Despite the strong seasonal fluctuations in living conditions in the Arctic, the animals living on the sea floor of the Arctic Ocean follow the same rhythm all year round – largely unaffected by the seasons.
The seasons in the Arctic mean drastic changes for flora and fauna on land and at the sea surface. A Norwegian-Polish research team was thus all the more surprised when they discovered that the macrobenthos – large animals living on the sea floor such as starfish, crustaceans, bristle worms and mussels – barely react to the strong seasonal fluctuations.
The study, published in Progress in Oceanography in December 2023, focussed on seven stations east of Svalbard and investigated the seasonal dynamics of life on the seafloor. Other than expected, the researchers found little variation in species composition and number of animals in these communities across the seasons.
“Many bottom-dwelling animals reproduce in the form of larvae that float with the water masses. After a while, these will settle down on the bottom and begin to grow. In seasonal bottom-dwelling animal communities, one would occasionally be able to find larger amounts of such recruits, but that was not the case in our study,” Henning Reiss, a professor at the Faculty of Biosciences and Aquaculture at Nord University in Bodø, Norway, and senior author of the study, said in a university press release.
These bottom-dwelling animals feed mainly on organic material that sinks from the upper water layers. This food supply is strongly influenced by the seasons, with a large phytoplankton bloom in spring, from which the zooplankton benefit and then grow rapidly. As soon as the sun disappears behind the horizon, the darkness of winter returns and the ice spreads, the zooplankton migrate to greater depths and wait for the next spring.
“The conditions in the Barents Sea change drastically through the seasons. We have the polar night, where the lack of light means no photosynthesis and thus no production of phytoplankton. Therefore, it was believed that winter here was a kind of ‘sleeping’ period for bottom-dwelling animal communities,” lead author Èric Jordà-Molina explained, who carried out the current study as part of his doctoral thesis at Nord University.
But now it turns out that more happens in the Arctic winter than previously assumed. The research team has investigated the seasonal changes in bottom-dwelling animal communities in the open Arctic Ocean for the first time.
“We want a better understanding of the rapid climate changes in the northern Barents Sea and the Arctic as a whole. In addition to looking at animal and plant communities, we are particularly interested in the timing and patterns of the processes that occur on the ocean floors,” Professor Reiss said.
The sediments on the sea floor, which serve as a stable food source during periods of low nutrient levels, could explain this resilience. “It is still just a hypothesis. But due to the high degree of advection from nutrient-rich waters into the Barents Sea and the relatively high primary production in the area, coupled with low water temperatures, marine sediments could accumulate deposited organic matter and act as food ‘fridges’,” Jordà-Molina said.
Despite the lack of seasonal variations, there were significant differences between the different stations: Environmental factors such as ice cover, depth, sediment type, salinity and seafloor temperature significantly influence bottom dwelling communities. South of the polar front, the researchers found a community with high diversity, possibly due to higher productivity in the water masses above and better access to food for soil organisms. “In contrast, the animal communities down in the Nansen Basin were quite different, with more specialised and unique animal groups adapted to great depths and limited food conditions,” Jordà-Molina said.
The researchers’ surprising discovery could mean that the larger benthic animals may have a kind of buffer against the current climate changes and will feel the effects of climate change less strongly and perhaps later.
“What we see is that these animal communities live independently of the seasonal variations in the water masses above them. It buys us time when it comes to changes in the Arctic environment. But it will not last forever, especially in a time where marine heatwaves and weakened ice cover occur increasingly frequently,” Jordà-Molina concluded.
The study was part of the Nansen Legacy project, an interdisciplinary collaboration involving researchers from northern Norway together with Akvaplan-NIVA.
Julia Hager, PolarJournal
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