6
3 Consequences for the Ice-Associated Ecosystem
The emerging physical properties of the new Arctic sea ice regime will have consequences for the ice-associated ecosystem. The retreat in Arctic sea ice extent has
already resulted in increased open ocean primary production in the Arctic (Arrigo
and Dijken 2015) while the thinner and more transparent summer ice cover facilitates under-ice phytoplankton blooms (Mundy et al. 2009; Arrigo et al. 2012). The
fate of ice algal production is less certain due to the limited data availability (Leu
et al. 2015). However, modelling studies indicate that the continuing retreat in
Arctic sea ice might result in a decline in ice algal production (Dupont 2012).
Changes in the timing, composition and magnitude of phytoplankton and ice algal
blooms will have cascading effects on higher trophic levels (Søreide et al. 2010).
Since the thinner sea ice is more susceptible to oceanic and atmospheric forcing,
sea ice deformation (ridging) and formation of leads (open areas in the ice pack)
will become more frequent. Our recent observations show that this more dynamic
ice cover had a positive effect on under-ice phytoplankton production already in
spring when the predominant FYI and second-year ice (SYI) were still covered by
0.3–0.5 m of snow that blocked >99% of the incoming sunlight from reaching the
underside of the sea ice and strongly limiting ice algal growth below these ice types.
However, the frequently formed leads acted as windows into the underlying water
column, enabling a large under-ice bloom dominated by the haptophyte algae
Phaeocystis pouchetii despite the thick snow cover (Assmy et al. 2017). We also
studied the ice algal community in the newly formed ice covering one of those
leads. The ice-associated diatoms that eventually dominated the algal community in
the refrozen lead originated from the adjacent thicker ice with thick snow cover.
These highly shade-adapted algae had to cope with the high light levels, both in the
visible and the UV range, below the refrozen lead by investing energy in synthesizing UV- and photo-protective pigments (sunscreens) while at the same time were
limited by recruitment from the adjacent thicker ice which likely explained their
mute response (Kauko et al. 2017). These observations also highlight the importance of older ice as a seed repository for ice algae and indicate that the observed
loss of MYI could have negative consequences for ice algal bloom formation (Olsen
et al. 2017). Model results suggest that maximum growth rates of ice algae will
increase whilst vertically integrated net primary production and biomass will
decrease under the thinner ice regime (Duarte et al. 2017). Furthermore, we identified pressure ridges as algal hotspots (Fig. 2; Fernández-Méndez et al. 2018) that
could contribute a significant if not the major share of ice algal stocks in a more
dynamic ice pack. The heavy snow load on the sea ice caused negative ice-freeboard
throughout the drift. In early June this led to infiltration of seawater through cracks
in the ice and growth of phytoplankton at the snow-ice interface (Fig. 3; FernándezMéndez et al. 2018). These snow-infiltration communities are common in the
Antarctic but have rarely been reported from the Arctic and could be another harbinger of “antarctification” in the Atlantic sector of the Arctic. The observed changes
in ice algal and phytoplankton bloom dynamics will potentially have cascading
effects on the entire Arctic food web and may alter the uptake and release of climaterelevant gases from ocean and sea ice.
M. A. Granskog et al.
3 Consequences for the Ice-Associated Ecosystem
The emerging physical properties of the new Arctic sea ice regime will have consequences for the ice-associated ecosystem. The retreat in Arctic sea ice extent has
already resulted in increased open ocean primary production in the Arctic (Arrigo
and Dijken 2015) while the thinner and more transparent summer ice cover facilitates under-ice phytoplankton blooms (Mundy et al. 2009; Arrigo et al. 2012). The
fate of ice algal production is less certain due to the limited data availability (Leu
et al. 2015). However, modelling studies indicate that the continuing retreat in
Arctic sea ice might result in a decline in ice algal production (Dupont 2012).
Changes in the timing, composition and magnitude of phytoplankton and ice algal
blooms will have cascading effects on higher trophic levels (Søreide et al. 2010).
Since the thinner sea ice is more susceptible to oceanic and atmospheric forcing,
sea ice deformation (ridging) and formation of leads (open areas in the ice pack)
will become more frequent. Our recent observations show that this more dynamic
ice cover had a positive effect on under-ice phytoplankton production already in
spring when the predominant FYI and second-year ice (SYI) were still covered by
0.3–0.5 m of snow that blocked >99% of the incoming sunlight from reaching the
underside of the sea ice and strongly limiting ice algal growth below these ice types.
However, the frequently formed leads acted as windows into the underlying water
column, enabling a large under-ice bloom dominated by the haptophyte algae
Phaeocystis pouchetii despite the thick snow cover (Assmy et al. 2017). We also
studied the ice algal community in the newly formed ice covering one of those
leads. The ice-associated diatoms that eventually dominated the algal community in
the refrozen lead originated from the adjacent thicker ice with thick snow cover.
These highly shade-adapted algae had to cope with the high light levels, both in the
visible and the UV range, below the refrozen lead by investing energy in synthesizing UV- and photo-protective pigments (sunscreens) while at the same time were
limited by recruitment from the adjacent thicker ice which likely explained their
mute response (Kauko et al. 2017). These observations also highlight the importance of older ice as a seed repository for ice algae and indicate that the observed
loss of MYI could have negative consequences for ice algal bloom formation (Olsen
et al. 2017). Model results suggest that maximum growth rates of ice algae will
increase whilst vertically integrated net primary production and biomass will
decrease under the thinner ice regime (Duarte et al. 2017). Furthermore, we identified pressure ridges as algal hotspots (Fig. 2; Fernández-Méndez et al. 2018) that
could contribute a significant if not the major share of ice algal stocks in a more
dynamic ice pack. The heavy snow load on the sea ice caused negative ice-freeboard
throughout the drift. In early June this led to infiltration of seawater through cracks
in the ice and growth of phytoplankton at the snow-ice interface (Fig. 3; FernándezMéndez et al. 2018). These snow-infiltration communities are common in the
Antarctic but have rarely been reported from the Arctic and could be another harbinger of “antarctification” in the Atlantic sector of the Arctic. The observed changes
in ice algal and phytoplankton bloom dynamics will potentially have cascading
effects on the entire Arctic food web and may alter the uptake and release of climaterelevant gases from ocean and sea ice.
M. A. Granskog et al.