Animal behavior in space and time is structured by the perceived day/night cycle. However, this is modified by the animals’ own movement within its habitat, creating a realized diel light niche (RDLN). To understand the RDLN, we investigated the light as experienced by zooplankton undergoing synchronized diel vertical migration (DVM) in an Arctic fjord around the spring equinox. We reveal a highly dampened light cycle with diel changes being about two orders of magnitude smaller compared to the surface or a static depth. The RDLN is further characterized by unique wavelength-specific irradiance cycles. We discuss the relevance of RDLNs for animal adaptations and interactions, as well as implications for circadian clock entrainment in the wild and laboratory.

The light climate in the Arctic is primarily controlled by seasonal changes in solar elevation. The region experiences extended periods of 24-hr daylight (Polar Day) and 24-hr darkness (Polar Night), resulting in extreme variations in both the magnitude and spectral composition of irradiance entering the ocean throughout the year. Whilst the sun remains below the horizon during the Polar Night, solar elevation still significantly controls spectral irradiance during much of this period through atmospheric scattering of light from the sun. However, throughout the darkest periods of the winter season, irradiance from the moon becomes the dominant source of illumination (Johnsen, Zolich, et al., 2021).