Retreating and reducing regional sea ice has played a major part in most of the climate change-related storylines taking place in the Arctic. From warming seas and an upsurge in sunlight absorption to the creation of fresh shipping pathways and the affected uptake and conversion of carbon dioxide by phytoplankton to fuel the foodweb, the impacts of a changing climate have spread across science and the public realm. However, whilst the driving force of physical processes such as temperature, atmosphere and photosynthesis in the globe's most unique area have been well-documented, studies haven't tended to consider the impact of light beyond the level of algae on the Arctic ecosystem. Until now.
The consequences of a greater amount of light entering the water at high latitudes is the innovative angle taken by scientists Øystein Varpe, Malin Daase, and Trond Kristiansen in a recently-published ICES Journal of Marine Science (IJMS) Food for Thought article. In it, they postulate the theory that as sea ice lessens in scope and thickness, more previously-reflected light will be delivered into the ocean. The effects of this extra influx will ripple through the pelagic and benthic ecosystems bringing about a host of changes such as visually-searching fish being able to hunt prey more efficiently and expanding further north towards the pole. Complex predator-prey interactions could also be influenced.
"There are several impressive studies already of how less sea ice and more light may impact primary producers," states Varpe. "These studies, along with our own experience from work on zooplankton-fish interactions and how the light regime impacts on visual predators, inspired us to extend the perspective to how the new Arctic lightscape could impact fish and other visual predators."
Amongst other things, Arctic sea ice (and snow, a more efficient light blocker) acts as a sort of shield, preventing the radiance from the sun, an essential ingredient for algal photosynthesis, reaching the waters underneath. As algae is the basis for all marine life, this trend suggests structural and functional changes that will resound through the foodweb and ecosystem. It's these broader effects, particularly the ones on fish that require light to locate prey, that the authors argue need to be taken into account.
To arrive at their conclusions, Varpe, Daase and Kristiansen coupled modelling work with an understanding of how light controls detection of prey by fish species. For the first part, based on projected greenhouse gas emission rises over the 21st century, the scientists used the Norwegian Earth System Model NORESM AR5 to calculate the relationship between declining sea ice and the amount of light hitting the water. The results showed that light intensity in the future Arctic Ocean will go up dramatically after 2040 if the predicted conditions in ocean, light and atmosphere take place.
The modelled outputs were then brought together with biological knowledge on fish in the region, like polar cod, that use light to pick up on their prey, a trait shared by other visual predators such as diving seabirds.
Light determines how efficiently visual predators like fish can find their prey. With fish being a major force in oceanic species interactions, how might the creatures' change in behaviour due to the spike in sunlight reaching the water column reflect on the foodweb? And how might new, more illuminated areas opening up impact on their habitats?
Up to this point, ideas about and observations of climate change's effect on Arctic ecology have been largely dictated by food availability and water temperature. Here, the authors contribute a third dimension: light, which could mean fish will be less constrained in their movements and by the calendar. An improvement in the feeding environment would see fish climbing latitudes and seasonal migrants visiting the Arctic during the summer more common.
From the top of the water column down and from the bottom up, other effects of a lighter Arctic would be felt. The increasing exposure of algae to light at the base of the ecosystem could trigger a boom in the growth of seafloor plants and a rise in open-water algae converting more atmospheric carbon dioxide to proteins and sugars (primary production).
Competition between polar cod and other species meanwhile might be stronger from the fresh northern presence of sub-Arctic creatures. More predatory fish will see larger plankton more easily and this selective eating may consequently favour smaller plankton. Higher algal growth and concentration, meanwhile, could actually reduce light reaching fish that inhabit deeper waters through shadowing.
"More light could also make prey fish more visible and more vulnerable to bigger visual predators," continues Varpe. "This illustrates the long list of ecological interactions impacted by changes in the light regime and that the biological responses are complex and can occur at several trophic levels at the same time. For instance, zooplankton, such as copepods and krill, have a large repertoire of anti-predator strategies and are likely to respond behaviourally to increased visual predation, for instance by vertical migrations."
The authors finish by making several recommendations on future research needs, include urging advancements in fields including visual ecology and prey detection modelling, knowledge on eye physiology and morphology of high-latitude species, studies on zooplankton body size variability, and data which allows spatial distribution to be analysed.
For Vaarpe, however, as important is the potential wider significance and application of the influential study.
"We focused on the Arctic in our article, but the physical and biological mechanisms we point to are relevant to other aquatic ecosystems as well such as the Southern Ocean and those of high-latitude and Alpine lakes."