Sea-ice and water dynamics and moonlight impact the acoustic backscatter diurnal signal over the eastern Beaufort Sea continental slope

Abstract

Abstract. A 2-year-long time series of currents and acoustic backscatter from an acoustic Doppler current profiler, moored over the eastern Beaufort Sea continental slope from October 2003 to September 2005, were used to assess the dynamics and variability of the sound-scattering layer. It has been shown that acoustic backscatter is dominated by a synchronized diel vertical migration (DVM) of zooplankton. Our results show that DVM timings (i) were synchronous with sunlight and (ii) were modified by moonlight and sea ice, which attenuates light transmission to the water column. Moreover, DVM is modified or completely disrupted during highly energetic current events. Thicker ice observed during winter–spring 2005 lowered the backscatter values but favored extending DVM toward the midnight sun. In contrast to many previous studies, DVM occurred through the intermediate water layer during the ice-free season of the midnight sun in 2004. In 2005, the midnight-sun DVM was likely impacted by a high acoustic scattering generated by suspended particles. During full moon at low cloud cover, the nighttime moonlight illuminance led to zooplankton avoidance of the subsurface layer, disrupting DVM. Moreover, DVM was disrupted by upwelling, downwelling, and eddy passing. We suggest that these deviations are consistent with DVM adjusting to avoid enhanced water dynamics. For upwelling and downwelling, zooplankton likely respond to the along-slope water dynamics dominated by surface- and depth-intensified flow, respectively. This drives zooplankton to adjust DVM by aggregating in the low or upper intermediate water layer for upwelling and downwelling, respectively. The baroclinic eddy reversed DVM below the eddy core.

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

“…The methodology used in this study is largely based on that applied by Petrusevich et al (2016Petrusevich et al ( , 2020 and Dmitrenko et al (2020). 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.…”

“…DVM significantly impacts the biological pump and the biogeochemistry of the twilight zone (e.g., Archibald et al, 2019), with the carbon transported by DVM accounting for up to 30% of the total particulate flux and as much as 50% of the metabolic carbon dioxide produced in the twilight zone (Bianchi et al, 2013). DVM has been studied throughout the world's Oceans, including the Arctic Ocean (e.g., Cottier et al, 2006;Berge et al, 2009;Wallace et al, 2010;Hobbs et al, 2018;Dmitrenko et al, 2020) and is known to be primarily driven by illuminance (e.g., Hobbs et al, 2021). Only recently was DVM observed to occur during the polar night (Berge et al, 2009(Berge et al, , 2015, and more recent studies have analyzed the role of moonlight in modifying DVM during polar night (Last et al, 2016;Petrusevich et al, 2016).…”

“…Only recently was DVM observed to occur during the polar night (Berge et al, 2009(Berge et al, , 2015, and more recent studies have analyzed the role of moonlight in modifying DVM during polar night (Last et al, 2016;Petrusevich et al, 2016). 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). Beyond light, the oceanographic factors that influence DVM in the seasonally ice-covered Arctic shelves remain poorly assessed (Hobbs et al, 2018).…”

“…Beyond light, the oceanographic factors that influence DVM in the seasonally ice-covered Arctic shelves remain poorly assessed (Hobbs et al, 2018). Recent studies revealed the role of wind-driven and tidal-driven water dynamics in modifying DVM (Petrusevich et al, 2016(Petrusevich et al, , 2020Dmitrenko et al, 2020). In general, significant progress has been achieved in assessing DVM in the Arctic Ocean; however, to date there have yet to be observations of DVM over the Siberian shelf and no analysis of how DVM is modified by the opening of a polynya.…”

“…The dramatic changes in the Arctic environment in recent years mainly associated with seaice decline may have wide significance for the Siberian shelf water dynamics and ecosystem (e.g., Bouchard and Fortier, 2008;Lalande et al, 2009;Wassmann et al, 2010;Barber et al, 2015). For example, the sea-ice reduction results in the loss of a shading effect and enhanced illuminance of the surface water layer impacting DVM (e.g., Hobbs et al, 2018;Dmitrenko et al, 2020). The Laptev Sea connected in the west with the Kara Sea and in the east with the East Siberian Sea (Figure 1A) is seasonally ice-covered.…”