Evolution of dissolved organic matter under lake ice in Lake Hoare, Antarctica

DOM is present in all aquatic ecosystems on Earth and plays an important role in the global carbon cycle. DOM is comprised of a complex, heterogeneous mixture of organic compounds derived from both microbial and terrestrial sources and is analyzed using fluorescence spectroscopy. Lake Hoare, located in the McMurdo Dry Valleys, Antarctica is a perennially ice covered lake that occupies a 4-km-long, closed-basin fed by a diurnal meltwater stream called Andersen Creek, runoff from the Canada Glacier, and overflow from an adjacent pond. Previous studies in Antarctica suggest that DOM evolves under lake ice as a function of solar radiation and biological processes. This study explores the evolution of DOM in Andersen Creek and Lake Hoare water measured over a 4-hour time period in mid-summer 2013 in response to changes in stream discharge, solar radiation, water temperature, and biological activity. Water samples were collected from 15 ice boreholes and 5 ice-free stream locations between 19:30 and 18:30 hours on 17 December, and again between 23:30 hours and 00:30 hours. Four boreholes in the perennial lake ice were sampled at two depths: (1) immediately below lake ice and (2) one meter below lake ice. All samples were analyzed using a Horiba-Jorbin Yvon Fluoromax 4 fluorometer and were blank, Raman, and inner-filter corrected. Indices were calculated from the resulting excitation emission matrices (EEMs). Samples were then analyzed using a biogeochemically diverse model, which gives information regarding the properties of DOM. Preliminary fluorescence index (FI) values show spatial and temporal variability, with higher FI values shown in stream samples than in lake samples. This could indicate higher microbial activity in stream water than below perennial lake ice. For shore ice boreholes near the stream inlet, the humification index (HIX) decreased with time suggesting that incoming stream water is less humified than lake water. In-lake processes are more likely to be a function of DOM evolution by solar radiation and/or water temperature, rather than stream water dilution. The redox index (RI) remained relatively constant over time in all samples, indicating an oxidizing environment below lake ice.