A new study by scientists at Plymouth Marine Laboratory (PML) has revealed a significant long-term decline in ocean productivity across large parts of the north-east Atlantic, raising concerns for marine food webs, fisheries and the ocean’s ability to absorb 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 that forms the foundation of marine ecosystems.
The study, led by PML’s Dr Gavin Tilstone and Dr Peter Land, examined satellite data spanning 1997 to 2018. 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.
Warming and ocean mixing are reshaping marine productivity
The research links these declines primarily to changes in sea surface temperature and mixed layer depth, the physical properties that control how nutrients and light are distributed in the upper ocean.
Dr Gavin Tilstone, Bio-optical Oceanographer at PML, said the ocean is often divided into layers based on temperature, and that as the ocean warms these layers become stronger and less likely to mix vertically, a process known as thermal stratification.
He added that ocean mixing helps transport nutrients from the depths to the surface, where phytoplankton use them to grow, and that microalgae productivity can decline when that nutrient supply is reduced.
Dr Peter Land, Remote Sensing Scientist at PML, said warming surface waters and altered mixing are reducing the conditions phytoplankton need to thrive in many regions, limiting the energy entering marine food webs with potential knock-on effects for fish stocks and ecosystem services.
Earlier spring blooms could disrupt marine food webs
In some areas, the timing of peak productivity has shifted earlier in the year, with the traditional spring bloom occurring weeks sooner than in previous decades.
This seasonal shift could disrupt the synchrony between phytoplankton, zooplankton and fish larvae, and may affect recruitment success in fish populations. Wider context on these pressures is explored in H2O Global News coverage of climate change and the oceans.
While the overall picture shows a net decline, the study highlights strong regional variability. Some areas, such as the Celtic Sea, showed stable or even increasing productivity, underlining the complexity of ocean responses to climate-driven change.
Why declining ocean productivity matters for carbon and fisheries
Microalgae play a vital role in the biological carbon pump, helping transfer carbon from the atmosphere into the ocean interior. A sustained reduction in primary production could weaken this natural carbon sink and reduce the ocean’s ability to draw down and store carbon dioxide.
The findings also carry implications for fisheries and coastal communities, where global averages can mask the local and regional changes that ecosystems actually experience. The relationship between healthy seas and human activity is examined further in H2O Global News reporting on water and the ocean.
The authors caution that the satellite record is still relatively short in climate terms, and stress the importance of maintaining long-term Earth observation programmes to detect and understand these trends. The team is now extending the analysis using around 30 years of satellite data to test whether similar patterns appear elsewhere in the Atlantic Ocean.
The study provides one of the most detailed regional assessments to date of long-term changes in ocean productivity in the north-east Atlantic, a region that includes the western English Channel, where PML maintains a 120-year data time series through its Western Channel Observatory.
FAQs
What did the Plymouth Marine Laboratory study find?
The study found a significant long-term decline in ocean primary production across much of the north-east Atlantic between 1997 and 2018, based on more than two decades of satellite observations.
What is ocean primary production?
Ocean primary production is the process by which microscopic marine plants, or phytoplankton, convert sunlight and carbon dioxide into organic matter. It forms the base of marine food webs and supports the ocean carbon cycle.
Why is ocean productivity declining?
Researchers link the decline mainly to rising sea surface temperatures and changes in mixed layer depth, which strengthen ocean stratification and reduce the supply of nutrients that phytoplankton need to grow.
Why does this matter for fisheries and carbon storage?
Lower primary production can reduce the energy available to fish stocks and weaken the ocean’s biological carbon pump, limiting its capacity to absorb and store carbon dioxide.
