3.1.2 Changes in the Antarctic
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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