ИСТИНА

Young Sound is located in Northeast Greenland at ~74°N and represents a relatively deep (340 m) and long (~90 km) high latitude fjord with a 40 m deep sill at the mouth. Young Sound is covered by land-fast ice from late October to the beginning of July. During winter, the land-fast ice extending off the fjord mouth by 10-30 km completely eliminates wind stress on the water column. Further offshore in the transition zone between the land-fast ice and pack ice, one of the most prominent polynyas in northeast Greenland is maintained during winter by sustainable northerly winds. During winter 2013-14, 4 land-fast ice tethered oceanographic moorings equipped with 300 KHz ADCP were deployed in Yong Sound. The backscatter intensity time series from one of ADCP mooring deployed in the fjord interior shows signature of the zooplankton diel vertical migration during polar night below the land-fast ice 20-110 cm thick with snow cover up to 50 cm. Furthermore, there was observed interaction of vertical migration with water dynamics. During the period of polynya induced estuarine-like circulation, the vertical migration signal is substantially disrupted. During mooring deployment and recovery, we observed a subsurface warm layer centered at 13/28 m which could be the target of the zooplankton as a potential source of phytoplankton. Normally diel (diurnal) vertical migration of zooplankton occurs in 24 hour period when the organisms move to illuminated zone (epipelagic or daylight) at night and return to twilight (mesopelagic) zone during the day following the daylight cycle. In the case of Young Sound mooring, such migration was observed during the polar night below the land-fast ice covered with thick snow cover. Different explanations for the mechanism of this migration during the polar night period were proposed in literature . One of explanations given in recent papers was that even during the polar night, Diel vertical migration is regulated by diel variations in solar and lunar illumination, which are at intensities far below the threshold of human perception. However, for Young Sound this mechanism is very unlikely taking into account snow thickness and ice depth. So we are looking into better explanation of diel vertical migration during the polar night season in Young Sound case.