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.
Highlights
Zooplankton migrates vertically through the water column according to the daily cycle of illuminance through a process known as diel vertical migration (DVM)
Annual actograms of the modeled under ice illuminance and mean volume backscatter strength (MVBS) were computed for various depths at both Yana (Figure 8) and Lena (Figure 9) based on total sky illuminance, sea ice concentration, sea ice thickness, and snow depth
The MVBS diurnal signal follows the seasonal variability of the sun illuminance except for the period of polar day and polar night when the sun is above and below the horizon 24 h a day, respectively
Summary
Zooplankton migrates vertically through the water column according to the daily cycle of illuminance through a process known as diel vertical migration (DVM). 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). In the Arctic Ocean, the ice cover attenuates light transmission and modifies DVM (Hobbs et al, 2018, 2021; Dmitrenko et al, 2020). Recent studies revealed the role of wind-driven and tidal-driven water dynamics in modifying DVM (Petrusevich et al, 2016, 2020; Dmitrenko et al, 2020). Significant progress has been achieved in assessing DVM in the Arctic Ocean; 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
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