Longer productive season and food base composition
change/mismatch
Sea-ice thinning, reduction of the snow cover, earlier melt and later
freeze-up of seasonal sea ice will increase the amount of sunlight
reaching the marine ecosystem (Alou-Font et al. 2013, Aumack et al.
2014) and likely drive ecological tipping points in which primary
producers flourish and outcompete dark-adapted species (Kortsch et al.
2012). In the areas where sea ice still persists, a longer productive
season is predicted to benefit the ice-algae, phytoplankton and
phytobenthos in both polar regions (Goldsmit et al. 2021). Higher
competition will likely lead to widespread shifts from invertebrate- to
algal-dominated states on coastal polar seabeds, reducing biodiversity
and altering ecosystem functioning. Future extension of the productive
season might drive some endemic shallow invertebrate communities into
small refugia where sea-ice duration is maintained, impeding genetic
connectivity. In many non-coastal areas, increased light will only allow
phytoplankton blooms (Montes-Hugo et al. 2009). Thus, some invertebrate
species may persist at depths where the light is too low for benthic
algae irrespective of sea-ice.
Changes in sea-ice duration and timing of major bloom events are
expected to negatively affect food base composition, particularly for
secondary producers. Primarily herbivorous zooplankton which transfer
energy-rich lipid compounds and essential fatty acids to higher trophic
levels, strongly depend on the sea-ice algae blooms (Ershova et al.
2021). During spring, ice algal-derived carbon is essential for the
maturation and reproduction of pelagic grazers before phytoplankton is
available for their offspring (Leu et al. 2011). An earlier onset of the
ice-associated and pelagic blooms is likely to create a mismatch in
carbon source availability and grazer occurrence (Ji et al. 2013). Krill
and Calanus copepods are a keystone species and a primary food
resource for many fish, seabirds, and marine mammals (Cavan et al. 2019,
Will et al. 2020). As many species are highly dependent on them,
declines in abundance could have cascading effects throughout the food
web. Large increases in gelatinous zooplankton biomass could imply an
energetic turning point and trophodynamic restructuring.
In regions still covered by seasonal sea ice, more and fresher organic
material is exported to the deep sea by the fast-settling ice-associated
diatoms and surface-born microbial clades in comparison to slow-settling
Phaeocystis aggregates in the ice-free regions (Savidge et al. 1995,
Reigstad et al. 2008, Lalande et al. 2019, Fadeev et al. 2021).
Weakening of the pelagic-benthic coupling through the decrease in
vertical export efficiency and amount of sinking labile organic matter
will have repercussions on arctic deep-sea ecosystems. Indeed, benthic
biomass and diversity were found to be higher in the seasonally
ice-covered areas (Pecuchet et al. 2022) with a shorter but with higher
modularity food web. Modelling results indicate that climate-related
changes in phytodetrital inputs can lead to important shifts in benthic
biomass, community structure, and functional diversity, with loss of
various common taxa (Lovvorn et al. 2016). Further warming and reduction
in sea-ice coverage will most likely also continue to negatively affect
arctic fish species whose food availability largely depends on the
coupling between sea-ice cover and the benthic production (e.g.
demersal). Most deposit-feeders (and, in turn, their consumers),
however, rather depend on the total amount of settling microalgae, and
not so much the timing.
Freshwater, nutrient, and
sediment inputs. Stratification and vertical transport by buoyant plume
Sea ice and glacier ice melt release large volumes of freshwater. It can
have stimulating effects on primary production early in the season by
nutrient input and meltwater-induced stratification, which provides
favourable conditions for phytoplankton growth and development (Detoni
et al. 2015), but prevents the vertical export to the bottom, especially
of small cells. Icebergs also leave a trail of trace nutrient enrichment
as they gradually melt, enhancing phytoplankton blooms in their path.
This phytoplankton bloom fertilisation increase bloom intensity, and
increase the potential blue carbon capture, but whether this increases
any storage or sequestration by benthic organisms depends on where and
when this occurs (Death et al. 2014, Wadham et al. 2019). However, as
outlet glaciers continue to retreat, many will lose their connection to
the ocean. This decoupling of glaciers (and their meltwater) from the
ocean will have important consequences for the timing and spatial
distribution of nutrient delivery and the carbon cycle (Meire et al.
2015, Hopwood et al. 2018). Glaciers’ retreat on land will also affect
seabirds as currently, submarine glacial plumes stun zooplankton by cold
and osmotic shock and raise them up to the surface, thus making
available to seabirds, in particular to surface-feeders over
benthic-feeders and pelagic pursuit-divers.
Meltwater discharge also transports dissolved and particulate matter
from land, leading to increased light attenuation and coastal waters
darkening/shading. The total primary production may drop drastically due
to intensive glacial melt (Hoffmann et al. 2019). Also, low light
availability by glacier melt inputs with high turbidity restricts large
phytoplankton productivity (diatoms). Whereas marine particulate organic
matter provides rich food to higher trophic consumers, terrestrial POM
is a low-quality food resource for marine organisms. Therefore, if
glaciers continue to melt because of climate change, an impoverishment
of the nutritional value of POM may be predictable impacting the food
webs of polar coastal regions. Many zoobenthic species were also found
to be sensitive to high sedimentation rates. Thus, if the increasing
intensity of glacial processes will continue in the upcoming years, the
diversity of the encrusting fauna, ascidians in the shallow sublittoral
could dramatically decrease (Krzeminska and Kuklinski 2018).
Ice scouring
Iceberg scouring affects around 30% of world’s coastline and leads to
significant mechanical disturbances (Conlan et al. 1998, Barnes and
Souster 2011a) that skews population structure of benthic communities
(Smale and Barnes 2008), contributes to the sediment resuspension and
increment of inorganic particulate matter on the water column (Moon et
al. 2015), and limits the potential immobilisation and, ultimately,
sequestration of benthic blue carbon (Barnes et al. 2018). The
probability of scouring effect is intertwined with sea ice dynamics,
since coastal fast-ice losses and low coverage of seasonal sea ice can
facilitate the movement of icebergs, thus producing high-frequency ice
scour disturbance events (Zwerschke et al. 2022). After the space
clearance, mobile and pioneer benthic organisms (via larval recruitment)
are the first to recolonise the scour marks and their surroundings. Slow
recolonisation and recovery post-disturbance which follows highly
complex successional pathways (approximately 10 to 250 years), is a
general tendency for polar benthic ecosystems due to slow growth and
life cycles (Gutt and Starmans 2001, Zwerschke et al. 2022). The
magnitude of iceberg-driven disturbance varies within and, most
contrastingly, between polar regions and the peak effect is
asynchronous. Abundance, disturbance area, and maximum age of ice scours
are comparatively lower in the Arctic (Gutt et al. 1996). Icebergs
drifting in the northern hemisphere are significantly smaller than those
in the southern, and since the contributory glaciers and ice shelves
have retreated more severely in the Arctic, massive disturbances from
mega-icebergs should only be expected in the vicinity of the main ice
shelves/glaciers in the Canadian high Arctic and Greenland. On the other
hand, almost 90% of glacier fronts have retreated along the western
Antarctic Peninsula since the early 1950’s (Cook et al. 2005). On
ecological time scales, benthos will be increasingly affected in such
areas, while ice scour disturbance will drastically drop in Antarctic
shallows in the centuries to come; this asynchronous ice disturbance
history in polar regions indicate that the Arctic may anticipate future
Antarctic.
Human activity
The opening of polar seaways is facilitating opportunities among all
types of vessels in areas that were previously covered by MYI,
supporting the strong increasing statistical correlations observed
between sea ice change and shipping trends in both polar regions
(Constable et al. 2022). This will have an impact on ice-associated
species by ice breakup, underwater noise, increase the risks of lethal
and sublethal collisions of marine megafauna, and risks of invasive
species. Loss of marine ice is also related to increasing human activity
of other forms, e.g. fisheries and the use of other marine resources.
Although the largest more sustainable fisheries are in polar waters,
more periodical reassessment of the stocks will be needed particularly
considering the imbalance that can be triggered by further changes of
the sea ice, and the increase of demand of highly nutritious protein
(Constable et al. 2022). Bottom trawling in polar shelves also makes the
stored carbon therein more prone to be released back into the carbon
cycle, thus blocking the long-term storage (Bax et al. 2021). However,
an overarching management framework to protect benthic communities from
bottom fisheries in high polar seas is the identification and
designation of VMEs (Ashford et al. 2019). Oil, gas, and deep-sea mining
exploration and exploitation in Arctic seas is dissonant with the Paris
Agreement, and unlike Southern Ocean’s governance of The Antarctic
Treaty System (ATS) through the Commission for the Conservation of
Antarctic Marine Living Resources (CCAMLR), the northern polar region is
more vulnerable to direct human interference.
Cumulative response to
environmental stressors
Our ecosystem based risk assessment suggests that future marine ice loss
is likely to pave the way for autotroph-dominated polar ecosystems with
higher productivity and carbon-capture. The cumulative score of all the
stressors shows that marine ice loss will be detrimental to secondary
producers, i.e. zooplankton and zoobenthos, and particularly for
primarily herbivorous copepods and krill. It is mostly driven by changes
in the food base composition and timing after sea ice loss as well as
sediment discharge with glacial meltwater that will also affect carbon
storage and sequestration as well as regional biodiversity. These
effects will likely suppress the effect of colonisation of pioneer
species and biodiversity shifts in the newly open coastal zones becoming
exposed in the near future. In the long term, there will be regions
where ice-algal production will decline with the disappearance of the
ice all together leading to new pelagic conditions, while in other areas
sea ice will expand over previously ice-shelf covered waters. Such
nonlinear responses complicate the prediction of future polar ecosystem
dynamics. There is no doubt, however, that the consequences of the large
abiotic changes in the polar regions are expected to be severe for
ice-obligate species such as polar bears, pinnipeds and seabirds.
Currently, they often find refuge at the front of marine terminating
glaciers and on ice shelves, or slowly adapt to land habitats. Moreover,
climate change-related biodiversity redistribution and range extensions
open up new and large-scale opportunities for the recruitment of
non-native, invasive species and human activity that have the potential
to affect ecosystem functioning and ecosystem services.
The patterns of species loss or change in biomass will likely differ
substantially in Arctic and Antarctic regions due to habitat differences
related to the nature of the ice-cover, which is floating in the Arctic
and land-associated in the Antarctic. Our analysis suggests that the
nature of the changes that will occur with marine ice loss in polar
regions may differ somewhat in magnitude, with the Arctic having lower
cumulative scores and thus being more prone to species loss by the end
of this century. Arguably, for all of the environmental stressors
assessed here, the Arctic seems to be close to the tipping point of
marine ice loss related shifts. In many environmental aspects and to a
certain degree, the ongoing changes in the Arctic could be used to
foresee possible ecosystem phenologies in Antarctic seas.
The current review shows that there is a great value in a synergistic
approach held in compliance with recent state-of-the art assessment and
predictions of the environmental stressors. Future reviews and
meta-analyses would be facilitated if source studies focussing in
species also provide some sort of functional classification of their
study subjects that can be used as input for building score matrices or
clustering type of responses based (pressure-response).