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

Abstract

In the North Atlantic, euphausiids (krill) form a major link between primary production and predators including commercially exploited fish. This basin is warming very rapidly, with species expected to shift northwards following their thermal tolerances. Here we show, however, that there has been a 50% decline in surface krill abundance over the last 60 years that occurred in situ, with no associated range shift. While we relate these changes to the warming climate, our study is the first to document an in situ squeeze on living space within this system. The warmer isotherms are shifting measurably northwards but cooler isotherms have remained relatively static, stalled by the subpolar fronts in the NW Atlantic. Consequently the two temperatures defining the core of krill distribution (7–13 °C) were 8° of latitude apart 60 years ago but are presently only 4° apart. Over the 60 year period the core latitudinal distribution of euphausiids has remained relatively stable so a ‘habitat squeeze’, with loss of 4° of latitude in living space, could explain the decline in krill. This highlights that, as the temperature warms, not all species can track isotherms and shift northward at the same rate with both losers and winners emerging under the ‘Atlantification’ of the sub-Arctic.

Global Distribution of Zooplankton Biomass Estimated by In Situ Imaging and Machine Learning

“…This is consistent with our predictions of higher biomass in the epipelagic zone (0.058 PgC) compared to the mesopelagic (0.049 PgC), and low values predicted for the Arctic Ocean. The high Eumalacostraca biomass predicted in the North Atlantic also consistent with other observations that reported high abundances of krill in this region (Edwards et al, 2021). Euphausia superba and Euphausia mucronata have been respectively described as keystone species of the Antarctic and the Humboldt Current System (Antezana, 2010).…”

Stepping stones towards Antarctica: Switch to southern spawning grounds explains an abrupt range shift in krill

“…With some notable exceptions in parts of the Arctic and subarctic (Dalpadado et al, 2020;Edwards et al, 2021;Ershova et al, 2021;Fossheim et al, 2015), polar data sets rarely have sufficient temporal and spatial coverage to understand how rapid polar warming has translated into range shifts (Wassmann, 2011). Poor understanding of the mechanisms behind past change challenges our confidence in the future projections (Pinsky et al, 2020).…”

Foraging Behaviours of Breeding Arctic Terns Sterna paradisaea and the Impact of Local Weather and Fisheries

“…During later breeding stages they are likely to be even further restricted in the distances that they travel, so local prey availability will have a profound impact on their success or the choice to abandon the breeding attempt. Vigfúsdóttir et al (2013) concluded that a key issue causing low fledging success in arctic terns breeding in Iceland was food shortages and if this is the case, we have demonstrated that poor fledging rates are likely not as a result of directly competing for prey with commercial fisheries, but could, however, be due to changes in climatic forcing factors altering ocean structuring and overturning, with knock-on effects for ocean productivity and subsequent prey distribution (Hoegh-Guldberg and Bruno, 2010;Doney et al, 2012;Sydeman et al, 2015;Edwards et al, 2021). We also show that changes in wind strength and direction are unlikely to directly affect arctic terns, but further investigations are warranted both in Iceland and other areas where the breeding successes of arctic terns are poor in order to establish the underlying causes of population declines.…”