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).