North Sea gliders reveal hidden subsurface phytoplankton blooms

Researchers using autonomous ocean gliders have discovered significant subsurface phytoplankton blooms in the North Sea that were largely undetected by satellite monitoring during a June 2023 marine heatwave.

While satellite observations indicated only a brief increase in phytoplankton near the ocean surface, the gliders revealed a much more sustained and intense biological response deeper in the water column. In the subsurface chlorophyll maximum, concentrations rose from typical post-bloom levels of 1–2 mg per cubic meter to over 3 mg per cubic meter, with temporary peaks exceeding 9 mg per cubic meter. These elevated levels persisted for more than a week.

The study, led by the National Oceanography Centre and published in Communications Earth & Environment, highlights a critical blind spot in current marine monitoring. Because satellites can only observe the ocean surface, they may miss major ecosystem shifts occurring beneath the surface. Scientists emphasize that continuous monitoring of the entire water column is essential for understanding how marine ecosystems respond to increasingly frequent climate extremes.