Comparison of zooplankton vertical migration in an ice-free and a seasonally ice-covered Arctic fjord: An insight into the influence of sea ice cover on zooplankton behavior

We present observations of zooplankton diel vertical migration (DVM) over a period of 2 yr in an ice-free and a seasonally ice-covered Arctic fjord. The contrasting environments permitted assessment of the influences of physical and biological factors on temporal variability in DVM patterns and a test of the hypothesis that a reduction in summer sea ice extent and thickness following climatic warming will lead to changes in DVM via the loss of a shading effect on the pelagic marine environment. Acoustic backscatter and vertical velocity data from moored 300-kHz acoustic Doppler current profilers were used to derive DVM patterns, while measurements of sea ice and snow thickness enabled assessment of the underwater light environment and consideration of its potential to influence DVM. Pronounced differences in DVM between the sites emerged during summertime when intermittent synchronized DVM was observed at the ice-free site, whereas the ice-covered site was characterized by asynchronous behavior, thereby highlighting the influence of sea ice. Notably, the 0.8-m-thick sea ice cover was deemed important for its influence on water column properties such as food, turbidity, and zooplankton species composition, as indicated by biological samples from moored sediment traps, rather than a simple shading of the ocean and direct modification of zooplankton DVM. Loss of Arctic sea ice is likely to have an indirect but substantial effect on zooplankton DVM via modification of hydrography and composition of the pelagic community.