Environmental controls on macrozooplankton and fish distributions over diurnal to seasonal time scales in the northern Barents Sea

We present acoustic observations obtained by bottom mounted echosounders at two locations in the northern Barents Sea and a third on the northern continental slope of the Barents Sea. Data collected over a period of approximately two years reveal significant variability in the density and depth distribution of pelagic macrozooplankton and fish. Variability over diurnal to seasonal timescales is related to light conditions, prevailing currents, water column characteristics (temperature, stability, and pycnocline depth) and to sea ice conditions. Of particular importance is the relative volume of Polar Water and Atlantic influenced water present at the mooring sites. On the northern continental shelf, above average concentrations of macrozooplankton and fish were observed during late summer and early autumn following ice melt, and during winter. Minimum densities of macrozooplankton and fish were observed during spring and early summer. These data demonstrate the persistent presence of a macrozooplankton community throughout the winter months in the northern Barents Sea. On the Great Bank full depth diel vertical migrations were observed throughout the Polar Night and under sea ice. At the two more northerly stations the density of the sound scattering layers performing diel vertical migrations and the vertical range of the migrations was greatest during the transition period between the polar night and mid-night sun periods. Superimposed on the seasonal variability in density distribution was significant shorter-term variability driven by hydrographic processes occurring over synoptic time scales. Significant mixing events, or changes in the prevailing current direction that lead to a change in water mass characteristics, forced changes in the concentration and depth distribution of macrozooplankton over periods of hours that were on occasion of similar magnitude to the seasonal variability. On the northern continental slope, a mesopelagic sound scattering layer more than 300 m thick at its seasonal maximum, was located for most of the year within the warm core of the Atlantic Water Boundary Current, ascending into the epipelagic zone for 5–6 weeks during summer. The density and thickness of this sound scattering layer were greatest during the late summer/early autumn and lowest during early spring. Fish-like scatterers were present, concentrated within the deeper regions of the sound scattering layer, during the winter months and during July.