New modeling has produced a surprising result: Most photosynthesis in the Arctic Ocean happens under the ice rather than in the open ocean.
Marine phytoplankton are the solar panels of the sea, soaking up the Sun’s rays to make energy that powers ocean ecosystems. These single-celled organisms photosynthesize like plants, sucking carbon out of the atmosphere and producing about half of the world’s oxygen. Scientists consider phytoplankton to be the ocean’s most important primary producers, because they take energy directly from the Sun and make it available to the rest of ocean life in such vast quantities.
The role of phytoplankton as the ocean’s favorite vegetarian snack means scientists can learn a lot about an ecosystem by measuring annual phytoplankton blooms. But that’s easier said than done in the ice-covered Arctic Ocean. Satellites, which researchers typically use for the task, cannot penetrate the ice to the water below, and ice-breaking research vessels can peek at tiny slivers of the ocean at only certain times of the year. Some research voyages in ice-breaking vessels, however, have found evidence that phytoplankton can grow in abundance under meter-thick ice. If true across the Arctic, this would mean that the Arctic Ocean has more primary production than researchers thought possible.
In new research, Kinney et al. use a sophisticated 3D model of the Arctic’s atmosphere, land, and sea to estimate phytoplankton levels across the entire Arctic Ocean. The research revealed something surprising: Most photosynthesis in the Arctic Ocean is taking place in ice-covered waters. In fact, the model estimates that over 40% is taking place in areas with more than 85% ice coverage, and the total amount of photosynthesis has increased by about 5% per decade in the past 40 years. As climate change warms the ocean, ice thin enough for blooms to form underneath is becoming more common.
“What we’re seeing now is thinner sea ice and earlier snowmelts, so there’s more light actually reaching through the ice into the surface of the ocean than there used to be,” said Jaclyn Kinney, an oceanographer with the Naval Postgraduate School in Monterey, Calif. “These little algal cells can grow better than they used to.”
The revelation that most of the Arctic’s phytoplankton can be found in ice-covered waters means that previous estimates of Arctic primary production are likely too low. The model’s results also imply that climate change is already affecting production in the Arctic Ocean, but it’s unclear exactly how this impact will change over time. These findings can help direct future field observations, which can, in turn, help fine-tune future models.
“I would not expect that production will continue to increase indefinitely,” Kinney said. “I think at a certain point, stratification and nutrient limitation will alter the trend.” (Journal of Geophysical Research: Oceans, https://doi.org/10.1029/2020JC016211, 2020)
—Rachel Fritts (@rachel_fritts), Science Writer