A synthesis study shows that phytoplankton can flourish under Arctic sea ice

Small photosynthetic seaweeds are a key component of the Arctic marine ecosystem, but their role for the ecology of the Arctic Ocean has been underestimated for decades. This is the conclusion of a team of scientists who have synthesized more than half a century of research on the presence, extent and composition of phytoplankton blooms under Arctic sea ice. The results were published in a special issue of Frontiers in marine science dedicated to research on the Arctic Ocean.

Phytoplankton are floating microscopic organisms, most of which are unicellular algae. Like terrestrial plants, they use photosynthesis to transform light into chemical energy by consuming carbon dioxide (CO₂) and nutrients in the water. Phytoplankton is the basis of the marine food web and plays a fundamental role in the carbon cycle by absorbing CO₂ from the atmosphere.

Up until about a decade ago, most scientists assumed that phytoplankton remained in some sort of stasis throughout the winter and spring until sea ice broke. There is now a growing body of evidence suggesting that phytoplankton under-ice blooms (UIBs), such as a sudden spring bloom in a garden, can occur in low-light environments under sea ice.

“There has long been speculation that what was happening under the sea ice in the water column was almost ‘paused’ during the polar night and before the seasonal sea ice retreat, which is apparently not the case.” said lead author Dr. Mathieu Ardyna, Marie Sklodowska-Curie postdoctoral fellow at Stanford University.

The revelation means that phytoplankton production in some regions of the Arctic Ocean could be an order of magnitude larger than originally predicted. This is important for climate modellers who want to know how much atmospheric carbon is absorbed by these algae.

Few places on Earth are transforming as rapidly as the Arctic due to climate change. Over the past 30 years, the Arctic has warmed to about double the global average. One of the most visible signs of that change has been the decline in sea ice floating on the ocean’s surface, with this year’s ice cover shrinking to the second lowest level on record.

Not surprisingly, the thinning of the ice cover allowed phytoplankton, which requires light for photosynthesis, to thrive. What surprised Ardyna and her colleagues is that the UIB phenomenon occurred long before climate change hit Arctic sea ice.

“Digging research from the 1950s and earlier shows that blooms, albeit not very large, were occurring under a thick layer of ice in the central Arctic,” he explained. “I think this fact surprised many of us, as the models had suggested that this was not the case.”

The historical observations included a couple of studies during the International Geophysical Year, a global campaign that ushered in the modern scientific era. The authors noted: “The end result of this work was nothing more than an amazing first glimpse of UIBs occurring in the central Arctic.”

The article goes on to describe the variability between UIB events across the Arctic Ocean in terms of occurrence, extent and even the type of organisms present. Some of these findings are based on scientific programs and expeditions dedicated to the specific study of UIBs. In many cases, observations have been based on autonomous floats, robotic gliders, and even remote-controlled vehicles that can swim under sea ice.

Ardyna said further observations to power new computer models will be key to more accurately predicting how the Arctic carbon cycle will change in the future.

“So many questions remain unanswered about this critical period of spring, for many arctic species, their food or their life cycle,” he said. “Given the remoteness of the Arctic, one way will surely be to develop more and better autonomous platforms to provide us with valuable information.”