"Blurring the boundaries - Pacific and Atlantic"

The melting of Arctic sea ice is blurring the biological boundaries between Pacific and Atlantic.

It was in May 1999, during routine monitoring, that the tiny diatom was first found drifting in the ocean currents. Not an unusual observation on a plankton survey, only the species was in the wrong ocean. The north-west Atlantic was thick with phytoplankton of a Pacific species on its first visit for 800,000 years.

"We were very familiar with the species in the Pacific," says Chris Reid, professor of oceanography at the Sir Alister Hardy Foundation for Ocean Science (SAFHOS) in Plymouth, UK, who led the survey. "But we had never seen it in the Atlantic before — it took a while for us to realise the significance."

Reid's explanation — based on analyses of sea ice coverage — is that Neodenticula seminae migrated from the Pacific to the Atlantic via the Arctic as a direct consequence of the Arctic's diminishing ice cover. Melting of ice is now opening up the Northwest Passage between the Arctic and Pacific Oceans during summer and could result in a seasonal ice-free state in the region as the climate continues to warm.

The threat is made all the more acute by recent satellite data which show the extent of Arctic sea ice at its lowest since satellite recording began in the early 1970s. With sea ice coverage reaching just 4.14 million km2 in September1, the route was more open this summer than in 1998 when Neodenticula slipped through. If Reid is correct, it is the first species to have become established via this trans-Arctic pathway for thousands of years and a sobering reminder of the extent to which our climate is changing.

Since its arrival, the diatom, commonly found in the most northerly reaches of the Pacific and the Bering Sea, has colonized the Labrador Sea between Greenland and Canada, as reported by Reid and colleagues in the September issue of Global Change Biology2. "An [ice] gate was opened in 1998 which has probably been closed for thousands of years and then it closed again immediately afterwards," he explains. "[The plankton] would have moved through the Bering Strait, through the [normally icy] Canadian archipelago and [south] into Baffin Bay." The completely open seawater in the summer months of that year — blown by winds accelerating the general east-west current flow — would have provided ideal conditions for the phytoplankton to grow and proliferate, he argues...

The true significance of the event lies not in the single species introduction but in a barrier being breached between the two oceans. Viable pathways through the Arctic ice mean that many more Pacific species could follow suit, posing a threat to northern north Atlantic species by competing for resources and potentially playing havoc with the ecosystem.

Although phytoplankton are among the ocean's smallest denizens, their size belies their impact. The phytoplankton species Coscinodiscus wailesii, which invaded the North Sea from the Indian and Pacific Oceans, for example, displaces indigenous species given the right conditions. And as many native phytoplankton feeders find it unpalatable, its presence has knock-on effects throughout the entire food web. "This is the trickle before the flood," says Reid, describing plankton as a "tremendous indicator" of what is happening in the ocean. "We could well see a complete reorganization of the fauna of a large part of the northern north Atlantic."

...Neodenticula's journey may also reveal new evidence for the unprecedented nature of today's warming climate. Fossil records show the only other time the species appeared in the north Atlantic was between 1.2 million and 800,000 years ago, introduced during an interglacial period. "It died out probably because of severe cooling," explains Reid, adding that oddly there is also no evidence of its presence in the north Atlantic during the Pliocene 'trans-Arctic interchange' of about 3.5 million years ago, when there was a huge extinction as Pacific species invaded the Atlantic....