Dear MARMAM community, On behalf of my co-authors, I am pleased to share our recently published article in Functional Ecology, where we test the influence of light regime on diel activity patterns in a marine predator, using beluga whales as a model predator inhabiting a natural experimental setting (extreme light regimes of the Arctic). Storrie, L., Hussey, N. E., MacPhee, S. A., O'Corry-Crowe, G., Iacozza, J., Barber, D. G. & Loseto, L. L. (2022) Empirically testing the influence of light regime on diel activity patterns in a marine predator reveals complex interacting factors shaping behaviour. Functional Ecology. https://doi.org/10.1111/1365-2435.14172 Abstract Diel cycles in marine predator diving behaviour centre around the light-mediated diel vertical migration (DVM) of prey, and are considered critical for optimizing foraging and limiting competition across global seascapes. Yet, our understanding of predator diel behaviour is based primarily on examining relative depth usage between constant day/night cycles with no formal investigation of how varying light regimes interact with abiotic factors to shape diel activity. The extreme seasonal light regimes (midnight sun, polar night, day/night cycle) in the Arctic provide a unique natural experimental setting to empirically investigate the occurrence and intensity of diel behaviour in marine predators relative to changing light levels while concomitantly assessing interacting abiotic factors. Depth time series data from satellite-linked tags deployed on six beluga whales (Delphinapterus leucas) for up to 12 months were used to quantify diel behaviour by calculating dissimilarity in time-at-depth between periods of low and high solar altitude on each day. Generalized additive mixed effects models were used to examine the influence of hours of daylight across extreme light cycles, coupled with bathymetry and sea ice concentration; focal diel patterns were further examined relative to the thermal structure of the water column. As predicted, belugas exhibited cathemerality during the midnight sun, and initiated diel behaviour with the onset of the fall day/night cycle, with a marked increase in its intensity with the progression to equal day/night length. Occurrence of diel patterns, however, was complex; ceasing in regions with seafloor depths <700 m, and occurring with greatest intensity when the water column was thermally homogeneous within the upper 150 m. Through empirical investigation, this study demonstrates that the onset of day/night light cycles and presumably associated prey DVM can modulate predator diel dive behaviour under certain circumstances, but highlights how the complex interaction of abiotic factors with light regime shape dynamic spatiotemporal patterns. These findings, building on a body of recent work, emphasize that the traditional view of the ubiquitous occurrence of diel behaviour tied to DVM at the base of the food web oversimplifies vertical predator–prey interactions, identifying the need for more structured investigation. This article is freely available at https://doi.org/10.1111/1365-2435.14172 All the best, Luke Storrie PhD Candidate University of Manitoba email: storriel@myumanitoba.ca