Ecological responses to climate perturbations and minimal sea ice in the northern Bering Sea

Abstract

The winter of 2017/2018 saw a composite of weather events that delayed sea ice formation in the northern Bering Sea (NBS) into early 2018. Residual warmth in the water column and strong southerly (i.e., warm) winds in February resulted in the lowest ice extent on record. Salinity has historically driven vertical stratification of the water column in the NBS, but with little sea ice formation and rejection of salty brine, there was a greatly diminished contribution of salinity to the stratification of the water column. The reduction of sea ice extent and duration likely resulted in a reduction in the amount of ice algae, which is an important subsidy for both the pelagic and benthic food webs. In 2018, the NBS had low abundances of large, lipid-rich copepods, while there were above average numbers of small, lipid-poor copepods. Shifts in the distribution of crab and fish populations over the eastern Bering Sea shelf occurred in response to the unusually warm sea temperatures in winter and spring 2018. More than 50% of Pacific cod biomass in the eastern Bering Sea was found over the northern shelf in 2018 concurrent with unexpectedly high abundance of snow crab in the NBS. A seabird die-off event in summer 2018 was unprecedented in terms of spatial and temporal scale and widespread reproductive failures also occurred. High numbers of dead marine mammals were found along the shorelines of the NBS and an Unusual Mortality Event, an official designation for marine mammals, was declared for bearded, ringed, and spotted seals in September 2019. The 2018 events indicate that when climate warming results in extended periods of reduced sea ice cover in the NBS, there may be long-term changes in energy flow and ecosystem structure. Following the low sea ice conditions of 2018, winter 2018/2019 brought a second year of low sea ice. Although it is clear that the climate perturbations experienced in 2018 and 2019 had acute impacts on many components of the NBS marine ecosystem, it is less clear what the long term impacts of this event will be if there is a return to “normal” sea ice cover in future years.