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Atlantic Amphipods Spreading Into The Arctic Ocean

December 19, 2013
Image Caption: Found in the sediment traps: Three different developmental stages of the amphipod Themisto compressa - juvenile (top), sub-adult (middle), and a mature adult (bottom). Credit: Angelina Kraft, Alfred-Wegener-Institut

redOrbit Staff & Wire Reports – Your Universe Online

Amphipods typically found in the Atlantic Ocean are now reproducing in the Arctic waters west of Spitsbergen, Norway, and according to research appearing in the journal Marine Ecology Progress Series, the discovery could signify a shift in the region’s zooplankton community.

The presence of these creatures was discovered by biologists from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), who believe that the main victims of this invasive species could be marine birds, fish and whales. The primary reason for this, they explained in a statement last week, is that the amphipods are smaller than Arctic zooplankton and less nutritious to predators.

AWI plankton expert Dr. Eva-Maria Nöthig explained that the amphipods were easy to detect because they were found hiding in “sediment traps which have been suspended for 13 years in HAUSGARTEN, the AWI long-term observatory in the Fram Strait.”

“We had originally anchored our funnel-shaped traps at a depth of some 300 meters there in the West Spitsbergen Current in order to catch downward floating material such as algae or excrement from zooplankton,” she added. “However, from the start we also found several amphipods in the traps. The sample containers are full to the brim, especially in summer months. We therefore believe that the animals are actively swimming into the traps.”

As it turns out, the accidental catch turned out to provide Dr. Nöthig with a valuable group of samples, because as years went on, they were able to detect changes in the number of amphipods captured as well as the species composition. During the first four years, they exclusively caught Arctic and sub-Arctic zooplankton. However, in June 2004, they found an increasing number of a smaller, Atlantic-native species known as Themisto compressa.

“During subsequent years what had begun as an exception turned into a seasonally recurrent rule,” the Institute said. “From this time scientists documented ever more examples of the Atlantic species Themisto compressa, especially in summer months. Despite this, scientists at that time believed water in the West Spitsbergen Current, with its average temperature of 3 to 3.5 degrees Celsius, to be too cold to permit the animals from the southern part of the North Atlantic, which have a greater sensitivity to cold, to reproduce there.”

However, recent discoveries have proven that assumption incorrect, according to Dr. Nöthig. During the months of August and September 2011, their hauls included “ovigerous females and recently hatched juveniles of the Atlantic species for the first time,” she said. “Moreover in following months we were able to provide evidence of the migrating amphipod in all stages of development,” even though the current’s warm phase had already come and gone.

Based on their calculations, the water masses of the northward-running West Spitsbergen Current requires roughly 150 days to travel from the northern part of the Atlantic Ocean to the Arctic Ocean – too much time for females already bearing eggs to travel from their native habitat in time for their larvae to hatch near Norway’s west coast.

That led the authors to conclude that the Atlantic amphipods are having their offspring in the eastern Fram Strait, meaning that they reach sexual maturity and reproduce after they arrive in the Arctic, Dr. Nöthig said. She and her colleagues view this discovery as an indication that there’s a shift in the strait’s ecosystem.

“We know from our long-term measurements in the Fram Strait and at HAUSGARTEN as well as from scientific literature that there have always been phases in the past in which comparably warm Atlantic water has advanced far northwards,” Dr. Nöthig said. “However, we have been unable to find a single indication that conditions ever changed as fundamentally as to permit these Arctic waters to serve as a nursery ground for Atlantic amphipods.”

At this point, the investigators are unsure whether or not the migrant creatures will continue spreading northward and begin competing for the habitat with the two types of amphipods native to the region. However, they do anticipate changes in the range of species and the food web in the Arctic. In fact, the authors report that amphipod predators will likely need to consume about five times the number of Atlantic species in order to ingest the same amount of energy typically acquired from Arctic amphipods.


Source: redOrbit Staff & Wire Reports - Your Universe Online



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