The fragility of thin Clark‐type glass microelectrodes used in aquatic eddy covariance measurements of benthic oxygen fluxes is a challenge when using this powerful technique. This study presents a new fast‐responding dual oxygen‐temperature sensor for eddy covariance measurements that is far more robust. Response time tests in the lab, where the sensor was inserted from air into water, revealed 90% response times of 0.51 s and 0.34 s for oxygen and temperature measurements, respectively. In wave tank tests, the new sensor showed no stirring sensitivity in contrast to Clark‐type microelectrodes. Other tests in a flume and in a particle‐free water tank revealed how close the sensor can be positioned to the measuring volume of the Acoustic Doppler Velocimeter without disturbing velocity recordings. In field tests at river sites, all > 24 h, the new sensor recorded high‐quality eddy covariance data for the entire deployment. Similar positive results were obtained in deployments at a marine site with unidirectional current flow overlaid with minor wave action. Concurrently deployed eddy covariance systems based on the new sensor and a traditional Clark‐type microelectrode revealed that they recorded statistically equivalent fluxes and similar velocity‐oxygen cospectra until the microelectrode broke after 2 h. The significant increase in robustness of the new sensor was achieved by relying on a larger sensor tip. This put some constrains on how the sensor should be deployed and fluxes extracted, but given the substantial gain in performance, it is a viable alternative for eddy covariance measurements in many aquatic environments.

Recent studies have shown that in a number of humic-rich surface waters in North America, NH 4 + is released when dissolved organic matter (DOM) is exposed to sunlight. However, photochemical NH 4 + production has not been observed in all surface waters, and factors that contribute to it are not well understood. We hypothesized that the presence or absence of NH 4 + photoproduction may be affected by the light exposure history of DOM. The present study was undertaken to determine whether DOM from surficial groundwaters, with minimal light exposure history, would produce labile nitrogen (N) photoproducts more consistently. In this study, estuarine surface waters and surficial groundwaters, collected just adjacent to estuaries in Georgia and South Carolina, USA, were exposed to sunlight to quantify the photochemical production of NH 4 + , dissolved primary amines (DPA), and NO 2 -. The photoproduction of NH 4 + was observed in 4 of 5 irradiated estuarine surface water samples but in only 2 of 13 groundwater samples. In contrast, NH 4 + concentrations decreased in 5 of 13 groundwater samples when exposed to sunlight. The results indicate that a small amount of NH 4 + may be lost from waters in which groundwater-derived DOM is first exposed to sunlight. No consistent trends were observed in the photoproduction or loss of DPA and NO 2 -.

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 O 2 eddy correlation systems were used to examine O 2 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 O 2 flux associated with release of O 2 depleted melt water was less than 13 % of the average daily O 2 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 O 2 spectra and their cospectra revealed characteristic fingerprints of well-developed turbulence. From the measured O 2 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 O 2 m −2 d −1 and a respiration rate of −2.13 mmol O 2 m −2 d −1 leading to a net ecosystem metabolism of −1.45 mmol O 2 m −2 d −1 . This application of the eddy correlation technique produced high temporal resolution O 2 fluxes and ice melt rates that were measured without disturbing the in situ environmental conditions while integrating over an area of approximately 50 m 2 which incorporated the highly variable activity and spatial distributions of sea-ice communities.

We report primary production and respiration of Posidonia oceanica meadows determined with the non-invasive aquatic eddy covariance technique. Oxygen fluxes were measured in late spring at an open-water meadow (300 m from shore), at a nearshore meadow (60 m from shore), and at an adjacent sand bed. Despite the oligotrophic environment, the meadows were highly productive and highly autotrophic. Net ecosystem production (54 to 119 mmol m −2 d −1) was about one-half of gross primary production. In adjacent sands, net primary production was a tenth-to a twentieth smaller (4.6 mmol m −2 d −1). Thus, P. oceanica meadows are an oasis of productivity in unproductive surroundings. During the night, dissolved oxygen was depleted in the open-water meadow. This caused a hysteresis where oxygen production in the late afternoon was greater than in the morning at the same irradiance. Therefore, for accurate measurements of diel primary production and respiration in this system, oxygen must be measured within the canopy. Generally, these measurements demonstrate that P. oceanica meadows fix substantially more organic carbon than they respire. This supports the high rate of organic carbon accumulation and export for which the ecosystem is known.

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We quantified oxygen flux in a coastal stream in Virginia using a novel combination of the conventional open water technique and the aquatic eddy covariance technique. The latter has a smaller footprint (sediment surface area that contributes to the flux; 10 m 2 ), allowing measurements to be made at multiple sites within the footprint of the open water technique ( 1000 m 2 ). Sites included an unvegetated stream pool with cohesive sediment, a macrophyte bed with sandy sediment, and an unvegetated sand bed with rippled bedforms. Nighttime eddy covariance oxygen uptake was always smaller than uptake produced by the open water technique. At the pool and unvegetated sand bed sites, nighttime eddy covariance uptake was 20-fold smaller than open water uptake. At the macrophyte bed site, gross primary production quantified with the two techniques was similar but eddy covariance uptake was 2.4-fold smaller. The difference in oxygen uptake between eddy covariance and open water techniques could not be accounted for by uncertainties in the gas transfer velocity but could be accounted for by anoxic groundwater inflow through stream banks outside of the eddy covariance footprint. Nighttime oxygen uptake was also measured with eddy covariance in a tidal freshwater part of the stream, where pore space in the sandy sediment near the sediment-water interface was flushed with stream water at peak water velocities. As a result of this advective hyporheic exchange, nighttime oxygen flux increased fourfold with a doubling of water velocity.

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, <i>n</i>=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, <i>n</i>=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, <i>n</i>=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