Phytoplankton decline hints trouble for north Atlantic food webs

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.”