Heatwave off Greenland permanently altered the marine ecosystem.
An extraordinary heat event in 2003 profoundly and permanently changed the marine ecosystem off Greenland. As a research team led by Karl Michael Werner from the Thünen Institute of Sea Fisheries reports in the journal Science, the warming at that time triggered an abrupt shift in species composition in the subpolar North Atlantic—one that has persisted to this day.
In a meta-analysis, the scientists examined around 100 time series of biological indicators from all parts of the ecosystem, from the seafloor to the open water column. They identified a “perfect storm” of several climatic factors: an unusually weak subpolar gyre southwest of Iceland allowed warm subtropical water to advance far northward, while at the same time significantly less cold Arctic water flowed south through the Fram Strait.
As a result, sea ice cover declined sharply and surface temperatures rose markedly—an warming signal detectable down to depths of 700 meters. The entire North Atlantic between Greenland and Norway was affected. At the same time, record atmospheric temperatures were measured over the region, coinciding with devastating heatwaves in Europe.
The ecological consequences in the ocean were severe: cold-water specialists were displaced by species from warmer regions. A key species in the food web, the capelin, shifted its spawning grounds northward, with fatal consequences for its population, as eggs and larvae there are exposed to unfavorable conditions. At the same time, species such as cod and plaice expanded further north. Humpback whales returned to the southern coast of Greenland after about 150 years of absence, and killer whales also moved into the now less ice-covered areas. Arctic species such as narwhals and hooded seals, by contrast, experienced significant population declines.
The impacts extended until 2005 and spread over thousands of kilometers as far as the Fram Strait. Warm water carried new organisms into the region, triggering further changes in the food web—for example, increased algal blooms, whose sinking biomass on the seafloor promoted mass occurrences of invertebrates.
What long-term consequences these upheavals will have for the North Atlantic remains unclear. For lead author Werner, however, one thing is certain: “Our results show that unexpected extreme events can trigger unpredictable ecological cascades.”
Heiner Kubny, PolarJournal