Scientists have detected a significant long-term decline in the productivity of waters around the UK and across much of the north-east Atlantic, raising concerns for fisheries, marine food webs and the ocean’s ability to absorb carbon dioxide from the atmosphere.
The research, led by Dr Gavin Tilstone and Dr Peter Land at Plymouth Marine Laboratory, used more than two decades of satellite data spanning 1997 to 2018 to track changes in microalgae production, the process by which microscopic marine plants known as phytoplankton, use sunlight and carbon dioxide to grow. These tiny organisms form the foundation of marine ecosystems, feeding everything from small fish to whales.
After a brief increase in productivity in the late 1990s and early 2000s, the study found a steady decline across much of the region, particularly in the waters around north-west Europe, the Irish Sea, the North Sea, the western English Channel and parts of the Norwegian Sea.
The researchers link this decline primarily to warming sea temperatures and changes in how ocean water mixes between deeper and surface layers. As the ocean warms, it becomes more layered, with warmer water sitting on top of cooler water below in a process known as stratification. This makes it harder for nutrients from deeper water to reach the surface, where phytoplankton need them to grow.
Dr Tilstone explained: ‘As the ocean warms, these layers become stronger and less likely to vertically mix. This matters because the mixing of ocean waters helps transport nutrients from the depths to the surface, where phytoplankton can use them to grow. When that supply is reduced, microalgae productivity can decline.’
Dr Peter Land, Remote Sensing Scientist at PML, added: ‘In many regions, warming surface waters and altered mixing are reducing the conditions phytoplankton need to thrive. This limits the energy entering marine food webs and can have huge knock-on effects for fish stocks and ecosystem services.’
The study also found that in some areas, the timing of the seasonal spring bloom, when phytoplankton numbers surge each year, has shifted earlier than in previous decades. This could disrupt the delicate timing between phytoplankton, the small creatures that feed on them, and the fish larvae that depend on this food chain, potentially affecting how successfully young fish survive.
Not every area showed the same pattern. The Celtic Sea, for example, showed stable or even increasing productivity, highlighting how varied the ocean’s response to climate change can be from region to region.
Dr Tilstone said: ‘Global averages can mask what’s really happening at local and regional scales, which is where ecosystems, fisheries and coastal communities actually feel the impacts.’
The findings also matter for the climate more broadly. Phytoplankton play a key role in transferring carbon from the atmosphere into the deep ocean, a process known as the biological carbon pump. A long-term decline in their productivity could weaken the ocean’s ability to act as a natural carbon sink.
The researchers say the satellite record, while valuable, is still relatively short in climate terms, and stress the importance of continuing long-term ocean monitoring to track how these changes unfold and to identify which regions are most at risk in the years ahead.