North Atlantic phytoplankton populations have remained stable since the industrial era, new research finds, contradicting previous research on declines and reports of North Atlantic phytoplankton declines likely being greatly exaggerated. A well-known 2019 study using ice cores from Antarctica showed that ocean productivity in the North Atlantic has declined by 10% during the industrial era, a trend that is worryingly likely to continue.

A University of Washington study analyzing ice cores from 800 years ago shows that phytoplankton populations in the North Atlantic have remained stable since the industrial era. The discovery challenges previous assumptions about large declines in phytoplankton and highlights the impact of industrial pollutants on atmospheric chemistry.

But new research led by the University of Washington shows that the North Atlantic's marine phytoplankton—on which large organisms throughout marine ecosystems depend—may be more stable than thought. The team's analysis of an ice core dating back 800 years suggests that a more complex atmospheric process may explain recent trends.

The research was recently published in the Proceedings of the National Academy of Sciences.

Satellites can detect reflections of chlorophyll in organisms that use chlorophyll for photosynthesis. This image shows reflections of phytoplankton in the North Atlantic as they swirl with ocean currents. While a previous ice core study suggested that phytoplankton in the North Atlantic had declined by 10 percent since the mid-19th century, the new study finds that these phytoplankton populations may have remained stable. Source: NASA

Tiny floating photosynthetic organisms called phytoplankton form the basis of marine ecosystems. These tiny creatures are also important to the Earth as a whole, producing about half of the oxygen in the Earth's atmosphere.

Because phytoplankton are difficult to count, scientists have tried other methods to measure their abundance. Phytoplankton release dimethyl sulfide, an odorous gas that gives beaches their distinctive smell. Once in the air, dimethyl sulfide is converted into methanesulfonic acid (or MSA) and sulfate. This material eventually falls to land or snow, making ice cores a way to measure past populations.

Becky Alexander holds ice drilled from an ice sheet that preserves atmospheric conditions from previous centuries in a freezer at the University of Washington's Plasma Laboratory. Her team analyzed ice cores from central Greenland and showed that emissions from photosynthetic marine organisms have remained stable since the mid-1800s. Source: Mark Stone/University of Washington

"Greenland ice cores show that MSA concentrations declined during the industrial era, which is thought to be a sign of declining primary productivity in the North Atlantic," said first author Ursula Jongebloed, a doctoral student in atmospheric sciences at the University of Washington. "But our study of sulfates in Greenland ice cores shows that MSA alone doesn't tell the whole story when it comes to primary productivity."

Factories and tailpipes have also been spewing sulfur-containing gases into the air since the mid-1800s. These gases have slightly different forms of sulfur atoms, allowing marine and terrestrial sources to be distinguished in ice cores.

The new study goes a step further than previous research by measuring several sulfur-containing molecules in ice cores from central Greenland spanning the years 1,200 to 2006. The authors' research shows that human-generated pollutants change the chemical composition of the atmosphere. This in turn changes the fate of the gases emitted by the phytoplankton.

"While studying ice cores, we found that sulfate production by phytoplankton increased during the industrial era," Jongebloed said. "In other words, the concurrent increase in phytoplankton-derived sulfate 'offset' the decline in MSA, suggesting that phytoplankton-derived sulfur emissions have remained generally stable."

Ursula Jongebloed of the University of Washington Plasma Laboratory used a machine called a stable isotope mass spectrometer to measure sulfur isotopes in Greenland ice cores. Sulfur isotopes in ice cores reveal how sulfate sources - including marine phytoplankton, fossil fuel burning and volcanic emissions - have changed over the past few centuries. Source: MarkStone/University of Washington

If you factor this balance into your calculations, phytoplankton populations appear to be fairly stable since the mid-1800s. However, researchers caution that marine ecosystems are still under threat from many fronts.

"Simultaneously measuring MSA and sulfate production by phytoplankton gives us a more complete picture of how emissions from ocean primary producers have changed - or not changed - over time," said senior author Becky Alexander, a professor of atmospheric sciences at the University of Washington.

"Ice core measurements, paired with other independent estimates of phytoplankton abundance, such as chlorophyll measurements, and modeling studies that help us estimate how atmospheric chemistry and climate change over time, can help us understand how ocean productivity has changed in the past, and how productivity may change in the future."

Compiled source: ScitechDaily