When a marine heatwave strikes, scientists often turn to satellites to see how the ocean is responding. From space, changes in ocean colour can reveal shifts in phytoplankton, microscopic organisms that form the foundation of marine food webs and play a crucial role in the global carbon cycle. But satellites have a fundamental limitation: they can only see the ocean's surface.
That became clear during an extreme marine heatwave in the North Sea in June 2023, when researchers discovered a striking biological response unfolding below the surface.
Using an autonomous ocean glider, researchers from the UK's National Oceanography Centre (NOC) continuously monitored the water in the northwestern North Sea as the heatwave developed. What they found was very different from the picture emerging from satellite observations.
While satellites detected only a brief increase in phytoplankton near the surface, the glider recorded a much stronger and more sustained increase deeper in the water column.
The findings, published in Communications Earth & Environment, reveal how significant changes in marine ecosystems can take place beneath the surface, effectively hidden from the view of satellites.
A hidden bloom
Phytoplankton are tiny, photosynthetic organisms that drift in the ocean. Although individually microscopic, they support marine ecosystems on a huge scale, providing food for organisms ranging from zooplankton to fish and whales.
In seasonally stratified seas such as the North Sea, phytoplankton can gather in a layer known as the subsurface chlorophyll maximum. This layer can sit below the surface, where conditions such as light and nutrients are favourable for growth.
During the June 2023 heatwave, phytoplankton concentrations in this layer increased dramatically.
Typical post-bloom concentrations in the region would be around 1–2 milligrams per cubic metre. During the heatwave, concentrations rose above 3 milligrams per cubic metre, with short-lived peaks exceeding 9 milligrams per cubic metre.
Crucially, these elevated concentrations persisted for more than a week and were considerably higher than historical observations for June in the region. Yet much of this change would have been largely invisible from space.
The study, led by researchers at the UK's National Oceanography Centre in collaboration with Bangor University and other partners, highlights the importance of combining satellite observations with autonomous ocean technologies to understand how marine ecosystems respond to climate extremes.
"The 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," says Dr Charlotte Williams, physical oceanographer at NOC and lead author of the study.
"These technologies are becoming increasingly important for monitoring how our oceans respond to climate extremes."
Tom Rippeth, Professor of Physical Oceanography, Bangor University, a co-author added, "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.
"This change impacts two of the key ingredients sustaining life in the ocean: the availability of light and the availability of dissolved nutrients."
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.