During an extreme June 2023 marine heatwave in the northwestern North Sea, autonomous ocean gliders recorded a sustained subsurface phytoplankton bloom that satellites largely missed. Researchers found that chlorophyll concentrations in the subsurface layer rose above 3 milligrams per cubic meter, with short-lived peaks exceeding 9 milligrams per cubic meter.
Autonomous Gliders Uncover Hidden North Sea Algal Blooms
When marine heatwaves strike, scientists traditionally rely on satellites to observe shifts in ocean color and surface phytoplankton. However, space-borne instruments possess a fundamental limitation: they can only view the ocean’s surface. That blind spot became apparent during an extreme marine heat wave in the North Sea in June 2023.
Using an autonomous ocean glider, researchers from the United Kingdom’s National Oceanography Centre continuously monitored the water column in the northwestern North Sea as the heatwave developed. The glider recorded a much stronger and more sustained phytoplankton increase deep in the water column than satellites detected near the surface. The findings were published in Communications Earth & Environment.
According to the study, severe environmental shifts can take place underwater while marine heatwaves unfold, demonstrating why tracking the entire water column is crucial as climate anomalies grow more common.
Subsurface Chlorophyll Spikes Beyond Historical Averages
During the June 2023 heat wave, concentrations within this layer surged well beyond normal parameters.
Typical post-bloom concentrations in the region hover between 1 and 2 milligrams per cubic meter. As the heat wave unfolded, concentrations climbed past 3 milligrams per cubic meter, featuring short-lived peaks exceeding 9 milligrams per cubic meter. These elevated levels persisted for more than a week, significantly outpacing historical June observations for the area.
Evaluating Stratification and the Limits of Satellite Monitoring
Researchers involved in the study emphasize that the primary driver of biological impact during such heat waves is not merely the rise in water temperature itself. The researchers suggest the increase may link to enhanced light availability or horizontal transport during the event. Had gliders not been deployed, a substantial portion of the biological activity during the 2023 heatwave would have gone entirely unnoticed. Dr. Charlotte Williams, a physical oceanographer at the National Oceanography Centre and lead author of the study, highlighted the necessity of combining multiple observation methods.
Autonomous ocean gliders allowed us to observe the marine heat wave as it happened and reveal biological changes that would have been difficult to capture using traditional ship surveys or satellites alone. These technologies are becoming increasingly important for monitoring how our oceans respond to climate extremes.
Dr. Charlotte Williams, physical oceanographer at the National Oceanography Centre
Broader Research Context and Future Monitoring
This investigation took place under the Physics to Ecosystem Level Assessment of Impacts of Offshore Windfarms (PELAgIO) initiative, which is a component of the Ecological Consequences of Offshore Wind (ECOWind) framework. Financial backing for the program comes jointly from the Natural Environment Research Council (NERC) and the Offshore Wind Evidence and Change (OWEC) Programme via grant NE/X008835/1, alongside supplementary funding awarded to Jo Hopkins through NERC grant NE/Y005589/1.