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.
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’.
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