Photo by Smithsonian
Ocean productivity across large parts of the north-east Atlantic has been declining steadily for two decades, according to a new study by scientists at Plymouth Marine Laboratory (PML) published in Frontiers in Remote Sensing. The findings raise significant concerns about the future health of marine food webs, fisheries and the ocean’s capacity to absorb and store carbon dioxide.
Using more than two decades of satellite observations, researchers analysed changes in microalgae net primary production – the process by which microscopic marine plants convert sunlight and carbon dioxide into organic matter, forming the foundation of marine ecosystems. The study, led by Dr Gavin Tilstone and Dr Peter Land, examined satellite data spanning 1997 to 2018. It 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, particularly in north-west European coastal waters, the Irish Sea, North Sea, western English Channel and parts of the Norwegian Sea.
The mechanism: warming waters, thinner mixing
The research links these declines primarily to rising sea surface temperatures and changes in what oceanographers call mixed layer depth – the depth to which wind and waves stir the upper ocean, distributing nutrients from deeper water to the sunlit surface where phytoplankton can use them to grow.
Dr Tilstone, Bio-optical Oceanographer at PML, explained the underlying dynamic: “While the ocean may appear to be one giant body of water, it is often divided into layers based on temperature. 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 Peter Land, Remote Sensing Scientist at PML, added that the consequences reach far beyond the microscopic. “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.”
Cascading consequences
Phytoplankton sit at the base of virtually every marine food chain. When their numbers decline, the effects ripple upwards through zooplankton to fish and marine mammals. The study also highlights a second area of concern: the timing of the annual spring bloom has shifted earlier in many areas, occurring weeks sooner than in previous decades. This seasonal displacement could disrupt the synchrony between phytoplankton, zooplankton and the larvae of commercially important fish species, potentially reducing recruitment success and contributing to the long-term decline of fish stocks.
Microalgae also play a central role in the ocean’s biological carbon pump, capturing atmospheric CO₂ through photosynthesis and transporting it into the deep ocean when cells die and sink. A sustained reduction in primary production could therefore weaken this natural carbon sink, reducing the ocean’s ability to buffer climate change at a time when that buffering capacity is more important than ever.
Dr Tilstone called for urgent scientific investment in response. “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.”
Regional complexity
The picture is not uniform. The Celtic Sea showed stable or even increasing productivity over the study period, highlighting the complexity of ocean responses to climate change at the sub-regional scale. Dr Tilstone cautioned against over-reliance on broader averages: “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.”
Nonetheless, the study found statistically significant declines across the majority of the eleven distinct regions into which the north-east Atlantic was divided for analysis, including the Irish Sea, the northern and southern North Sea, the Kattegat, the western English Channel and the Faroe-Shetland Channel.
The researchers also note a potential secondary effect in areas where productivity remains higher: as food resources decline across neighbouring regions experiencing the sharpest declines, fish and marine mammals may migrate into productive zones such as the Celtic Sea in search of food, increasing pressure on those ecosystems in turn.
A monitoring imperative
The study was built on data from the Ocean Colour Climate Change Initiative, funded by the European Space Agency, and forms one of the most detailed regional assessments of long-term productivity change in the north-east Atlantic to date. PML’s Western Channel Observatory — which has maintained a continuous in-situ time series for 120 years – provided critical context and validation for the satellite findings.
The authors acknowledge that the satellite record, spanning just over two decades, is still relatively short in climate terms. They are conducting further work extending to approximately 30 years of satellite data to assess whether the patterns identified in the north-east Atlantic are replicated across other parts of the Atlantic Ocean.
Dr Land underlined the urgency of maintaining that observational infrastructure: “With continued satellite monitoring, we can better track how climate change is reshaping ocean productivity and identify regions most at risk. That knowledge is essential for managing marine ecosystems in a changing climate.”