Reduction in Arctic sea ice cover affect marine food webs through a multitude of direct and indirect effects

Climate changes can affect a species directly through a modification of the environment (e.g. higher temperature) that leads to either positive or negative development of the abundance of the species. However, climate changes can also affect a species indirectly, through effects on its predators, competitors or prey. Indirect effects on a species have often a time lag before the effect is seen as an increase or decrease in the abundance of the species in question.

In a recent study, Nansen Legacy scientists analyzed direct and indirect effects of climate variability, more concretely variability in sea ice cover, on the pelagic food web in the central and northern Barents Sea. The statistical analysis was based on observations of the year-to-year variations in the biomasses of the various fish species and zooplankton groups over the last 35 years (see figure below).

A schematic presentation of how climate, as represented by winter sea ice cover, influence the food web in the central and northern Barents Sea. Climate affects the species directly as well as indirectly through predators, competitors and prey.

The analysis showed that the biomasses of krill and capelin tended to increase in years with low sea ice cover, whereas the biomasses of copepods and amphipods tended to decrease.

The dominant krill species have their main living area further south, and increased warming in the central and northern Barents Sea is expected to expand the habitable space for krill northwards. However, the analysis showed that, as capelin biomass also tends to increase in warm years, the boost in krill biomass may only be temporary, as predation pressure on the krill may increase. Whether krill biomass will increase or decrease in a warmer climate therefore depends on what happens to their main predator, the capelin.

For the predominantly arctic copepods in the region, it is simpler. The direct effect of warming is shown to be negative. In addition, they are likely to face increased predation in a warmer climate. For these copepods, both the short-term and the longer-term effects of warming are therefore negative.

This shows the importance of assessing potential feedback mechanisms between higher and lower trophic levels when studying climate effects on marine ecosystems.

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