RELATIONSHIPS BETWEEN STREAM DISCHARGE AND SHALLOW TURBULENCE IN AN ICE-COVERED POLAR LAKE USING CTD AND ADCP OBSERVATIONS, LAKE HOARE, McMurdo DRY VALLEYS, ANTARCTICA

Lakes in the McMurdo Dry Valleys, Antarctica, host some of the lowest-temperature aquatic ecosystems on Earth. This study explores the relationship between stream input and shallow turbulance within Lake Hoare, a 4.2-km-long, ice-covered, proglacial lake. The lake receives input at its east end from Andersen Creek, an ephemeral glacial stream, and at its west end from overflow from an adjacent pond. Lake Hoare only loses water by ice ablation and moat evaporation. On 2 Jan. 2010, the discharge volume and water density of Andersen Creek showed diurnal fluctuations with a minimum ρ = 999.84 kg/m³ measured at 2:00 hrs and a maximum ρ = 999.95 kg/m³ measured at 13:00 hrs. This discharge should form an interflow layer between the lake ice (ρ ~ 916.8 kg/m³) and denser water below. Conductivity-Temperature-Depth (CTD) data measured on 3 Jan. 2010 in an ice bore hole, 900 m south-southwest of the mouth of Andersen Creek, showed that ρ increased linearly from 999.84 kg/m³ at the piezometric water surface to 999.86 kg/m³ at the ice-water interface at 2.25 m. Below this depth, ρ gradually increased to 999.91 kg/m³ at 2.8 m and then sharply increased to 999.99 kg/m³ at 3.1 m. These data identify a pycnocline at 2.95 m that separates sub-ice water from deeper water. On 7 Jan. 2010, we placed an Acoustic Doppler Current Profiler (ADCP) at the ice-water interface within the bore hole. At 10-second intervals for 32.5 hours, the ADCP measured horizontal speed, vertical velocity, and current direction in six 50-cm-thick cells between 2.75 and 5.75 m. The average horizontal speed at 3.0 m was 23.8 cm/sec. This agreed with speeds of 20.1 cm/sec measured at 1.8 m on 26 Dec. 2009 using a handheld Acoustic Doppler Velocity meter. The average horizontal speed decreased to 15.9 cm/sec at 3.5 m and remained at this speed to 5.5 m. Each cell had a downward vertical velocity that decreased with depth from 0.71 cm/sec at 3.0 m to 0.63 cm/sec at 5.5 m. The average current direction was 180° East of North in each cell; away from the mouth of Andersen Creek. The Richardson number across the pycnocline was 0.035 indicating turbulent conditions. We hypothesize that Andersen Creek generates interflow between the ice-water interface and the pycnocline, and that the movement of water along this boundary generates shallow turbulence.