A habitat of frozen brine
Sea ice is not a solid block. As seawater freezes, it expels salt, creating a maze-like network of microscopic liquid channels and pockets. This hypersaline liquid, called brine, can be several times saltier than normal seawater and remain liquid at temperatures far below 0°C. Within this network lives a specialized community of microscopic life, dominated by single-celled algae. These organisms, primarily diatoms like Fragilariopsis cylindrus and various Nitzschia species, are adapted to cold, high salinity, and darkness for much of the year.
The abundance of algae within the ice can be immense, often reaching concentrations three orders of magnitude higher than in the water column below. They concentrate in the bottom few centimeters of the ice, often turning the underside a distinct brownish-green. These algae produce extracellular polymeric substances, slimy compounds that act as a natural antifreeze and help them manage the extreme salinity of the brine channels. Some of these diatoms are not passive inhabitants; they are capable of movement, actively gliding through the icy channels to position themselves for optimal light and nutrient access. This ability to move within the ice matrix allows them to seed new blooms under the ice.
The engine of the spring bloom
For most of the winter, the algae exist in a state of near-dormancy. As the sun returns to the Arctic in spring, light begins to penetrate the snow and ice. Sea ice scatters light intensely, but it allows a broad spectrum of wavelengths to pass through. The algae's photosynthetic pigments are adapted to make use of this wide range of colors in the very low light conditions found at the bottom of the ice sheet. They can resume growth at daily average light levels as low as 0.04 μmol photons m⁻² s⁻¹, which is 10 to 100 times lower than previously thought and near the theoretical minimum for photosynthesis.
This early-season growth is important in the Arctic calendar. The ice algae bloom can contribute up to 40% of the primary production in some ice-covered locations. As the ice melts from the bottom up, it releases this concentrated, nutrient-rich pulse of algae directly into the upper layer of the ocean. This release "seeds" the much larger spring phytoplankton bloom in the open water. The timing is perfect for zooplankton like copepods and krill, which have reproductive cycles aligned with this predictable, high-fat food source. This first feast of the year supports the entire Bering Sea food web, from tiny crustaceans to pollock, Arctic cod, seals, walruses, and whales.