Ice algae

Ice algae are any of the various types of algal communities found in annual and multi-year sea or terrestrial ice. On sea ice in the polar oceans, ice algae communities play an important role in primary production. The timing of blooms of the algae is especially important for supporting higher trophic levels at times of the year when light is low and ice cover still exists. Sea ice algal communities are mostly concentrated in the bottom layer of the ice, but can also occur in brine channels within the ice, in melt ponds, and on the surface. Ice algae are any of the various types of algal communities found in annual and multi-year sea or terrestrial ice. On sea ice in the polar oceans, ice algae communities play an important role in primary production. The timing of blooms of the algae is especially important for supporting higher trophic levels at times of the year when light is low and ice cover still exists. Sea ice algal communities are mostly concentrated in the bottom layer of the ice, but can also occur in brine channels within the ice, in melt ponds, and on the surface. Because terrestrial ice algae occur in freshwater systems, the species composition differs greatly from that of sea ice algae. These communities are significant in that they often change the color of glaciers and ice sheets, impacting the reflectivity of the ice itself. Microbial life in sea ice is extremely diverse, and includes abundant algae, bacteria and protozoa. Algae in particular dominate the sympagic environment, with estimates of more than 1000 unicellular eukaryotes found to associate with sea ice in the Arctic. Species composition and diversity vary based on location, ice type, and irradiance. In general, pennate diatoms such as Nitschia frigida (in the Arctic) and Fragilariopsis cylindrus (in the Antarctic) are abundant. Melosira arctica, which forms up to meter-long filaments attached to the bottom of the ice, are also widespread in the Arctic and are an important food source for marine species. While sea ice algae communities are found throughout the column of sea ice, abundance and community composition depends on the time of year. There are many microhabitats available to algae on and within sea ice, and different algal groups have different preferences. For example, in late winter/early spring, motile diatoms like N. frigida have been found to dominate the uppermost layers of the ice, as far as briny channels reach, and their abundance is greater in multi-year ice (MYI) than in first year ice (FYI). Additionally, dinoflagellates have also been found to dominant in the early austral spring in Antarctic sea ice. Sea ice algal communities can also thrive at the surface of the ice, in surface melt ponds, and in layers where rafting has occurred. In melt ponds, dominant algal types can vary with pond salinity, with higher concentrations of diatoms being found in melt ponds with higher salinity. Because of their adaption to low light conditions, the presence of ice algae (in particular, vertical position in the ice pack) is primarily limited by nutrient availability. The highest concentrations are found at the base of the ice because the porosity of that ice enables nutrient infiltration from seawater. To survive in the harsh sea ice environment, organisms must be able to endure extreme variations in salinity, temperature, and solar radiation. Algae living in brine channels can secrete osmolytes, such as dimethylsulfoniopropionate (DMSP), which allows them to survive the high salinities in the channels after ice formation in the winter, as well as low salinities when the relatively fresh meltwater flushes the channels in the spring and summer. Some sea ice algae species secrete ice-binding proteins (IBP) as a gelatinous extracellular polymeric substance (EPS) to protect cell membranes from damage from ice crystal growth and freeze thaw cycles. EPS alters the microstructure of the ice and creates further habitat for future blooms. Surface-dwelling algae produce special pigments to prevent damage from harsh ultraviolet radiation. Higher concentrations of xanthophyll pigments act as a sunscreen that protects ice algae from photodamage when they are exposed to damaging levels of ultraviolet radiation upon transition from ice to the water column during the spring. Algae under thick ice have been reported to show some of the most extreme low light adaptations ever observed. Extreme efficiency in light utilization allows sea ice algae to build up biomass rapidly when light conditions improve at the onset of spring. Ice algae play a critical role in primary production and serve as part of the base of the polar food web by converting carbon dioxide and inorganic nutrients to oxygen and organic matter through photosynthesis in the upper ocean of both the Arctic and Antarctic. Within the Arctic, estimates of the contribution of sea ice algae to total primary production ranges from 3-25%, up to 50-57% in high Arctic regions. Sea ice algae accumulate biomass rapidly, often at the base of sea ice, and grow to form algal mats that are consumed by amphipods such as krill and copepods. Ultimately, these organisms are eaten by fish, whales, penguins, and dolphins. When sea ice algal communities detach from the sea ice they are consumed by pelagic grazers, such as zooplankton, as they sink through the water column and by benthic invertebrates as they settle on the seafloor. Sea ice algae as food are rich in polyunsaturated and other essential fatty acids, and are the exclusive producer of certain essential omega-3 fatty acids that are important for copepod egg production, egg hatching, and zooplankton growth and function. The timing of sea ice algae blooms has a significant impact on the entire ecosystem. Initiation of the bloom is primarily controlled by the return of the sun in the spring (i.e. the solar angle). Because of this, ice algae blooms usually occurs before the blooms of pelagic phytoplankton, which require higher light levels and warmer water. Early in the season, prior to the ice melt, sea ice algae constitute an important food source for higher trophic levels. However, the total percentage that sea ice algae contribute to the primary production of a given ecosystem depends strongly on the extent of ice cover. The thickness of snow on the sea ice also affects the timing and size of the ice algae bloom by altering light transmission. This sensitivity to ice and snow cover has the potential to cause a mismatch between predators and their food-source, sea ice algae, within the ecosystem. This so called match/mismatch has been applied to a variety of systems. Examples have been seen in the relationship between zooplankton species, which rely on sea ice algae and phytoplankton for food, and juvenile walleye pollock in the Bering Sea.