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waters with potential feeding benefits for visually searching fish.Here, we present an extensive and previously unpublished dataset on the vertical distribution of zooplankton in the topographically rugged Barents Sea, a continental shelf region hosting some of the largest fish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.

Pan-Arctic Depth Distribution of Diapausing Calanus Copepods

“…8; Table 3). Region-specific studies have also shown that diapausing Calanus tends to be found deeper and more dispersed with increased bottom depth and closer to the sea bottom at shallower locations (Dupont and Aksnes, 2012;Aarflot et al, 2018;Krumhansl et al, 2018). Our study suggests that on a pan-arctic scale, diapausing Calanus populations are blocked by shallow topography-creating potential feeding hotspots for zooplanktivores (Aarflot et al, 2018).…”

“…Nevertheless, both species are found deeper in winter than in summer, and the increase in WMD with bottom depth suggests that individuals seek out deep diapause habitat. In deep areas, suitable diapause habitat is available over a broad depth range, while in shallow shelves, Calanus is topographically blocked in winter (Genin, 2004;Aarflot et al, 2018). Furthermore, our pan-arctic analysis shows that C. hyperboreus descends deeper than its smaller congener in winter, possibly due to a higher size-selective predation pressure.…”

“…Finally, because of its larger size, C. hyperboreus may need to descend deeper than its congeners to avoid being detected by visual predators (Aarflot et al, 2018). Predators may influence the depth distribution of prey both via depth-specific predation (e.g., increased mortality close to the surface) and behav-ioral responses in the vertical distribution of prey (Kaartvedt, 1996).…”

“…Similarly, we expect that as the availability of favorable diapause habitat increases with bottom depth, the depth distribution of diapausers will broaden. For example, Krumhansl et al (2018) observed denser near-bottom winter aggregations of Calanus on the Canadian Atlantic shelf than in deeper areas, indicating that diapausers seek out depths that are unavailable on the shelf, while optimal diapause habitat is found across a broader depth range in deeper areas (see also Aarflot et al, 2018).…”

Suitable habitats of fish species in the Barents Sea

“…Another variable that could be considered in future studies is light availability. Light constrains numerous processes like primary production, visual predation (Langbehn & Varpe, 2017), or zooplankton depth in the Barents Sea (Aarflot, Aksnes, Opdal, Skjoldal, & Fiksen, 2019). It might change the limiting factors for boreal species entering the Barents Sea, as light conditions might be a particularly strong tradeoff at those latitudes (Poloczanska et al., 2016).…”