Scientists at Plymouth Marine Laboratory have identified a significant long-term decline in ocean productivity across large parts of the north-east Atlantic, raising concerns about the future of marine food webs, fisheries, and the ocean’s capacity to absorb carbon dioxide.

Usings more than two decades of satellite data – spanning 1997 to 2018 – the study has tracked changes in microalgae net primary production – the process by which microscopic marine plants convert sunlight and CO₂ into organic matter at the base of the marine food web.

The study leads, Dr Gavin Tilstone and Dr Peter Land, found that after a brief period of increasing productivity in the late 1990s and early 2000s, primary production declined steadily across much of the region. The drop was most pronounced in north-west European coastal waters, including the Irish Sea, North Sea, western English Channel, and parts of the Norwegian Sea.

It has linked the declines primarily to rising sea surface temperatures and changes in mixed layer depth – physical properties that control how nutrients and light are distributed in the upper ocean. As surface waters warm, the ocean stratifies into more distinct layers that are less likely to mix vertically, cutting off the supply of nutrients from depth that phytoplankton depend on to grow.

“While the ocean may appear to be one giant body of water, it is often divided into layers based on temperature,” said Dr Tilstone. “As the ocean warms, these layers become stronger and less likely to vertically mix – a process known as thermal stratification. 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 Land said the effects extend through the entire food web. “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 the timing of the spring bloom – the seasonal surge in phytoplankton growth – has shifted earlier in some areas, by as much as several weeks. Researchers warn this could disrupt the synchrony between phytoplankton, zooplankton and fish larvae, with potential consequences for fish recruitment.

Not all areas showed decline. The Celtic Sea recorded stable or increasing productivity, highlighting the regional complexity of ocean responses to climate change. “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,” said Dr Tilstone.

Beyond fisheries, the findings carry implications for climate. Microalgae play a central role in the biological carbon pump, transferring carbon from the atmosphere into the deep ocean. A sustained fall in primary production could weaken this natural sink, diminishing the ocean’s ability to buffer rising CO₂ levels.

The authors acknowledge the satellite record is relatively short in climate terms, but stress the importance of sustained Earth observation to detect and understand these trends. Their work is continuing, using around 30 years of data to assess whether similar patterns exist across the wider Atlantic.

“With continued satellite monitoring, we can better track how climate change is reshaping ocean productivity and identify regions most at risk,” said Dr Land. “That knowledge is essential for managing marine ecosystems in a changing climate.”

Dr Tilstone added: “A concerted research effort on determining the capacity of regional seas to continue to regulate the climate is required, in the shadow of climate extremes such as more and prolonged heat waves, decreasing seawater pH and an increase in oxygen minimum zones in the ocean.”

The study is one of the most detailed regional assessments of long-term productivity change in the north-east Atlantic to date. It covers an area that includes the western English Channel, where PML operates the Western Channel Observatory – a continuous data time-series stretching back 120 years.

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