Polar ocean ecosystems in a changing world

Polar organisms have adapted their seasonal cycles to the dynamic interface between ice and water. This interface ranges from the micrometre-sized brine channels within sea ice to the planetary-scale advance and retreat of sea ice. Polar marine ecosystems are particularly sensitive to climate change because small temperature differences can have large effects on the extent and thickness of sea ice. Little is known about the interactions between large, long-lived organisms and their planktonic food supply. Disentangling the effects of human exploitation of upper trophic levels from basin-wide, decade-scale climate cycles to identify long-term, global trends is a daunting challenge facing polar bio-oceanography.

Polar marine ecosystems are located at the ends of planetary gradients in daily and annual solar radiation and are ice-covered for varying lengths of the year. They harbour, or have until recently, large stocks of conspicuous animal life — birds, seals and whales — which led to the conclusion that polar ecosystems channel a greater proportion of primary production to warm-blooded predators than those at lower latitudes1. This early view was explained by short, low-diversity food chains in polar regions, evoking images of simple systems dominated by a few key organisms.

Research conducted over the past two decades has shown that the concept of short, low-diversity polar food chains is overly simplistic. Although comparatively few species do provide the bulk of food to polar marine predators, the planktonic base of their food supply is equivalent in phylogenetic diversity to the planktonic base in temperate climate zones2, 3, implying that the structure and functioning of pelagic (open-water) food webs are broadly similar across all latitudes. But the key prey organisms for vertebrates vary between polar ecosystems: benthos and fish are the main prey in the north whereas crustaceans are in the south. This indicates that cold adaptation has not favoured a specific food chain. If air-breathing predators play a greater role in polar-ecosystem functioning than they do in lower latitudes, then a decrease in their stocks should have cascading effects down the food chain and lead to marked shifts in ecosystem structure. The evidence for such cascading effects is controversial, however, because of both the absence of baselines against which to assess change4 and our poor understanding of the carrying capacity of pelagic food webs for higher trophic levels.

Are seasonally ice-covered pelagic ecosystems fundamentally different from their counterparts in adjacent ice-free waters and how will they be affected by the retreat of sea ice in a warming world? Clearly the organisms that live in the sea ice or are dependent on it to complete their life cycles will be most severely affected, but some organisms may actually benefit from sea-ice retreat and overall productivity might actually increase5.

In this review we examine the possible effects of a warming world on polar ecosystems and consider only the seas and oceans directly influenced by sea ice and its melting. Because human-mediated change has influenced polar ecosystems at both ends — thinning and retreat of the ice cover and heavy exploitation of top predator populations — unravelling the effects of bottom-up and top-down forcing on pelagic ecosystems is an immediate task facing polar bio-oceanographers.