#Capelin2022: ​Understanding the implications of the new Arctic lightscape

Arctic sea i​ce and snow are melting at an increasing rate leading to an increase in light reaching the waters of high-latitude oceans.
Tom Langbehn talks about the impacts of this changing Arctic lightscape on species distribution in Bergen.
Published: 11 October 2022

​​​​Capelin experts gather this week in Bergen, Norway as the Institute of Marine Research hosts the second-ever ICES Capelin symposium

Capelin's responsiveness to changes in the marine environment led Rose to suggest in 2005 that it could be the sea “canary” for the northern boreal marine ecosystem.

​This four-day ICES symposium on capelin will seek to revise and expand our knowledge base on capelin biology, ecology, and its role in Arctic and Sub-Arctic ecosystems, focussing specifically on capelin stocks in the Barents Sea, waters around Iceland–East Greenland–Jan Mayen, the Newfoundland–Labrador Shelf, and the Gulf of Alaska and the Bering Sea. ​

Arctic lightscapes

Tom Langbehn, University of Bergen, opened Theme session 2: Capelin ecology and response to climate variation on Monday afternoon with his talk "The Arctic lightscape and its role in pelagic interactions and species distributions ​". Here, Langbehn provides an overview of his plenary.​

Global climate change is rapidly changing the environmental conditions in the Arctic - and organisms' response to this change will most likely have profound effects on the entire ecosystem. The Arctic is warming at three times the global average. Consequently, sea i​ce and snow are melting at an increasing rate. 

All the better to see you​

Sea ice, especially when snow-covered, acts as a lid and prevents light from reaching the water column; less ice implies more light reaching the waters of high-latitude oceans. Where nutrients are not limited, this increase in light availability can boost primary production in the pelagic zone and alter its seasonal timing. 

The bottom-up effect of changes in bloom dynamics for higher trophic life is well-acknowledged. However, light also directly impacts organisms higher up in the food chain. Many predators, such as amphipods, birds, fish, and even some baleen whales, depend on vision, and therefore light, to successfully find their prey. More light therefore not only means potentially higher production but also increased top-down control through visual foragers. Large zooplankton will be detected at larger distances than their smaller congenerics and are therefore more susceptible to visual predation. 

New foraging hotspots

In western Greenland, the decline of sea ice coincides with a shift in the community size structure of Arctic copepods towards smaller and less fat individuals or species. However, the underpinning mechanisms of these changes are poorly understood. One would expect large copepods to survive where visual predation is reduced i.e., in dim habitats below sea ice​​​ and where bottom depth does not constrain diel vertical migration. An analyse spatially resolved, long-term survey data of the mesozooplankton community in the Barents Sea seem to confirm this support this hypothesis (Langbehn et al., in review). 

Large copepods were predominantly found in the deep troughs that intersect the shelf south of the polar front, or at shallower depths in seasonally ice-covered waters, northeast of Svalbard. 

On the shallow banks, large copepods are largely absent while smaller copepods appear to survive. This suggests that large copepods are constantly advected from outside the Barents Sea onto the shelf and eventually the banks where they are then decimated by visual predators. Therefore, advection and topographic blockage are key mechanisms in explaining the high productivity in shallow subarctic and Arctic shelf seas, and as such, the receding ice edge may open up new foraging hotspots across the Arctic shelf seas. 

Surviving the dark northern winter

Globally, species are on the move due to climate warming and species are typically predicted to shift poleward, tracking shifts in their preferred temperature. However, temperature is not the only factor that dictates where a species can thrive and survive, and where it cannot. Species shifting towards higher latitudes will experience an increasingly seasonal light regime. While the endless daylight during the midnight sun period allows for efficient prey search 24/7, darkness during the polar night will continue to limit visual foraging at least for parts of the year. This extreme light environment favours seasonal migrants but may act as a barrier for species expanding their range poleward​. To survive at high latitudes, other organisms must take advantage of the light and productive summer season and need strategies to tackle the dark winter season, with no or limited access to food, factors not currently considered in projections from most species' distribution models. 

Understanding the implications of the new Arctic lightscape for pelagic interactions and species distributions is key to our understanding of the future Arctic Ocean.

Tom Langbehn is one of thirteen early career scientists that ICES has funded to take part in the Capelin Symposium.​​​​

The Capelin Symposium Bergen continues from 10–13 October 2022. ​Follow the news coming from Bergen @CSB_2022



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#Capelin2022: ​Understanding the implications of the new Arctic lightscape

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