Figure 3. Risk assessment (RA) for ecosystem components (left) and services (right) in response to stressors related to marine ice loss in the Arctic (top, circles) and Southern Ocean (bottom, diamonds). Ecosystem components; planktic flagellates (PF), planktic diatoms (PD), ice algae (IA), microphytobenthos (MiPB), macrophytobenthos (MaPB), primarily herbivorous zooplankton (HZ), omnivorous zooplankton (OZ), ice-associated fauna (IAF), sediment sensitive macrozoobenthos (SSZB), sediment tolerant macrozoobenthos (STZB), subpolar (SFi) and polar fish (PFi), seabirds (SB), polar bears and pinnipeds (PB&P), baleen whales (W). Ecosystem services (excluding cultural); regulating: carbon storage and climate regulation (CR); supporting: genetic and biological diversity (D), nutrient cycling (NC), habitat (H), primary production (PP); provisioning: fisheries (F). Stressors and their expected direction shown in arrows: sea-ice habitat (SI), marine habitat (MH), productive season (PS), nutrient input with meltwater (MI), food base composition change/mismatch (FB), sediment discharge (SD), vertical transport by buoyant plume (VT), stratification (ST), IS (ice scouring), human activity (HA). See more detailed information on Table S2.
Loss of sea ice habitat
An ongoing change from a MYI to less stable FYI, particularly in the Arctic Ocean, is unfavourable for the sympagic community (see Box 1). The highly variable under-ice topography of MYI allows enough light penetration for ice algae (Lange et al. 2017), enables richer biodiversity and species abundances compared to FYI (Melnikov et al. 2002), and offers an attractive feeding ground and refuge for ice-associated species year-round (Gradinger 2001) Furthermore, MYI can act as long-term storage for carbon and other elements, given internal biomass layers and has a potential for seeding spring bottom-ice algal communities in the following year, compared to FYI that undergoes a complete annual cycle of growth and melt. It leads to release of ice fauna and their low abundance and biomass (Ehrlich et al. 2020).
The sea-ice boundary will shift dramatically in both polar regions with further sea ice decline. Loss of FYI in the Arctic and Antarctica will lead to more open water within the ice pack and to lower primary production by ice algae (Lange et al. 2017). In the Arctic, sea ice retreat has led to an expansion of boreal fish and marginalisation of typical Arctic fish species (Aune et al. 2018). At the current state, it is increasing species richness and evenness, and the functional diversity of the fish assemblage. Even though higher diversity is often interpreted as being positive for ecosystem health, the observed trend may be temporal as subpolar species threaten Arctic species via predation and competition and ultimate loss of Arctic species will result in a reduction in functional diversity (Frainer et al. 2021). In the case of marine mammals and seabirds, sea-ice decline results in loss of shelter from inclement weather, open-water predators, but also represents loss of foraging habitats, platforms for birthing, nursing, resting and moulting (Lydersen et al. 2014). However, recent observations suggest that they are increasingly using land habitats in some parts of their range, where they have minimal access to their preferred prey, which has the potential to increase nutritional stress and interactions with humans (Rode et al. 2018). Despite some signs of adaptation, it is generally agreed that the ice-obligate species cannot survive absent sea ice. The abrupt loss of sea ice is creating a dichotomy between ice-dependent mammals and seabirds that are losing habitat (Iverson et al. 2014, Trathan et al. 2020), and some cetaceans that appear to be thriving during periods of rapid sea ice loss with extended open-water seasons enabling the establishment of macrozooplankton and fish populations (Moore et al. 2022).