Ship travel across the Arctic Ocean will become easier as the ocean’s ice cover dwindles, but will these changing conditions also remove barriers for plankton traveling from the Pacific to the Atlantic? New computer models of the currents moving past Alaska, Siberia, and Greenland suggest that the answer is yes.
The Pacific and Atlantic oceans are separate ecosystems, even though it would seem easy for organisms that go with the flow, such as plankton, to move between them. The average level of the Pacific is higher than that of the Atlantic because of differences in the waters’ salinity and density, maintained by the global system of ocean currents. This difference in levels causes water to flow through the available connections such as the Bering Strait in the north and the Drake Passage at the southern tip of the Americas.
Because the ice covering the Arctic Ocean makes the water beneath it cold and often dark, the long trip along the northern route is evidently too much for plankton. For instance, the remains of a species of diatoms that is plentiful in the northern Pacific, Neodenticula seminae, have been conspicuously absent from sediment cores taken from the Atlantic seabed. It seemed that these unicellular creatures were not present in the Atlantic for the last 800,000 years. Or were they?
In 1999, a surprising trove of remnant silicate cell walls of these organisms turned up in the northwest Atlantic, leading the researchers who discovered it to suppose that a pulse of Pacific water somehow got through with the diatoms surviving.
Stephen Kelly, a physical oceanographer at the University of Southampton in the United Kingdom, took up the question of how this could have happened. He had been modeling the fate of virtual particles—for instance, oil droplets—released in polar waters that might result from ship accidents along newly opened northern routes.
To see, instead, how Pacific diatoms could traverse the Arctic Ocean, he dropped his virtual particles in the Bering Strait under the environmental conditions of 10 or more years ago. Then, with his model inferring the movement of ocean waters, the outflow of rivers, and the development of ice from the actual weather conditions for the decade beyond his chosen starting year, he followed the particles’ trajectories. He then looked to see how many virtual particles released during conditions present from 1980 to 2000 reached the Atlantic and how many years it had taken each one.
Kelly’s findings seem to confirm the notion that under some circumstances Pacific water can reach the Atlantic unusually quickly. He presented preliminary results last month at the General Assembly of the European Geosciences Union in Vienna.
Sign of a Shortcut?
There was one year when his model showed many particles taking a shortcut not seen in other years. Normally, after passing through the Bering Strait, they would exit into the Atlantic via the Fram Strait between Greenland and Svalbard, with or without a stint circulating the Arctic Ocean in the Beaufort Gyre, typically taking 4 to 5 years to complete the journey. But some particles released in that particular year made it to the Atlantic faster by passing through the Canadian Arctic Archipelago, getting there after 2 to 3 years, possibly allowing the diatoms to survive the journey. That year was 1998.
Does the model explain the real-world Atlantic appearance in 1999 of Neodenticula seminae? “Looking at the timescales involved, the answer is probably no,” Kelly told Eos. According to his model, the Pacific diatoms could not have been spotted in the Atlantic so soon. “But the real interesting thing is that we found this shortcut pathway,” he said. “It’s a quicker route and geographically very different, along the coast, in an area where the ice disappears quicker in the summer and the growing conditions of the ice are different as well. So regardless of whether or not it can explain the appearance of this diatom, to which the answer appears to be no, it is interesting that this route is opened, coinciding with the first year of significantly ice free waters to the north of the Canadian Archipelago.”
Warming May Accelerate Journeys
In a follow-up study, Kelly has also tried to predict whether this shortcut, seen in only one year in his simulations from 1980 to 2000, is a one-off coincidence or if it will appear more often as global temperatures rise. “In 1998, having an ice-free Arctic just north of the Archipelago was absolutely shocking news. Now it’s, depressingly, every year,” he said.
For now, he has had to work with a coarser input for his model to generate these predictions because it is forecasting ocean currents as well as conditions in the Arctic for the next few decades, instead of hindcasting currents with known conditions. With those lower-resolution data, he hasn’t seen the shortcut appear so far. But his results, which he hopes to publish later this year, do show a general decrease in the time it will take for particles to reach the Atlantic. “We need to do more to work out why it is accelerating,” he said.
Kelly is not a biologist, so it is not for him to say whether new oceanic pathways opening between the two basins would be detrimental to life in the Atlantic. He does, however, note that Neodenticula seminae is very similar to Atlantic diatoms, and its recent incursion into the Atlantic doesn’t seem to have caused any problems. “I’m not sure it would be a bad thing, but it would be a thing. Obviously, invasive species in general can cause lots and lots of problems.”
Species in Transit
Even though the models don’t yet explain the 1999 appearance of Neodenticula seminae in the Atlantic, Kelly’s research is extremely interesting, said Robert Spielhagen, a paleoceanographer at the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany. “From the point of view of microbiology, we want to know how such species, which apparently do not occur in the central Arctic, can make their way. Modeling studies such as these are probably the only way to answer this question.”
Spielhagen says there are similar riddles throughout the world. “For instance, there’s a certain species of planktic foraminifera which is living only in the two polar areas. DNA studies have shown the two populations are very similar, so there must have been some exchange in the last several hundreds of thousands of years, although the poles have been climatically separated for millions of years. How were genes exchanged across the equator?”
Spielhagen was not involved with Kelly’s work on pathways through the Arctic, but he made a possible contribution to it during the meeting in Vienna. He commented, after the presentation, that he had found remains of Neodenticula seminae, the Pacific diatoms Kelly tried to trace to the northwestern Atlantic, in sediments from the Fram Strait, which connects the Atlantic and Arctic oceans between Greenland and Svalbard, in layers dating from around 1989. This observation has not yet been published.
The Fram Strait is a section of the normal route from the Pacific to the Atlantic, not the shortcut route discovered by Kelly, who told Eos he is now eager to do detailed modeling studies of flows through the strait around that time. “We can look at more things—data like temperature, salinity, sunlight—to see whether or not there’s anything that might have made that period more favorable for that route.”
—Bas den Hond (email: [email protected]), Freelance Journalist