Visual predation risk and spatial distributions of large Arctic copepods along gradients of sea ice and bottom depth

Abstract

Changes in the community size structure of Arctic copepods toward smaller and less fat individuals or species have been linked to environmental changes. The underpinning mechanisms are, however, poorly understood. We use a two‐step hurdle regression model to analyze spatially resolved, long‐term survey data of the Barents Sea mesozooplankton community along gradients of water mass properties, sea ice, and bottom depth. We test the hypothesis that reduced visual predation, and hence increased survival in dim habitats, explains the distribution of large copepods. We expect the presence and biomass of large copepods to increase with increasing bottom depth and the occurrence of seasonal ice‐cover. The patterns and drivers that emerge from our analysis support our hypothesis: in the Barents Sea large copepods were predominantly found in deep troughs that intersect the shelf south of the polar front, or at shallower depths in seasonally ice‐covered waters northeast of Svalbard. On the banks, large copepods are largely absent whereas smaller copepods appear to survive. Top‐down control provides one plausible explanation for these distributions. Large copepods survive where sea‐ice shades the water or deep habitats permit escape from visual predators through vertical migrations. However, when upwelled onto shallow banks or flushed out from below the ice they are decimated by visual foragers. Therefore, advection and topographic blockage of vertical zooplankton distributions are key mechanisms for the efficient energy transfer and productivity in subarctic and Arctic shelf seas. New prolific foraging grounds may open up for planktivores where the ice‐edge recedes under a changing climate.

Response of the copepod community to interannual differences in sea-ice cover and water masses in the northern Barents Sea

“…Large copepods, such as Calanus spp., experience a reduced visual predation risk and subsequent increased survival rate where sea ice shades the water. The increased predation risk in open waters can therefore shift the community to a dominance of smaller species (Aarflot et al, 2019;Langbehn et al, 2023), which is also in accordance with our observations.…”

“…While our data indicates that differences in bloom phenology and food availability between the two years may explain the observed changes in community composition from larger to smaller species, the presence of sea ice itself and its effect on visual predation risk may have played an important role. A recent study from the Barents Sea suggests that the prevalence of large copepods in deeper troughs and under sea ice is best explained by top-down control (Langbehn et al, 2023). Large copepods, such as Calanus spp., experience a reduced visual predation risk and subsequent increased survival rate where sea ice shades the water.…”

Abundance and biomass of copepods and cladocerans in Atlantic and Arctic domains of the Barents Sea ecosystem

“…The biomass of C. hyperboreus is generally low in the Barents Sea, which probably reflects its vulnerability to predation from visual predators such as capelin ( Aarflot et al. , 2022 ; Langbehn et al. , 2023 ).…”

“…A reason for their low importance in the Barents Sea could be vulnerability to predation by visual fish predators due to their large size. This makes them easy targets for visual predators in shallow shelf waters ( Langbehn et al. , 2023 ) and could explain their tendency to be mesopelagic ( Mauchline, 1995 ).…”