| Summary: | Solar elevation (degrees relative to horizon) at Ny-Ålesund, Svalbard (Kongsfjorden) during Polar Night, December 2017 to February 2018. Grayscale bar represents daily moon fullness (black = 0%, new moon; light gray = 100%, full moon). Atmospheric irradiance (E PAR ) at Ny-Ålesund is replotted from [ 4 ]. Red dashed vertical lines denote the month of January 2018, during which hydroacoustic observations were conducted with ADCPs. Acoustic MVBS (dB re 1 m −1 ) detection of zooplankton in the water column is plotted during January for Kongsfjorden and Rijpfjorden (Svalbard). Period analysis ( S1 Table ) shows significant diel rhythmicity at all depths and throughout January. Missing acoustic data between approximately 100 and 130 m is due to a “blind zone” of upward/downward facing ADCPs. For data, see S1 Data . We see evidence at the community level that the cyclic light we measured during January 2018 influences in situ migration behavior of marine zooplankton. We examined acoustic backscatter throughout a water column of approximately 200 m in Kongsfjorden and Rijpfjorden to monitor cyclic changes of zooplankton biomass over time and with depth (Fig 2, dashed red box). These biomass changes reflect both DVM and LVM of zooplankton [ 15 , 18 , 32 – 34 ], which is commonly triggered by the ambient light cycle [ 35 , 36 ]. Period analysis of acoustic backscatter revealed significant migrations at both stations in January when the maximum solar elevation at midday was only −6.2° ( S1 Table ). Significant periods in the circadian range for biomass movement were detected throughout the month of January in Kongsfjorden (57% of depth bins) and in Rijpfjorden (61% of depth bins). These data agree with previous observations (e.g., [ 15 ]) showing migrations that continue during the Polar Night in January are driven by solar and lunar cycles. While we do not have net samples from the acoustic mooring location to confirm the identities of the zooplankton migrators, previous net sampling coincident with acoustic surveys in these fjords during Polar Night show that krill ( Thysanoessa spp.) are the dominant migrators and contribute >90% of macrozooplankton biomass [ 37 , 38 ]. Our own net sampling at this time of year further confirms this observation. ADCP, acoustic doppler current profiler; MVBS, mean volume backscatter.
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