Fueled by previously unappreciated links between climate and ecology, the North Sea has undergone a radical ecological shift in the last half-century, say scientists.

The very shape of the food web has changed, from plankton on up to the cod and flatfish that once dominated the icy waters, supporting rich commercial fisheries. They've been largely replaced by jellyfish and crabs.

The full scope of the change has gone relatively unnoticed, and could foreshadow changes in waters around the world.

"Climate-driven changes in the biology of the sea are largely hidden from view," said Richard Kirby, a University of Plymouth marine biologist and Royal Society Research Fellow. "If similar changes occurred in a temperate forest, we would be shocked."

In a study published in the upcoming December Proceedings of the Royal Society B, Kirby and Gregory Beaugrand, an oceanologist at the Lille University of Science and Technology, analyze decades of climate and ecosystem data gathered in the North Sea, a pocket of ocean bordered by the United Kingdom and Scandinavia.

Though relatively small, the North Sea has historically been a fabulously fertile fishing ground. Even now, it provides about five percent of the global fish harvest — but that's barely a third of what it yielded just a century ago.

Declining stocks have been blamed almost entirely on overfishing. However, though fishing pressures have indeed been intense, some scientists have suspected that water temperatures are also a factor.

Over the last quarter-century, the North Sea's upper layers have warmed by about 2 degrees Fahrenheit. That seems like little, but in the North Sea, summer and winter water temperatures differ by just a few degrees. Even a few degrees of change is relatively profound, and enough to disrupt aquatic organisms accustomed to functioning in a very narrow thermal range.

Whether the warming is man-made or not, it's a sign of times to come. Global ocean temperatures are expected to experience a comparable or greater rise during the next century. And the consequences, as anticipated by the North Sea, have been relatively unacknowledged. Most discussions of climate change impacts focus on the terrestrial. When ocean life is mentioned, it's in the context of of coral reef bleaching or acidifying waters.

Both those threats are grave, but the possibility of oceans completely changing their character, independent of acidification or reef effects, may be just as troubling.

"The effect of climate on the marine food web, the way small changes can be amplified through the web, that's the moral of the story here," said Kirby. "And food webs everywhere will be affected in a similar way."

At the heart of Kirby and Beaugrand's findings is data from the Continuous Plankton Recorder Survey, which has been run in the North Atlantic since 1931, when explorer Alister Hardy invented the recorder — a specialized box that's dragged behind commercial ships, allowing researchers to take sea-wide samples of plankton and juvenile members of other species.

Combined with temperature records, the CPRs provides the most comprehensive climate-ecosystem dataset of any ocean, if not the entire world. And as temperatures have changed, so has every part of the food web, starting with its foundation.

"If you were to divide zooplankton into those that prefer warmer southern waters, and those that prefer colder northern waters, and look at the boundaries between those groups, it's moved north by over 700 miles in the last 40 years," said Kirby. "That's one of the largest range shifts, if not the largest, that's been recorded."

The distribution of hundreds of species have changed, in every niche from plankton up to the North Sea's top predators. Cod and flatfish numbers have plummeted, and tuna have vanished. The ecological roles they once played are now occupied by jellyfish and bottom-dwelling crabs.

"The North Sea has fundamentally changed. It's a totally different ecosystem from what it was," said Kirby.

When Kirby and Beaugrand crunched the numbers describing these patterns with equations designed to separate cause from coincidence, they found that temperature drove the changes. They also found evidence for what they call "trophic amplification."

"Because temperature acts on different components of the food web, the gross effect is amplified," said Kirby. "It affects the phytoplankton that copepods feed on; it affects the copepods; it affects the predators who eat the copepods; and all those effects, magnified, are much greater than any one alone." This compounding dynamic is responsible for the extreme rapidity of the shift, he added.

"The findings seem plausible to me," said Marten Scheffer, a Wageningen University ecologist who specializes in ecosystem-wide transitions. Scheffer, who was not involved in the study, also said that marine shifts are notoriously difficult to study. "Compared to work on lakes, or terrestrial grazing systems, there is little scope for experimental testing," he said.

According to Kirby, models by fisheries managers need to incorporate these dynamics and and policymakers contemplating global warming need to consider the magnitude of the change.

A similar dynamic may be at work in the Sea of Japan, which in recent years has become dominated by giant jellyfish.

"Marine ecosystems have always changed, but people don't realize how responsive they are, and how rapidly they may change," he said. "Humans shouldn't forget that we don't live in isolation from the food web."

Images: 1. Flickr/PhillipC 2. A model of North Sea ecosystem dynamics, from Richard Kirby and Gregory Beaugrand.

See Also:

*Citation: "Trophic ampliﬁcation of climate warming." By Richard R. Kirby and Gregory Beaugrand. Proceedings of the Royal Society B, Vol. 276 No. 1676, December 7, 2009. *

*Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecosystem and planetary tipping points. *