A team of UK and French scientists have shown dramatic changes in the abundance of phytoplankton in the North Atlantic over the last 60 years driven primarily by climate variability and North Atlantic warming. In particular, the scientists focused on the important group of phytoplankton collectively known as diatoms. This major phytoplankton group contributes approximately one-fifth of all of Earth’s photosynthesis and up to 30-40% of the global marine primary production each year. As such diatoms are extremely important contributors to marine primary production and to the ocean carbon cycle. In the North Atlantic and its adjacent seas, primary production is primarily driven by these diatoms which produce vast spring blooms that cover the whole ocean every year and fuel the highly productive marine food-webs found there. They also transfer a significant part of the produced energy as carbon to the deep ocean contributing to a significant drawdown of carbon from the atmosphere.
In the study the authors showed that anthropogenic warming and climate variability (including natural climate oscillations and wind) over a multidecadal scale have had important consequences for the productivity and spatial/temporal dynamics of these phytoplankton. The authors used multidecadal diatom abundance data (>60 years) for large areas of the North Atlantic and the North Sea to show significant spatial and temporal correlations over these scales between diatoms and climate variability. They also examined 50 phytoplankton species individually to investigate seasonal and life-cycle (phenology) patterns at the species level. In summary, the study found that climate warming is having a huge impact on the total abundance of diatoms and species in the North Atlantic over the period of this study.
Martin Edwards from Plymouth Marine Laboratory who led the study said ‘some of the most important findings in this study include showing an increasing diatom population in northerly systems, but deceasing populations in more southerly systems. We also discovered major phase shifts in diatom abundance synchronous with multidecadal trends in Atlantic climate variability that occurred after the mid-1990s’.
Over the whole area of study there has been an increase in phytoplankton biomass during spring and autumn (where diatoms dominate) with increasing temperatures in cooler regions but a decrease in phytoplankton biomass in warmer regions. The authors suggest that this is possibly due to increased phytoplankton metabolic rates caused by warming temperatures in colder regions but conversely a decrease in nutrient supply in warmer regions (where warming can enhance stratification and limit nutrient replenishment and hence diatom growth in the surface layers). Gregory Beaugrand from CRNS in France and a co-author of the study also said ‘that the that autumnal diatom abundance is positively correlated with Sea Surface Temperatures and the increase in Northern Hemisphere Temperatures seen over the last few decades’. The study also found that regional climate warming in some areas of the North Sea has been linked to an increase in certain diatoms that are associated with Harmful Algal Blooms (HABs). Diatom growth in such well mixed areas may be enhanced by temperature as these regions are not inhibited by stratification and hence nutrient availability. These dramatic changes in such a fundamental primary producer for marine food-webs in the North Atlantic will have large on-going ramifications for other marine life from fish to whales found in these oceans.
More information: Edwards, M., Beaugrand, G., Kléparski, L. et al. Climate variability and multi-decadal diatom abundance in the Northeast Atlantic. Commun Earth Environ 3, 162 (2022). https://doi.org/10.1038/s43247-022-00492-9