3.2.1 Changes in the Arctic
The changes in the sea ice and at the ocean’s surface in general also affect life at the sea-floor. First and foremost, what is important is the amount of food that reaches the seafloor from the productive, well-lit layer near the surface, providing energy for the organisms living there, the so-called benthos. Light also plays an important role in shallow ocean areas. When the sea ice disappears, more light reaches the seafloor, which can e.g. lead to increased algal growth. In the shallow coastal areas in the Arctic, the amount of large brown algae, several-metre-long kelp that offers important habitats for fishes and marine mammals, has increased in the last two decades. The reasons for this are the greater availability of light due to the thinner ice cover, and the warmer water masses. In addition, the plants are less frequently scraped off the seafloor by pressure ridges or icebergs (IPCC, 2019).
The algae that sink to the seafloor from the ice are an important energy source for the benthos. Added to this is e.g. excrement from small crustaceans. Especially large amounts of food reach the seafloor when, in early spring, the diatom Melosira arctica rapidly reproduces. This creates long strands beneath the ice, which then gradually detach and sink to the depths (Lannuzel et al., 2020).
In the future, meltwater ponds could become a dominant feature of the sea ice in spring. Since they allow more light to penetrate the ice, this could promote the development of dense Melosira colonies. In turn, the sinking algal strands could amplify the episodic nutrient input at the seafloor. However, the farther the ice retreats, the less ice-algae mass will be produced. Therefore, experts assume that the amount of available biomass for the benthos will decrease in the future. Although in many regions of the Arctic, primary production by free-swimming algae will increase due to areas of open water and greater light availability, this won’t be enough to compensate for the loss of algal mass (Lannuzel et al., 2020). The result will be a decrease in biomass at the seafloor.
Such a development has already been observed in the Bering Sea and Chukchi Sea in the North Pacific. In the Barents Sea, north of Norway, on the other hand, it has been observed that the composition of the benthic fauna has been changing for several years. It is assumed that several factors interact here: sea-ice retreat, greater mixing of the water, ocean acidification, and higher temperatures at the seafloor. Something similar is currently happening in the fjords of Svalbard. Here the typical Arctic fauna at the stony seafloor are changing because, for years, a number of fjords have been ice-free in summer and the water temperature has increased (IPCC, 2019).
How significantly the changes at the seafloor influence the stocks of animals that live there and feed on benthic organisms is still uncertain. Off Newfoundland, Labrador and Nova Scotia, the number of snow crabs has decreased. At the same time, stocks in the Barents Sea are increasing. It is unclear how changes in the benthos will affect mammals like the walrus and grey whale, which mainly find their food at the seafloor (IPCC, 2019).
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