Coastal Polynya Disrupts the Acoustic Backscatter Diurnal Signal Over the Eastern Laptev Sea Shelf

Abstract

The diel vertical migration (DVM) of zooplankton is one of the largest species migrations to occur globally and is a key driver of regional ecosystems and the marine carbon pump. The dramatic changes in the Arctic environment in recent years, mainly associated with sea-ice decline, may have wide significance for the Arctic shelf ecosystems including DVM. Observations have revealed the occurrence of DVM in ice-covered Arctic waters, however, there have yet to be observations of DVM from the extensive Siberian shelves in the Eurasian Arctic and no analysis of how the sea-ice decline may affect DVM. Here, 2 yearlong time series of acoustic backscatter, collected by moored acoustic Doppler current profilers in the eastern Laptev Sea from August 1998 to August 1999, were used to examine the annual cycle of acoustic scattering, and therefore the annual cycle of DVM in the area. The acoustic time series were used along with atmospheric and oceanic reanalysis and satellite data. Our observations show that DVM did not occur during polar night and polar day, but is active during the spring and fall transition periods when there is a diurnal cycle in light conditions. DVM began beneath the fast ice at the end of polar night and increased in intensity through spring. However, the formation of a large polynya along the landfast ice edge in late March 1999 caused DVM to abruptly cease near the fast ice edge, while DVM persisted through spring to the start of polar day at the onshore mooring. We associate this cessation of synchronized DVM ∼1 month ahead of polar day with a predator-avoidance behavior of zooplankton in response to higher polar cod abundance near the polynya. During polar day, the intensity of acoustic scattering was attributed to the riverine suspended particles. Overall, our results highlight the occurrence of DVM on the Siberian shelves, the cessation of synchronized DVM when a polynya opens up nearby, and the potential impact of significant trends toward a more extensive Laptev Sea polynya as part of changing ice conditions in the Eurasian Arctic and their impact on the Arctic shelf ecology.

Contrasting two major Arctic coastal polynyas: the role of sea ice in driving diel vertical migrations of zooplankton in the Laptev and Beaufort Seas

“…The methodology used in this study is largely based on that applied by Petrusevich et al (2016Petrusevich et al ( , 2020 and Dmitrenko et al (2020Dmitrenko et al ( , 2021. We analyzed the year-long time series of acoustic backscatter measured by the ADCPs to reveal modifications of the diurnal signal that is the result of DVM.…”

“…In summer, the larger species are present in the regions with depth >50 m (Kosobokova et al, 1998), but by winter they migrate to the deeper regions of the Laptev Sea continental slope. DVM of zooplankton in the shallow Laptev Sea seems to be conducted by copepods Pseudocalanus spp., Acartia spp., and mysids (Dmitrenko et al, 2021), while in the deeper regions (>50 m depth) Calanus spp. and M. longa are known to perform DVM in summer (Arashkevich et al, 2018).…”

“…The impact of the Laptev Sea polynya on DVM has not been sufficiently assessed. Only recently Dmitrenko et al (2021) revealed that the DVM disruptions were attributed to polynya opening. However, for the shallow and strongly stratified water column of the Laptev Sea, the comprehensive analysis of the polynya impact on DVM requires further evaluation.…”

“…To our knowledge, the impact of the Cape Bathurst polynya on DVM of fish and zooplankton was never assessed. Overall, among all DVM studies in the Arctic shelves, only Dmitrenko et al (2021) evaluated the role of coastal polynyas in disrupting DVM over the Laptev Sea shelf. They suggested a predator-avoidance behavior of zooplankton conditioned by higher polar cod abundance attracted by the polynya opening.…”

“…In the Arctic Ocean, the ice cover attenuates light transmission and therefore modifies DVM (Hobbs et al, 2018(Hobbs et al, , 2021Dmitrenko et al, 2020;Flores et al, 2023). Thereby, over the Arctic and sub-Arctic shelves, seasonally covered with perennial sea-ice, DVM is also partially controlled by the ice cover (Wallace et al, 2010;Dmitrenko et al, 2021). For this reason, one may assume that small-and large-scale disruptions of the sea-ice cover could impact DVM locally and regionally.…”