This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: When a marine heat wave strikes, scientists often turn to satellites to see how the ocean is responding. From space, changes in ocean color 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 heat wave 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 U.K.'s National Oceanography Centre (NOC) continuously monitored the water in the northwestern North Sea as the heat wave 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.
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 favorable for growth. During the June 2023 heat wave, phytoplankton concentrations in this layer increased dramatically. Typical post-bloom concentrations in the region would be around 1–2 milligrams per cubic meter.
During the heat wave, concentrations rose above 3 milligrams per cubic meter, with short-lived peaks exceeding 9 milligrams per cubic meter. 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.
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