An extreme marine heatwave in the North Sea triggered a dramatic increase in phytoplankton below the ocean surface that would have been largely invisible to satellites, new research has found.
The study, led by the National Oceanography Centre (NOC), and published in Communication Earth & Environment , reveal how major ecosystem changes can occur beneath the surface during marine heatwaves, highlighting the importance of observing the ocean throughout the water column as climate extremes become more frequent.
Phytoplankton form the foundation of marine food webs, meaning changes in when and where they grow can have consequences for organisms higher up the food chain, as well as wider marine ecosystems and biogeochemical cycles.
During the extreme marine heatwave of June 2023, researchers used an autonomous ocean glider to continuously monitor conditions in the northwestern North Sea. While satellite observations showed only a brief increase in phytoplankton near the surface, the glider revealed a much stronger and more sustained response deeper in the water.
Phytoplankton levels in the subsurface chlorophyll maximum – a layer where phytoplankton are concentrated below the ocean surface – rose from typical post-bloom levels of 1–2 mg per cubic metre to more than 3 mg per cubic metre, with short-lived peaks of more than 9 mg per cubic metre. These higher levels lasted for more than a week and were well above what is normally seen in the region in June.
Dr Charlotte Williams, physical oceanographer at the NOC and lead of 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. These technologies are becoming increasingly important for monitoring how our oceans respond to climate extremes.”
The study highlights a potential blind spot in how marine heatwaves are currently monitored. Satellites provide valuable information about changes at the ocean surface but cannot see the full water column. Without the glider observations, much of the biological response recorded during the 2023 heatwave would have gone undetected.
The researchers could not definitively identify the processes responsible for the increase in subsurface phytoplankton but suggest it may have been linked to increased light availability and/or enhanced horizontal transport during the heatwave.
As marine heatwaves become more frequent and intense in a warming ocean, understanding these hidden responses will be increasingly important for predicting how marine ecosystems, fisheries and ocean biogeochemical cycles could change.
Read the full study: Chlorophyll extremes during a surface marine heatwave in a seasonally stratified shelf sea | Communications Earth & Environment
The research was conducted as part of the Physics to Ecosystem Level Assessment of Impacts of Offshore Windfarms (PELAgIO) project, part of the Ecological Consequences of Offshore Wind (ECOWind) programme. ECOWind is co-funded by the Natural Environment Research Council (NERC) and the Offshore Wind Evidence and Change (OWEC) Programme under grant NE/X008835/1. Jo Hopkins was supported by NERC grant NE/Y005589/1.
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