Record-breaking sea temperatures in 2023 may have triggered an unprecedented global decline in the growth of phytoplankton and algae, which form the base of the marine food web.
Last year, swathes of the world’s oceans were gripped by extreme marine heatwaves, caused by climate change and a transition to hotter El Niño conditions in the Pacific Ocean.
On average, on any one day in 2023 nearly a third of the global ocean was experiencing a marine heatwave, the World Meteorological Organization reported last month.
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The spikes in sea surface temperatures sparked a slump in the production rate of phytoplankton, algae and bacteria that sustain ocean ecosystems, known as net primary productivity (NPP), a new study suggests.
at Louisiana Universities Marine Consortium and his colleagues used 21 years of satellite data measuring the colour of the sea surface, which is affected by the amount of green pigment in photosynthesising organisms, to estimate the rate of NPP in the world’s oceans.
In April 2023, when global sea surface temperatures were at record levels, NPP dropped by 22 per cent compared to the 21-year average.
Equatorial regions of the Atlantic and Pacific, as well as the northern Atlantic, saw the largest drops in productivity rates, the study found. Those regions also saw some of the most severe marine heatwaves – in parts of the North Atlantic, water temperatures were 4°C above normal in June 2023.
NPP rates remained lower than normal throughout the northern hemisphere summer, according to the study. The global average sea surface temperature peaked in August 2023, and in September the global rate of NPP was almost 16 per cent lower than the 21-year average for the same month. Almost three-quarters of the global ocean surface area saw a decline in NPP during September 2023, the researchers found.
The study, which hasn’t yet been peer-reviewed, argues that the decline in NPP was caused by the increase in sea surface temperatures over the period, pointing out that similar – although less dramatic – declines in productivity have happened in the past when ocean temperatures have spiked.
Increased ocean temperatures don’t always lead to a reduction in NPP, says at the UK’s National Oceanography Centre. “In tropical regions, if you have a marine heatwave the productivity is generally going to be less. But if it’s occurring in the polar regions, it might well be increased,” she says.
Marine ecosystems can usually recover from short “blips” in productivity rates, Jacobs adds, so 2023’s extreme weather may not cause lasting damage. But she says the findings are “very concerning” and should act as an “early warning bell”, given the increased frequency of extreme heat events occurring under climate change.
at the University of Southampton, UK, says the decrease in NPP reported by the study was substantial. If sustained, it could cause disruption throughout the wider marine ecosystem, he says. “A 20 per cent reduction for a couple of years is quite a significant difference in the amount of energy that’s available for the rest of the marine food web,” he says.
The study only considered data up to September 2023, but global sea surface temperatures .
However, Moore urged caution over the precise figures presented in the study, and said more research is needed before it is possible to blame water temperatures alone for a drop in the NPP rate.
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