Arctic zooplankton and the Lightscape of Fear: Behavioural Responses to Light Buffer Diel and Seasonal Changes in Visual Predation Risk

Light plays a critical role in zooplankton ecology, with brighter conditions making individuals more vulnerable to visual predators. Consequently, avoidance of light is a central driver in behavioural responses. The Arctic has a highly seasonal light environment, presenting challenges to typical paradigms of zooplankton behaviours such as diel vertical migration.

Here we investigate the community-level vertical movement of Arctic zooplankton in response to ambient light across diel and seasonal time scales. We have created an underwater light model using an annual record of light measurements at Ny-Ålesund, Svalbard (79° N). We approximate light attenuation using chlorophyll as measured by fluorometers on a mooring, and observe zooplankton vertical positioning using acoustics.

We find that the upper edge of zooplankton vertical distribution follows a specific isolume (the value of which is comparable to the visual detection limits of Arctic Calanusspp.) across diel and seasonal time scales, suggesting that the light-mediated predation risk (potential mortality from visual predators such as fish) is kept constant by zooplankton at all times. The depth of the isolume is altered by the presence of phytoplankton through shading. We investigate the various ways in which phytoplankton can affect zooplankton behaviours: as both a prey source and as a shading factor.

Changing light fields (through populations moving north and sea-ice loss) have been suggested to affect zooplankton predation. From the results presented here, we pursue the idea that zooplankton might continue to follow specific isolumes, resulting in the predation risk remaining constant across a variety of lightscapes, but the trade-off becoming one of foraging and energy intake.