To paraphrase Mark Twain, reports of declining phytoplankton in the North Atlantic may have been greatly exaggerated. A prominent 2019 study used ice cores in Antarctica to suggest that during the industrial era, with worrying implications that the trend might continue.
But new research led by the 91探花 shows that marine phytoplankton 鈥� on which larger organisms throughout the marine ecosystem depend 鈥� may be more stable than believed in the North Atlantic. The team鈥檚 analysis of an ice core going back 800 years shows that a more complex atmospheric process may explain the recent trends.
The was published the week of Nov. 13 in the Proceedings of the National Academy of Sciences.
Tiny floating photosynthetic organisms known as phytoplankton form the base of the marine ecosystem. These microscopic creatures are also important to the planet as a whole, producing roughly half the oxygen in Earth鈥檚 atmosphere.
Since phytoplankton are hard to count, scientists attempt to measure their abundance in other ways. Phytoplankton emit dimethyl sulfide, an odorous gas that gives beaches their distinctive smell. Once airborne, the dimethyl sulfide converts to methanesulfonic acid, or MSA, and sulfate. These eventually fall out onto land or snow, making ice cores one way to measure past population sizes.
鈥淕reenland ice cores show a decline in MSA concentrations over the industrial era, which was concluded to be a sign of declining primary productivity in the North Atlantic,鈥� said lead author , a 91探花doctoral student in atmospheric sciences. 鈥淏ut our study of sulfate in a Greenland ice core shows that MSA alone can鈥檛 tell us the whole story when it comes to primary productivity.鈥�
Since the mid-1800s, factories and tailpipes have also been spewing sulfur-containing gases into the air. Those gases have slightly different forms of sulfur atoms that make it possible to distinguish the marine and land-based sources in ice cores.
The new study goes further back than the previous study by measuring several sulfur-containing molecules in an ice core from central Greenland with layers spanning the years 1200 to 2006. The authors show that human-generated pollutants changed the atmosphere鈥檚 chemistry. This, in turn, altered the fate of the gases emitted by phytoplankton.
鈥淲hen looking at the ice cores, we found that sulfate derived from phytoplankton increased during the industrial era,鈥� Jongebloed said. 鈥淚n other words, the decline in MSA is 鈥榦ffset鈥� by the simultaneous increase in phytoplankton-derived sulfate, indicating that phytoplankton-derived sulfur emissions have remained stable overall.鈥�
When that balance is included in the calculations, the phytoplankton populations seem fairly stable since the mid-1800s. The researchers caution, however, that marine ecosystems remain under threat from many directions.
鈥淢easuring both MSA and phytoplankton-derived sulfate gives us a fuller picture of how the emissions from marine primary producers have changed 鈥� or not changed 鈥� over time,鈥� said senior author , a 91探花professor of atmospheric sciences.
鈥淚ce core measurements along with other independent estimates of phytoplankton abundance (such as chlorophyll measurements) and paired with modeling studies (which help us estimate how atmospheric chemistry and climate change over time) can help us understand how marine productivity has changed in the past and how productivity might change in the future.鈥�
Other co-authors are research scientist , doctoral student and former undergraduates and at the UW; Jihong Cole-Dai and Carleigh Larrick at South Dakota State University; William Porter and Linia Tashmim at the University of California, Riverside; and Lei Geng at the University of Science and Technology of China.
The study was funded by the National Science Foundation and the National Natural Science Foundation of China.
For more information, contact Jongebloed at ujongebl@uw.edu or Alexander at beckya@uw.edu.