Caught in broad daylight: Topographic constraints of zooplankton depth distributions

Abstract

For visual predators, sufficient light is critical for prey detection and capture. Because light decays exponentially with depth in aquatic systems, vertical movement has become a widespread strategy among zooplankton for avoiding visual predation. However, topographical features such as seamounts have been shown to block their descent, trapping them in illuminated waters with potential feeding benefits for visually searching fish. Here, we present an extensive and previously unpublished dataset on the vertical distribution of zooplankton in the topographically rugged Barents Sea, a continental shelf region hosting some of the largest fish stocks in the world. By modeling the ambient light exposure of zooplankton in relation to the bathymetry, we find support for a similar blockage mechanism. During daytime, zooplankton are exposed to four orders of magnitude more light above shallow banks than in the deeper water surrounding the banks. We show that zooplankton depth distributions are highly related to zooplankton size and that the bottom constrains the vertical distributions. Consequently, zooplankton remain in the planktivores’ visual feeding habitat over the banks but not in deeper areas. Bottom topography and light absorbance are significant determinants of the seascape ecology across continental shelves with heterogeneous bathymetry.