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

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.

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

This study uses the eddy correlation technique to examine fluxes across the ice-water interface. 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 as driven by biological and physical processes. The research was conducted below 0.7 m thick sea ice in mid March 2010 in a southwest Greenland fjord and revealed low average rates of ice melt amounting to a maximum of 0.80 ± 0.09 mm d−1 (SE, n=31). The corresponding calculated 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 during certain time periods, amounting to 66 % of total deployment time. The identification of these conditions was evaluated by examining the velocity and the linearity and stability of the cumulative flux. The examination of unstratified conditions through velocity and O2 spectra and their cospectra revealed characteristic fingerprints of well-developed turbulence. From the observed 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 the 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 ± 0.02 mmol O2 m−2 d−1 (SE, n=4) and a respiration rate of −2.13 mmol O2 m−2 d−1 (no SE, see text for details) leading to a net primary production of −1.45 ± 0.02 mmol O2 m−2 d−1 (SE, n=4). Modeling the observed fluxes allowed for the calculation of fluxes during time periods when no O2 fluxes were extracted. This application of the eddy correlation technique produced high temporal resolution O2 fluxes and ice melt rates that were measured without disturbing the environmental conditions while integrating over a large area of approximately 50 m2 which encompassed the highly variable activity and spatial distributions of sea ice algal communities.

Processing Turbulence Data Collected on board the Helicopter Observation Platform (HOP) with the Empirical Mode Decomposition (EMD) Method

The Duke University Helicopter Observation Platform (HOP) has previously been shown to be a useful instrument for the measurement of turbulent atmospheric fluxes. As with all such measurements, especially those made from moving platforms, spurious signals, such as instrument noise and mesoscale atmospheric motions, are superposed on the desired signal. Empirical mode decomposition (EMD) is applied in a novel way to identify and separate out different signals represented by intrinsic mode functions (IMFs) in the HOP data and is shown to be an effective tool for the task. The method produces a basis that is adaptive, unique, and orthogonal, all of which are required for this type of data processing, and none of which are present in more traditional techniques. The results of applying EMD are shown to be nonlinear, and occasionally the removal of the correct number of IMFs increases the observed value of energy and fluxes calculated with the eddy correlation technique.

Application of Oxygen Eddy Correlation in Aquatic Systems

The eddy correlation technique is rapidly becoming an established method for resolving dissolved oxygen fluxes in natural aquatic systems. This direct and noninvasive determination of oxygen fluxes close to the sediment by simultaneously measuring the velocity and the dissolved oxygen fluctuations has considerable advantages compared to traditional methods. This paper describes the measurement principle and analyzes the spatial and temporal scales of those fluctuations as a function of turbulence levels. The magnitudes and spectral structure of the expected fluctuations provide the required sensor specifications and define practical boundary conditions for the eddy correlation instrumentation and its deployment. In addition, data analysis and spectral corrections are proposed for the usual nonideal conditions, such as the time shift between the sensor pair and the limited frequency response of the oxygen sensor. The consistency of the eddy correlation measurements in a riverine reservoir has been confirmed—observing a night–day transition from oxygen respiration to net oxygen production, ranging from −20 to +5 mmol m−2 day−1—by comparing two physically independent, eddy correlation instruments deployed side by side. The natural variability of the fluctuations calls for at least ∼1 h of flux data record to achieve a relative accuracy of better than ∼20%. Although various aspects still need improvement, eddy correlation is seen as a promising and soon-to-be widely applied method in natural waters.