Abstract
AbstractFor 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 Here, we present an extensive and previously unpublished dataset on the vertical distribution of zooplankton in the topographically rugged Barents Sea, a continental shelf region hosting some of the largest fish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.
Highlights
Interactions between vertically migrating pelagic species and ocean bathymetry may influence ecosystem structures. Trueman et al (2014) demonstrated that the diel vertical migrating (DVM) community is accessible to bottom feeding fish only at depths < 1000 m
Zooplankton display a range of vertical migration strategies related to diurnal (Bollens and Frost 1991; Ohman and Romagnan 2016) and seasonal (Bandara et al 2016) rhythms, which is an evolutionary adaptation to the trade-off between growth and survival in dynamic environments (Pearre 2003; Bandara et al 2018)
Bottom depth had a significant effect on the zooplankton WMD (p < 0.001), and slopes were different between the size classes (p < 0.001)
Summary
Interactions between vertically migrating pelagic species and ocean bathymetry may influence ecosystem structures. Trueman et al (2014) demonstrated that the diel vertical migrating (DVM) community is accessible to bottom feeding fish only at depths < 1000 m. Previous studies have demonstrated topographic blockage of zooplankton performing DVM (reviewed in Genin [2004]), where individuals ascending to surface layers at night are advected onto shallow topographies that block their morning descent to deeper waters. Seasonal migraters such as Calanus spp., the key herbivores in North-Atlantic ecosystems (Melle et al 2014), will be blocked by the topography if occupying shallow areas in periods of diapause (e.g., Krumhansl et al 2018). Survival of Calanus spp. during overwintering has earlier been related to access darkness for avoiding predation (Kaartvedt 1996; Dale et al 1999)
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