signatures into the region. Elevated surface irradiance, the greater light availability that can accompany the calm, clear conditions often linked to heatwave weather patterns, may also have contributed to the enhanced phytoplankton growth. The observations cannot definitively separate these mechanisms, but both point to a plausible explanation in which physical forcing during extreme warming amplifies biological productivity in ways that standard theory might not predict.

The implications ripple outward through the marine ecosystem. Phytoplankton are the foundation of nearly all marine food webs, converting sunlight and dissolved nutrients into the organic matter that sustains everything from copepods to fish to seabirds. A sustained, several-fold increase in subsurface chlorophyll during a heatwave means that the vertical distribution of food available to grazing zooplankton, fish larvae, and other organisms could shift substantially. In stratified seas, many animals forage at or near the thermocline precisely because that is where the food is. Altering the intensity and extent of the subsurface chlorophyll maximum could therefore restructure foraging behavior, growth rates, and ultimately the productivity of commercially important species. The research was conducted as part of the PELAgIO project within the Ecological Consequences of Offshore Wind programme, reflecting growing interest in how climate extremes and human infrastructure interact in shelf sea ecosystems.

There is also a sobering message here for how we monitor the ocean. Much of what we know about global phytoplankton trends comes from satellite ocean color imagery, which samples only the uppermost meters of the sea. If heatwave-driven changes concentrate in the subsurface, as this study shows they can, then satellite-based assessments would record a brief surface anomaly and miss the sustained ecological response below. The depth-integrated chlorophyll increase to 140 milligrams per square meter would be largely invisible to a satellite pass. Only platforms capable of repeated vertical profiling, such as gliders, Argo floats equipped with biological sensors, or dedicated shipboard sampling, can capture these subsurface dynamics. As marine heatwaves become more common, the case for investing in vertically resolved observing systems grows correspondingly stronger.

The findings also complicate our understanding of what marine heatwaves do to marine productivity. A common assumption holds that warm stratified waters should depress phytoplankton growth by tightening stratification and cutting off nutrient supply to the sunlit zone. The observations from this Category IV event run counter to that expectation in important ways, showing rapid surface enhancement and, more significantly, sustained subsurface intensification. The researchers suggest that horizontal transport processes, capable of moving water properties laterally across the shelf, may be part of the explanation, and they caution that the combined influence of enhanced irradiance and advective effects cannot be cleanly disentangled from the available data. This honesty about mechanism is itself valuable: it flags precisely which processes future studies, and future models, must resolve if we are to forecast how shelf sea ecosystems will fare under continued warming.

Shelf seas punch far above their weight in the global carbon cycle and in the fisheries that hundreds of millions of people depend upon, so understanding their response to climate extremes is far from an academic exercise. The 2023 heatwave observations demonstrate that the most dramatic ecological responses to ocean warming in these seas may occur out of sight, below the surface, sustained over timescales long enough to matter for the food web. As temperatures continue to climb and events of this severity grow more frequent, the hidden layers of the ocean are emerging as a critical frontier for climate science, and the gliders that probe them may prove to be among the most important instruments in the modern oceanographer’s toolkit. The study, published open access in Communications Earth & Environment, stands as a reminder that in a warming ocean, what we see at the surface is only the beginning of the story.

Subject of Research: Subsurface chlorophyll response of a seasonally stratified shelf sea to a Category IV marine heatwave observed by an autonomous ocean glider

Article Title: Chlorophyll extremes during a surface marine heatwave in a seasonally stratified shelf sea

Article References: Williams, C. A. J., Scott, B. E., Rippeth, T. P., Jacobs, Z. L., Wihsgott, J. U., Palmer, M. R., O’Hara Murray, R., & Hopkins, J. E. (2026). Chlorophyll extremes during a surface marine heatwave in a seasonally stratified shelf sea. Communications Earth & Environment. https://doi.org/10.1038/s43247-026-04118-2

Image Credits: AI Generated

DOI: 10.1038/s43247-026-04118-2

Keywords: marine heatwave, chlorophyll, phytoplankton, shelf sea, ocean glider, seasonal stratification, subsurface chlorophyll maximum, thermocline, ocean warming, satellite ocean color, marine ecosystem, Communications Earth & Environment

The study, published in Discover Oceans by Madhurima Paul, Rashmi R. Nayak, and Arun Chakraborty of the Indian Institute of Technology Kharagpur, draws on 263 depth-resolved profiles collected between March 2015 and December 2020 by a biogeochemical Argo float (WMO ID 2902174) deployed by the Indian National Centre for Ocean Information Services. Every ten days, the float dove to 2000 meters and ascended, measuring temperature, salinity, dissolved oxygen, and chlorophyll-a fluorescence along the way. Because the float stayed within a compact box between roughly 70.3°E and 73.35°E and 13.2°N and 17.2°N, the researchers could build a clean, location-specific climatology of how plankton biomass is distributed through the water column across the entire annual cycle.

The headline finding is deceptively simple. Surface chlorophyll-a in the CEAS climbs through the southwest monsoon months, peaking at about 0.31 milligrams per cubic meter in August, then plummets sharply in September before surging back to roughly 0.36 milligrams per cubic meter in October. Satellite ocean-color data and the float measurements agree closely, with a correlation of 0.86, and the September dip and October rebound appeared in every single year of the record. That consistency rules out a one-off anomaly and marks the pattern as a genuine climatological signature of the region, one that neither the persistently upwelling southeastern Arabian Sea to the south nor the freshwater-dominated northern waters display in the same way.

Why does September fail? The answer, according to the study, lies in the physics of the retreating monsoon. During the peak monsoon, strong winds associated with the low-level Findlater Jet push wind stress to around 0.13 to 0.14 newtons per square meter, driving turbulent mixing and positive Ekman pumping velocities of 1.5 to 2.5 times ten to the minus six meters per second. This Ekman suction draws cold, salty, nutrient-rich water upward, feeding the phytoplankton and deepening the mixed layer to about 40 meters. In September, however, wind stress collapses to roughly 0.06 newtons per square meter, the wind stress curl weakens, and the Ekman pumping velocity drops sharply. Vertical mixing falters, the upward nutrient conveyor stalls, and the phytoplankton bloom runs out of fuel despite the water still being relatively cool.

The float data add a crucial vertical dimension that satellites simply cannot capture. During the spring intermonsoon from March to May, intense solar heating stratifies the upper ocean, shoaling the mixed layer to around 20 meters. Surface nutrients are rapidly exhausted, and near-surface chlorophyll falls below 0.1 milligrams per cubic meter. Yet below the mixed layer, chlorophyll thrives, reaching about 0.2 milligrams per cubic meter in a pronounced subsurface chlorophyll maximum between 30 and 50 meters, where light from the deeply penetrating euphotic zone, extending to 100 to 140 meters, meets residual nutrients. Integrated biomass during this period is modest at about 17 milligrams per square meter, but most of it sits beneath the layer that satellites would count.

This vertical seesaw between the mixed layer and the water beneath it is the study’s central structural insight. Correlation analysis shows that mixed layer depth is positively linked with chlorophyll in the near-surface and mixed layers (a correlation coefficient of 0.48) and strongly negatively correlated with chlorophyll below the mixed layer (minus 0.87). In other words, when monsoon winds deepen the mixed layer and pump nutrients upward, biomass concentrates near the surface and thins at depth; when the ocean calms and stratifies, the opposite occurs, and phytoplankton migrate downward to a depth where light and nutrients overlap. The subsurface chlorophyll maximum, which the authors note can account for nearly one-fifth of total euphotic-zone biomass basin-wide, is a hidden reservoir that satellite chlorophyll maps systematically miss.

The October rebound has its own mechanism, and it is not simply the mirror image of the September collapse. As the southwest monsoon winds die away, sea surface temperature climbs to a secondary peak of about 29.2 degrees Celsius, the mixed layer shoals to roughly 17 meters, and sea surface salinity drops sharply to about 35.4 practical salinity units. That freshening reflects the poleward reversal of the West India Coastal Current, which begins transporting low-salinity water from the Bay of Bengal along the coast, alongside increased discharge from rivers such as the Periyar, Vaigai, and Tambraparni. These freshwater inputs, combined with northward-flowing surface currents, deliver nutrients and displace nutrient-rich water northward, driving the sharp rise in near-surface chlorophyll, concentrated mainly in the top ten meters of the water column.

The study also highlights how salinity structure shapes the region’s biology in less obvious ways. A barrier layer, the salt-stratified zone between the isothermal layer depth and the mixed layer, thickens markedly during the winter monsoon months of December through February, insulating the mixed layer from cooler subsurface water and influencing sea surface temperature evolution. Meanwhile, the depth of the 20 degree Celsius isotherm, a standard proxy for thermocline depth, deepens during winter, a feature the authors link to heat trapped by the Findlater Jet during the previous summer and redistributed by westward-propagating downwelling Rossby waves. As spring approaches, these waves decay, the thermocline shoals, and the trapped heat is ventilated upward, resetting the upper-ocean structure for the next annual cycle.

Ecologically, the findings help explain why the CEAS sits in an awkward middle ground. To the south, the southeastern Arabian Sea’s vigorous upwelling supports large phytoplankton, rich fisheries, and planktivorous fish communities; to the north, winter convective mixing sustains an extended growing season. The CEAS, by contrast, experiences weaker winds, reduced upwelling, and a deeper thermocline, resulting in lower chlorophyll and integrated biomass during winter and a weaker fishery dominated by different trophic structures. The region also acts as a perennial source of carbon dioxide to the atmosphere in some seasons, while river-fed productivity during the southwest monsoon can temporarily tip it toward being a carbon sink, underscoring how sensitive its biogeochemical balance is to the timing of winds, currents, and runoff.

Perhaps the study’s most important message is methodological. A single float, profiling faithfully every ten days for six years, exposed a seasonal pattern invisible to two decades of satellite ocean color and decades of shipboard sampling. As climate change alters monsoon winds, river discharge, and ocean stratification, the biological response of regions like the CEAS will depend on fine-scale vertical processes that only autonomous biogeochemical sensors can resolve. The September dip and October rebound documented here are more than a regional curiosity; they are a demonstration that the ocean’s hidden interior still holds surprises, and that the robotic observers drifting silently through it are our best hope of catching them before the baseline itself begins to shift.

Subject of Research: Seasonal variability of chlorophyll-a and phytoplankton biomass in the central eastern Arabian Sea observed by biogeochemical Argo floats

Article Title: Variability of chlorophyll-a in the central part of the eastern Arabian Sea using biogeochemical Argo observations

Article References: Paul, M., Nayak, R. R., & Chakraborty, A. (2026). Variability of chlorophyll-a in the central part of the eastern Arabian Sea using biogeochemical Argo observations. Discover Oceans, 3(1), Article 15. https://doi.org/10.1007/s44289-026-00128-2

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00128-2

Keywords: chlorophyll-a, Arabian Sea, BGC-Argo, phytoplankton, monsoon, upwelling, mixed layer depth, subsurface chlorophyll maximum, West India Coastal Current, ocean biogeochemistry, satellite ocean color, marine productivity