Oxygen exchange and ice melt measured at the ice-water interface by eddy correlation

Abstract

This study examined fluxes across the ice-water interface utilizing the eddy correlation technique. Temperature eddy correlation systems were used to determine rates of ice melting and freezing, and O₂ eddy correlation systems were used to examine O₂ exchange rates driven by biological and physical processes. The study was conducted below 0.7 m thick sea-ice in mid-March 2010 in a southwest Greenland fjord and revealed low rates of ice melt at a maximum of 0.80 mm d⁻¹. The O₂ flux associated with release of O₂ depleted melt water was less than 13 % of the average daily O₂ respiration rate. Ice melt and insufficient vertical turbulent mixing due to low current velocities caused periodic stratification immediately below the ice. This prevented the determination of fluxes 61 % of the deployment time. These time intervals were identified by examining the velocity and the linearity and stability of the cumulative flux. The examination of unstratified conditions through vertical velocity and O₂ spectra and their cospectra revealed characteristic fingerprints of well-developed turbulence. From the measured O₂ fluxes a photosynthesis/irradiance curve was established by least-squares fitting. This relation showed that light limitation of net photosynthesis began at 4.2 μmol photons m⁻² s⁻¹, and that algal communities were well-adapted to low-light conditions as they were light saturated for 75 % of the day during this early spring period. However, the sea-ice associated microbial and algal community was net heterotrophic with a daily gross primary production of 0.69 mmol O₂ m⁻² d⁻¹ and a respiration rate of −2.13 mmol O₂ m⁻² d⁻¹ leading to a net ecosystem metabolism of −1.45 mmol O₂ m⁻² d⁻¹. This application of the eddy correlation technique produced high temporal resolution O₂ fluxes and ice melt rates that were measured without disturbing the in situ environmental conditions while integrating over an area of approximately 50 m² which incorporated the highly variable activity and spatial distributions of sea-ice communities.