In the Arctic the sea-ice decline due to the early onset of melting in spring, the delayed ice growth in autumn and the generally increasing temperatures is very likely to have massive impacts on life in and on the sea ice. In this context, there will probably be winners and losers alike. In the Arctic the loss of sea ice and the increased amount of light penetrating the thinner ice mean that the primary production by phytoplankton in open water has increased. Satellite data shows that the reduced ice cover since 1998 has led to a 30 percent rise in annual net primary production in ice-free Arctic waters. Since larger areas are now ice-free, the wind can also more effectively mix the water, which churns nutrients up to the surface, making them available to the plankton living in the open water (IPCC, 2019).
There are also more extensive phytoplankton blooms beneath the thinner sea ice due to the increased amount of sunlight reaching the water. It was previously thought that blooms measuring several thousand square kilometres were limited to the areas around the ice edge and the open water, where there is abundant light. However, recently there have been growing indications that such blooms also form in regions where the ice is less thick, as well as underneath meltwater ponds and in cracks in the ice (Cavan et al., 2019). In addition to its effect on phytoplankton blooms in open water, the spread of seasonal ice also promotes the growth of ice algae as a result of the increased light availability (IPCC, 2019).
Initially, the increased light availability will certainly have a positive impact on the growth of algae in the sea ice and surrounding open waters. However, it is more difficult to estimate how the algae’s nutrient supply in and beneath the ice will change. Basically, nutrients are the key to algal growth – both in the sea ice and in the water column. But unlike light, it is still unclear how the nutrient situation will develop. The ultimate nutrient source in the sea ice is seawater. The nutrient concentration in the sea ice is governed by the circulation of brine and exchange with the nutrient-rich water from the depths, which lies below it. The influx of freshwater from the land could have the opposite effect, and this in turn could be amplified by the increased melting of ice masses on the continent. Freshwater is lighter and therefore forms a layer on top of the seawater. This hinders exchange with the nutrient-rich water below. When it comes to nutrients and primary production, there are opposing trends. Areas of open water together with wind can lead to mixing and a steady supply of nutrients. At the same time, thin ice results in greater light availability. The increased influx of meltwater from the land, driven by climate change, could, however, result in a lack of nutrients at the ocean’s surface due to stratification of the water (Lannuzel et al., 2020).
It is also difficult to predict future primary production in the sea ice because the nutrient concentration in the brine channels could be determined by various parameters in the future. Changes in the nutrient concentration in the sea ice are mainly influenced by the vertical flow of brine between the ice and seawater. The brine dynamics, in contrast, depend on the ice temperature and the salt content. However, in the winter months when the ice is forming, the ice temperatures could, on the one hand, rise due to the warmer atmosphere, but could also drop as a result of less snowfall, since without the protection of the snow layer, the ice surface cools more rapidly. Accordingly, there are still a number of uncertainties regarding the future nutrient supply in open waters and nutrient dynamics in the sea ice (Lannuzel et al., 2020).
However, what is certain is that the habitat for microalgae living in the ice will shrink as a result of the ice melting. Moderate melting in spring and summer could produce large hollow areas in the ice, which would offer more space for the ice algae, and their biomass could even increase as a result. On the other hand, if the ice melts completely, it could mean a total loss of habitat. Added to this is the fact that the amount of thick multiyear ice, which offers ice algae far more room for colonisation, is generally decreasing. In this respect, it is feared that the species diversity of the algal communities will decrease with continued warming of the Arctic.
At the same time, thanks to better light conditions and the growing areas of open water, more phytoplankton species from more southerly regions, like the widely spread foam alga Phaeocystis, are now penetrating the High Arctic. As the number of ice algae species decreases, such phytoplankton species will become more evenly distributed. This will be advantageous, above all, to those species that grow quickly and can thrive under more intensive light conditions and with low levels of nutrients and salt. The winners under such climatic conditions could include the dinoflagellates – unicellular organisms equipped with hooks and flagella, which can also photosynthesise. Diatoms, which currently dominate the ice, could become rarer. Populations of sea-ice specialists like the diatom Nitzschia frigida are expected to decline. This is especially problematic, since many of the diatoms living in the ice are among the most important producers of energy-rich fatty acids, which represent a major food source for zooplankton. Higher organisms such as fishes ingest these fatty acids via the food chain. It’s still uncertain how a lack of fatty acids due to the possible reduction in the diatom population could affect the food web as a whole. On the other hand, due to the high light availability in the vicinity of meltwater ponds, especially diatom species that live in open water, like Chaetoceros, Thalassiosira and Fragilariopsis, could thrive. But these provide far less energy than their counterparts living in the ice (Lannuzel et al., 2020).
However, the changes in primary production won’t be the same everywhere in the Arctic. In the western Arctic, where seasonal ice will largely replace multiyear ice, experts anticipate a general increase in primary production, but a decline in diatom biodiversity. In the eastern Arctic, it’s more difficult to estimate the increase in primary production because the future nutrient situation is still unknown – especially due to the unpredictability of meltwater flows from the land or greater mixing of open waters by the wind (Lannuzel et al., 2020).
The Southern Ocean around the Antarctic is the largest ocean region in the world that, despite its high nutrient content, is characterised by relatively low microalgal growth. Such regions are referred to as high-nutrient, low-chlorophyll (HNLC). The reason for the low algal growth is the lack of a single element, iron, a vital plant nutrient. In some regions there is also a lack of silicon, an important constituent of silicic acid, which diatoms need to form their shells. Although there are otherwise sufficient nutrients in the water, iron and silicon limit their growth. How climate change will affect primary production in the future therefore depends mainly on how the concentrations of iron and silicon in the water change (Henley, 2020).
Today, the biomass of phytoplankton is greatest north of the polar front; in the Atlantic sector of the Southern Ocean; near the sub-tropical front region of the West Pacific sector; and over the Antarctic continental shelf – for example in Prydz Bay, in the Ross Sea, and in the Amundsen and Bellingshausen Seas (Henley, 2020).
Primary production is lowest between the polar front and the southern boundary of the Antarctic Circumpolar Current, especially in the Indian sector within and north of the sea-ice zone. In the HNLC waters, where phytoplankton growth is limited by iron, the composition of the phytoplankton varies from season to season. Regions with extensive algal growth, on the other hand, tend to be dominated by blooms of diatoms, Phaeocystis or nanoplankton – like in the eddies behind islands or at the ice edge. In the northern part of the West Antarctic Peninsula, for example, record levels of chlorophyll at over 45 milligrams per cubic metre have been measured in diatom blooms (Henley, 2020).
Simulations indicate that, as a result of climate change, primary production in large parts of the Southern Ocean could increase by 50 percent. As the simulations show, various factors could contribute to this development, which might also lead to improved iron supplies. Today, iron finds its way into the Southern Ocean through air currents, particularly in dust or ash from bush or forest fires. Furthermore, iron is transported to the surface by gyres. In other regions, iron is transported here by ocean currents. Experts assume that climate change will intensify air currents to such an extent that in the future, more iron will be carried to Antarctic waters. Stronger ocean currents in the West Antarctic, on the other hand, could intensify gyres, causing iron-rich water masses from the subtropics to be carried toward the Antarctic. However, there is also a potential obstacle to the increased primary production due to the improved iron supply: rising temperatures could lead to increased cloud formation over the Southern Ocean, which would in turn mean less light during the Antarctic spring (Henley, 2020).
Like in the Arctic, it is expected that in the Antarctic, too, the composition of algae in and on the sea ice will be significantly affected by climate change. However, forecasts are difficult, since to date there have been comparatively few studies. There are indications that as the water masses become warmer, the free-swimming phytoplankton will spread southward toward the sub-Antarctic zone (Henley, 2020). In the past, the loss of sea ice in the Antarctic was less marked than in the Arctic. While the sea-ice extent especially decreased in the West Antarctic, it increased slightly in the east. More recently, however, the sea ice in the east also appears to have been shrinking (IPCC, 2019). With the poleward retreat of the sea ice, more space will become available for plankton communities in the open water, while the habitat for ice algae will shrink. In particular, the number of diatom species in the ice is also expected to decrease in the Antarctic, while other phytoplankton species such as flagellates will become more common (Henley, 2020).
When it comes to the species communities in the open water, however, the trend could well be very different: higher water temperatures and ice concentrations could offer more favourable conditions for diatoms and other species that form large algal blooms in the open water and at the ice edge. Ocean acidification caused by rising carbon dioxide concentrations in the atmosphere and other environmental factors could also change the species composition of the diatom communities. It is expected that in the coming decades, the biogeographic provinces of phytoplankton communities in the Southern Ocean will shift or fundamentally change (Henley, 2020).
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