Is Ambient Light during the High Arctic Polar Night Sufficient to Act as a Visual Cue for Zooplankton?

Abstract

The light regime is an ecologically important factor in pelagic habitats, influencing a range of biological processes. However, the availability and importance of light to these processes in high Arctic zooplankton communities during periods of 'complete' darkness (polar night) are poorly studied. Here we characterized the ambient light regime throughout the diel cycle during the high Arctic polar night, and ask whether visual systems of Arctic zooplankton can detect the low levels of irradiance available at this time. To this end, light measurements with a purpose-built irradiance sensor and coupled all-sky digital photographs were used to characterize diel skylight irradiance patterns over 24 hours at 79°N in January 2014 and 2015. Subsequent skylight spectral irradiance and in-water optical property measurements were used to model the underwater light field as a function of depth, which was then weighted by the electrophysiologically determined visual spectral sensitivity of a dominant high Arctic zooplankter, Thysanoessa inermis. Irradiance in air ranged between 1–1.5 x 10-5 μmol photons m-2 s-1 (400–700 nm) in clear weather conditions at noon and with the moon below the horizon, hence values reflect only solar illumination. Radiative transfer modelling generated underwater light fields with peak transmission at blue-green wavelengths, with a 465 nm transmission maximum in shallow water shifting to 485 nm with depth. To the eye of a zooplankter, light from the surface to 75 m exhibits a maximum at 485 nm, with longer wavelengths (>600 nm) being of little visual significance. Our data are the first quantitative characterisation, including absolute intensities, spectral composition and photoperiod of biologically relevant solar ambient light in the high Arctic during the polar night, and indicate that some species of Arctic zooplankton are able to detect and utilize ambient light down to 20–30m depth during the Arctic polar night.

Unexpected Levels of Biological Activity during the Polar Night Offer New Perspectives on a Warming Arctic

“…Thus, rates of growth, respiration, and reproduction for the examined benthic organisms were not lower during the polar night compared to other times of the year. In addition, measurements of a potential circadian cycle (24 hr) in both Chlamys islandica valve opening and vertical migration of zooplankton indicated that, despite 4 months of apparent darkness, activity cycles remain entrained to the existing but weak diurnal cycles [34] and cues (Table S1). …”

“…Irradiance (E) is extremely low during the winter polar night period [34], resulting in primary production rates close to zero [41] and in little newly produced food for pelagic and benthic grazers [42]. Low E is also assumed to hamper feeding by visual predators over much of the water column, including zooplankton, fishes, and seabirds [34,43].…”

“…Low E is also assumed to hamper feeding by visual predators over much of the water column, including zooplankton, fishes, and seabirds [34,43]. These factors have led to the paradigm of the polar night being a period of dramatically reduced activity in all components of the system.…”

Evidence of Diel Vertical Migration of Mesopelagic Sound-Scattering Organisms in the Arctic

“…Cronin et al, 2016), that can be particularly important during the Arctic winter. Recently published work from this region show that some organisms respond to even very weak variations in light level, and mass-vertical migration during Arctic winter driven by moonlight has been documented (Cohen et al, 2015;Last et al, 2016). However, these studies have focused on zooplankton in the upper 50 m.…”

Bioluminescence as an ecological factor during high Arctic polar night

“…In contrast to other seasons when atmospheric light is present for some or all of the diel period, the winter light field33 would provide far less interference with the production and perception of bioluminescence. The high Arctic during polar night was once understood to be biologically dormant until light and the spring bloom recovered the system with organic carbon through photosynthesis.…”