New study shows a 50% decline in Krill abundance in the North Atlantic

North Atlantic warming over six decades drives decreases in krill abundance with no associated range shift

A team of UK and French scientists have shown a huge decline in North Atlantic krill over the last 60 years driven primarily by climate variability and North Atlantic warming. Krill, are extremely abundant crustaceans present throughout the world’s oceans. In the North Atlantic, krill are numerically a significant component of the biomass of marine ecosystems particularly in the more boreal and Arctic waters of the North Atlantic. They are an important source of food for commercially exploited fish species, squid and marine mammals such as baleen whales and therefore represent a crucial component in North Atlantic food webs.

50% decline in krill abundance

Examining the data that used long-term observations of krill, the team led by Martin Edwards from Plymouth Marine Laboratory (PML) showed that across the whole North Atlantic basin there has been a 50% decline in krill abundance over the last 60 years. The findings, published in the journal Communications Biology https://www.nature.com/articles/s42003-021-02159-1 show this widespread and abrupt decline has been associated with the warming climate of the North Atlantic observed over the last six decades. This warming has particularly accelerated since the mid 1990s where there was an abrupt shift to warmer conditions in Atlantic waters.

Close up of krill, photo by Brett Wilks

Accelerated pace of changes in the Arctic

In the sub-polar regions of the North Atlantic, where krill are most abundant, concern is growing at the accelerated pace of these changes and the increasing ‘Atlantification’ (i.e warmer more saline Atlantic waters) of these more northern waters and their detrimental effects on Arctic systems. The Arctic sea regions, in particular, are experiencing the strongest warming on the planet (nearly three times as fast as the planetary average) and the loss of sea ice in recent decades has been very rapid. Many regional seas that were once considered as being inhabited exclusively by Arctic flora and fauna have become more influenced by more southerly species as these species move northward as the Arctic warms.

Martin Edwards said ‘as ocean temperature rise, we generally expect species distributions to track towards historically cooler regions in line with their preferred habitats. In this case we would expect the krill populations to simply shift northward to avoid the warming environment and find new habitats in cooler regions of the North Atlantic. However, this study shows for the first time in the North Atlantic that marine populations do not simply just shift their distributions northward due to shifting isotherms to re-establish new geographic habitats’.

Angus Atkinson also from PML said ‘while krill has declined in abundance by 50%, its core latitudinal distribution at ~55 oN has remained markedly stable over the 60 year period’. The study showed that the isotherms for the warmer temperatures are shifting steadily northwards, the cooler isotherms remain in place with an 8 degree difference in average latitudes of the 7-8°C and 12-13°C isotherms in 1958-1967 but only 4 degrees of latitude between the same temperatures in 2008-2017. This ‘habitat squeeze’ and a potential habitat loss of 4 degrees of latitude could be the main driver in the decline of krill populations seen in this study.  This highlights that, as the temperature warms, not all species will be able to tract isotherms as they shift northward and there will be particular species that will win or lose when establishing new habitats as more northerly regions like the Barents Sea and Arctic Ocean become increasingly warmer and ‘Atlantified’.

Humpback whale feeding on krill. Photo by Jean Tresfon

One of the main reasons for the lack of northerly movement is because the centre of krill populations is found in the North West Atlantic (south and east of Greenland) and populations can become spatially constrained due to ocean currents and strong thermal boundaries such as the polar front limiting their northward expansions.  Here, unlike the North East Atlantic which has unimpeded northward flow into the Norwegian and Barents Seas, this region is latitudinally stalled by the sub-polar gyre circulation which is geographically and temporally more robust and forms a thermal barrier to the rapid northward expansion of species.

Martin Edwards further added: ‘while temperature alone does not necessary explain all patterns observed in this study, as trophic interactions would also play an important role, we are currently exploring the mechanisms for these wide-scale changes. We also do not currently know the full ecological ramifications of this dramatic decline in krill but they would presumably have had major consequences for the rest of the marine food-web and will have important implications for ongoing fisheries in the North Atlantic’.

Get the Open Assess paper here: https://www.nature.com/articles/s42003-021-02159-1.pdf

Edwards, M., Goberville, E., Helaouet, P., Lindley, A., Atkinson, A., Burrows, M., Tarling, G. (2021). North Atlantic warming over six decades drives decreases in krill abundance with no associated range shift. Commun Biol 4, 644. https://doi.org/10.1038/s42003-021-02159-1

Record-high Arctic freshwater will flow to Labrador Sea, affecting local and global oceans

Freshwater is accumulating in the Arctic Ocean. The Beaufort Sea, which is the largest Arctic Ocean freshwater reservoir, has increased its freshwater content by 40% over the past two decades. How and where this water will flow into the Atlantic Ocean is important for local and global ocean conditions.

A new study shows that this freshwater travels through the Canadian Archipelago to reach the Labrador Sea, rather than through the wider marine passageways that connect to seas in Northern Europe. The open-access study was published in Nature Communications.

“The Canadian Archipelago is a major conduit between the Arctic and the North Atlantic,” said lead author Jiaxu Zhang, a UW postdoctoral researcher at the Cooperative Institute for Climate, Ocean and Ecosystem Studies. “In the future, if the winds get weaker and the freshwater gets released, there is a potential for this high amount of water to have a big influence in the Labrador Sea region.”

The finding has implications for the Labrador Sea marine environment, since Arctic water tends to be fresher but also rich in nutrients. This pathway also affects larger oceanic currents, namely a conveyor-belt circulation in the Atlantic Ocean in which colder, heavier water sinks in the North Atlantic and comes back along the surface as the Gulf Stream. Fresher, lighter water entering the Labrador Sea could slow that overturning circulation.

A simulated red dye tracer released from the Beaufort Gyre in the Artic Ocean (center top) shows freshwater transport through the Canadian Arctic Archipelago, along Baffin Island to the western Labrador Sea, off the coast of Newfoundland and Labrador, where it reduces surface salinity.

“We know that the Arctic Ocean has one of the biggest climate change signals,” said co-author Wei Cheng at the UW-based Cooperative Institute for Climate, Ocean and Atmosphere Studies. “Right now this freshwater is still trapped in the Arctic. But once it gets out, it can have a very large impact.”

Fresher water reaches the Arctic Ocean through rain, snow, rivers, inflows from the relatively fresher Pacific Ocean, as well as the recent melting of Arctic Ocean sea ice. Fresher, lighter water floats at the top, and clockwise winds in the Beaufort Sea push that lighter water together to create a dome.

When those winds relax, the dome will flatten and the freshwater gets released into the North Atlantic.

“People have already spent a lot of time studying why the Beaufort Sea freshwater has gotten so high in the past few decades,” said Zhang, who began the work at Los Alamos National Laboratory. “But they rarely care where the freshwater goes, and we think that’s a much more important problem.”

Using a technique Zhang developed to track ocean salinity, the researchers simulated the ocean circulation and followed the Beaufort Sea freshwater’s spread in a past event that occurred from 1983 to 1995.

Their experiment showed that most of the freshwater reached the Labrador Sea through the Canadian Archipelago, a complex set of narrow passages between Canada and Greenland. This region is poorly studied and was thought to be less important for freshwater flow than the much wider Fram Strait, which connects to the Northern European seas.

In the model, the 1983-1995 freshwater release traveled mostly along the North American route and significantly reduced the salinities in the Labrador Sea — a freshening of 0.2 parts per thousand on its shallower western edge, off the coast of Newfoundland and Labrador, and of 0.4 parts per thousand inside the Labrador Current.

The volume of freshwater now in the Beaufort Sea is about twice the size of the case studied, at more than 23,300 cubic kilometers, or more than 5,500 cubic miles. This volume of freshwater released into the North Atlantic could have significant effects. The exact impact is unknown. The study focused on past events, and current research is looking at where today’s freshwater buildup might end up and what changes it could trigger.

“A freshwater release of this size into the subpolar North Atlantic could impact a critical circulation pattern, called the Atlantic Meridional Overturning Circulation, which has a significant influence on Northern Hemisphere climate,” said co-author Wilbert Weijer at Los Alamos National Lab.

More information: https://www.nature.com/articles/s41467-021-21470-3

Long-term responses of North Atlantic calcifying plankton to climate change

Study on calcifying plankton and climate change published in Nature Climate Change

Abstract:

The global increase in atmospheric carbon dioxide concentration is potentially threatening marine biodiversity in two ways. First, carbon dioxide and other greenhouse gases accumulating in the atmosphere are causing global warming. Second, carbon dioxide is altering sea water chemistry, making the ocean more acidic.

Although temperature has a cardinal influence on all biological processes from the molecular to the ecosystem level, acidification might impair the process of calcification or exacerbate dissolution of calcifying organisms.

Here, we show however that North Atlantic calcifying plankton primarily responded to climate-induced changes in temperatures during the period 1960–2009, overriding the signal from the effects of ocean acidification. We provide evidence that foraminifers, coccolithophores, both pteropod and non-pteropod molluscs and echinoderms exhibited an abrupt shift circa 1996 at a time of a substantial increase in temperature and that some taxa exhibited a poleward movement in agreement with expected biogeographical changes under sea temperature warming. Although acidification may become a serious threat to marine calcifying organisms, our results suggest that over the study period the primary driver of North Atlantic calcifying plankton was oceanic temperature.

more information: https://www.nature.com/articles/nclimate1753