The Arctic in transformation

Following two oceanographic expeditions to the Arctic in 2015 as part of the international GEOTRACES programme, researchers from the Woods hole oceanographic institution (WHOI) and their international colleagues1 found that significant amounts of carbon and trace elements from rivers and sediments on the continental shelf were being brought near the North Pole by transpolar drift, a major surface current. Associated with global warming, this input of nutrients and contaminants could alter phytoplankton productivity in the Arctic.

Trace elements, such as iron, are essential nutrients for ocean life. They fuel the growth of phytoplankton, the microscopic algae that form the basis of the marine food chain. In general, more phytoplankton leads to more zooplankton (small fish and crustaceans), which can then be eaten by "top" ocean predators such as seals and whales. Contaminants, especially mercury, follow the same path and very high levels have been found in Arctic animals.

In 2015, oceanographers conducting research in the Arctic Ocean as part of the international GEOTRACES program found much higher concentrations of carbon and trace elements in surface waters near the North Pole than in those on either side of the Transpolar Drift, a large surface current capable of transporting water from the Siberian continental shelf across the Arctic Ocean via the North Pole.

The many trace elements that enter the world ocean from the rivers and sediments of the continental shelf are usually rapidly removed from the water column. In contrast, researchers have shown that trace elements in the Arctic Ocean are linked to the abundance of organic matter from rivers, allowing them to be transported by transpolar drift to the central Arctic, more than 1,000 kilometres from their sources.

As a result of global warming, researchers expect that the thawing of the ground will lead to increased runoff and thus the addition of trace elements previously trapped in permafrost, which could result in an increase in the amount of nutrients and contaminants reaching the central Arctic Ocean.
However, as the Arctic warms and large parts of the ocean become free of ice for long periods of time, seaweed becomes more productive. Therefore, more nutrients could further fuel this algal production.

At the moment, while researchers know that the structure of marine ecosystems is determined by the availability of nutrients, they can't say exactly what changes all this might bring about. Concerning mercury, whose biogeochemical cycle is closely linked to photochemistry and to the important exchange flows at the atmosphere-ocean interface, strong changes are also expected with the disappearance of sea ice in summer.

Although increased nutrients may boost Arctic marine productivity, researchers warn that the continued loss of sea ice will exacerbate global warming, which will have a broader impact on ecosystems.

Funding

Funding for Arctic GEOTRACES was provided by the United States National Science Foundation, the Alfred-Wegener Institute for polar and marine research (AWI), the Swedish Research Council Formas, the French National Research Agency (ANR) and Labex MER, the Netherlands Organisation for Scientific Research and the Danish Independent Research Fund. The Arctic GEOTRACES expeditions were supported by the captains and crews of the USCGC Healy and FS Polarstern.

Source

The Transpolar Drift as a Source of Riverine and Shelf-Derived Trace Elements to the Central Arctic Ocean, Matthew A. Charette et al, Journal of Geophysical Research-Oceans. 08 April 2020. https://doi.org/10.1029/2019JC015920