The sea ice is a powerful natural barrier between the atmosphere and ocean. As such, it keeps the biological, biochemical and physical processes at work above and below the ice separated. For example, the sea ice reduces the amount of light that reaches the ocean. It prevents the exchange of gases and heat between the air and water, and it influences the amount of freshwater input in the ocean (Arrigo, 2014). In addition, the sea-ice barrier has a critical influence on surface layering and mixing in the ocean.
The Arctic sea-ice landscape is rapidly changing. Increasing transparency can spark premature seasonal primary production. This early growth can be accompanied by an increase in biomass from ice algae and phytoplankton, which boosts the emission of dimethyl sulphide (DMS) and the binding of carbon dioxide (CO2). Secondary production in the shelf regions can also increase, although the loss of sea ice exacerbates the loss of sea-ice fauna, endemic fish species and megafauna. The loss of sea ice can be connected to the release of more methane into the atmosphere, but warmer ice may release fewer halogens, which means fewer ozone depletion events. As such, the net changes to the carbon cycle remain highly uncertain (Lannuzel et al., 2020).
The sea-ice cover changes throughout the year. The ice extent begins growing in autumn, spreads to cover vast expanses of the polar region in winter, and shrinks again when the spring melting begins. This dynamic fundamentally shapes the biogeochemical processes and material cycles in the Arctic, in particular the growth of ice algae, the basis of the Arctic food webs. Ice algae are especially an important food source for zooplankton, as they contain high concentrations of various unsaturated and other essential fats. In turn, the higher trophic levels profit from plentiful zooplankton (Arrigo, 2014). The reproduction of the ice algae begins in early spring, when there is little light. As a result of this intensive blooming, a large portion of the light is absorbed by the ice algae (Arrigo et al., 2014).
The light attenuation in snow, produced by scattering and absorption, is roughly one order of magnitude higher than that of the sea ice below it, which in turn is roughly an order of magnitude higher than that of the seawater. Consequently, snow-covered ice allows only very little light to penetrate more than 1 m below the surface of the snow, providing very little support for microbial biomass production. Even under snow-free conditions, the growth of photoautotrophic organisms living in the ice can be limited by the available light, especially in early spring, when the ice brine is still rich in nutrients. The light attenuation in the ice is further intensified by particle absorption, especially via sediments in coastal regions and microalgae containing pigment that grow on the underside of the ice (Arrigo, 2014).
Over the past several years public, political and economic interest in the Arctic has grown considerably. This is particularly due to the fact that, over the last few decades, the amount of sea ice in the summer has declined significantly. Climate models predict that this sea-ice retreat will intensify in the years to come, and that the Arctic will have predominantly ice-free (i.e., will have a sea-ice extent of less than 1 million km²) summers by the middle of this century. Experts believe this sea-ice retreat will produce dramatic changes in Arctic habitats. In addition, this development has sparked new commercial and geopolitical interest. After all, as the ice cover dwindles, the Arctic will become more accessible for shipping, raw material exploration and fishing, not to mention tourism.
Primary production with the aid of photosynthesis involves a complex sequence of processes, which includes light harvesting, electron transport and carbon fixation, whereby each process has its own sensitivities to environmental conditions and to cellular control processes. The environmental conditions investigated to date include light intensity and spectral quality, temperature, salinity and nutrient depletion. Salinity, light and available nutrients in particular are important factors for the growth of ice algae, whereas changes in acidity, CO2 concentration and UV radiation appear to be less relevant (Arrigo, 2017).
Just like land-based plants, both ice algae and free-floating microalgae (phytoplankton) need light for photosynthesis. Light is a main driver of algae growth in the sea-ice zone. In the high latitudes, a pronounced seasonality in the light cycle determines both when ice algae and phytoplankton blooms form, and their size. Since sea ice and snow have much higher albedos than seawater, the majority of sunlight is reflected back into space (Arrigo, 2014). The albedo is higher in thick ice with dense snow cover and lower when there is moisture in the snow, when meltwater pools form on the surface of the ice, or when patches of open water form between floes. The amount of available light in the sea ice declines exponentially with increasing depth. Depending on the sea-ice and snow conditions, between less than 1% and ca. 20% of sunlight reaches the ocean below (Lannuzel et al., 2020). It has been confirmed that – as a result of less snow, thinning ice, prolonged surface melting and a longer period with open water – the Arctic albedo declined by 4–6% from 1979 to 2011. Accordingly, the available light for ice algae and phytoplankton most likely increased in the same period, as model-based simulations suggest (Lannuzel et al., 2020).
Taken together, all these factors ultimately determine how much the ice algae and phytoplankton, and with them, so-called primary production, grow. Primary production is the basis of life for all other marine organisms. Compared to elsewhere, the level of phytoplankton production in the polar regions is fairly low. The annual sea ice primary production are quite similar in the Arctic and Antarctic. In the Arctic, the annual rate ranges from 0.001 to 23 grams of carbon per square metre; in the Antarctic, it lies between 0.3 and 38 grams (Arrigo, 2017). As such, the level of primary production in the sea-ice habitats is below 50 grams of carbon per square metre and year, an amount similar to the oligotrophic central gyres of the open oceans (Arrigo, 2017).
On the other hand, at times there can be extremely high production rates in the polar regions. The highest have been observed in the platelet ice layer, where the daily production value can reach up to 1.2 grams of carbon per square metre (Arrigo, 2017). This is because the platelet ice is the most porous type of sea ice known, allowing the relatively free exchange of nutrients with the underlying seawater. As such, it is also home to some of the largest microbial assemblages ever seen in the sea ice. The underside of the sea ice is also very productive. Due to its easy accessibility, proximity to seawater nutrients, and mild temperature and salinity gradients, the underside is often the most biologically productive habitat in the sea ice. In the Canadian Arctic, the highest rate recorded was 463 milligrams per square metre per day (Arrigo, 2017).
In the ice-covered regions, algae blooms form not only below the ice at the interface with the sea-water, but also on the edges of floes. Researchers have observed that the production rate rises directly when patches of open water increasingly form. These areas of open water (polynyas, leads) offer more intensive and direct light, which boosts primary production.
But the level of primary production depends not only on the light intensity. Another important factor are the nutrients in the seawater, which the algae need in order to grow, especially nitrate and phosphates, but also silicic acid for diatoms. Both in the sea ice and water column, it is believed that nutrients regulate blooms’ intensity and potentially also when they end. Blooms can also be ended by other processes, e.g. viruses. In comparison to light, however, there are still major uncertainties in our understanding of nutrient dynamics in the sea ice. The most essential source of nutrients in the ice is the seawater. The nutrient concentrations in the sea ice are determined by the brine circulation and exchanges with the underlying seawater, as well as biogeochemical processes like assimilation and remineralisation. Adsorption on the walls of the brine channels and biofilm processes most likely influence the availability and mobility of sea-ice nutrients. Nutrients in the seawater below are shaped by layering in and the provenance of the water masses (e.g. in the Arctic by nutrient-rich Pacific versus nutrient-poor Atlantic water masses), drainage from rivers and glaciers, as well as advection (Lannuzel et al., 2020).
Primary production is also influenced by the sea-ice cover. Increased meltwater and river input leads to intensified layering in the surface water, whereas thinner ice with a larger percentage of open water increases the exposure of the near-surface water to wind and waves, promoting mixing. These processes have conflicting and unknown effects on the supply of deeper and nutrient-richer water for phytoplankton and ice algae, and therefore on primary production. Earth-system model-based simulations indicate that, in the future, increased layering and declining nutrients will dominate the pelagic environment. Other models predict a rise in atmospheric deposition, which could be balanced out by the limited nutrient availability resulting from increased layering (Lannuzel et al., 2020). Another important factor: the characteristics of the sea ice itself. Changes in the nutrient concentrations in the sea ice are mainly influenced by vertical processes (e.g. brine dynamics and ice-ocean flows), and future brine dynamics depend on the ice temperature and salinity. Ice temperatures could rise due to a warmer atmosphere, or could sink due to reduced snow accumulation. The sea-ice salinity is expected to rise in autumn and winter – since seasonal ice contains more salt than multiyear ice does. In contrast, the salinity is expected to be lower in summer due to the earlier start of the melting phase. Assuming the nutrient concentrations in the seawater remain unchanged, more brine in winter would mean a higher nutrient concentration in the sea ice in spring, and would potentially increase sympagic (= inner-ice) productivity. However, these additional nutrients from the dynamics in the sea ice will be balanced out if the nutrient concentrations in the seawater decline (Lannuzel et al., 2020).
Changes in the previously discussed conditions regarding the light, nutrients and habitat influence the timing, composition and frequency of occurrence for primary producers, and especially the respective contributions of the ice algae and phytoplankton. Changes in primary production can subsequently impact secondary production (microbial and metazoic consumers), higher trophic levels, and carbon absorption in the ocean.
Arrigo, K.R. (2017): Sea ice as a habitat for primary producers. In: D. N. Thomas (ed.), Sea Ice, 3rd edition, Wiley-Blackwell, Chichester (UK) Hoboken (NJ), pp. 352-369
Lannuzel, D., L. Tedesco & M. van Leeuwe et al. (2020): The future of Arctic sea-ice biogeochemistry and ice-associated ecosystems. Nat. Clim. Chang. 10, pp. 983–992. https://doi.org/10.1038/s41558-020-00940-4