Abstract. The aquatic eddy covariance technique stands out as a powerful method for benthic O2 flux measurements in shelf environments because itintegrates effects of naturally varying drivers of the flux such as current flow and light. In conventional eddy covariance instruments, the timeshift caused by spatial separation of the measuring locations of flow and O2 concentration can produce substantial flux errors that aredifficult to correct. We here introduce a triple O2 sensor eddy covariance instrument (3OEC) that by instrument design eliminates theseerrors. This is achieved by positioning three O2 sensors around the flow measuring volume, which allows the O2concentration to be calculated at the point of the current flow measurements. The new instrument was tested in an energetic coastal environment with highly permeablecoral reef sands colonised by microphytobenthos. Parallel deployments of the 3OEC and a conventional eddy covariance system (2OEC) demonstrate thatthe new instrument produces more consistent fluxes with lower error margin. 3OEC fluxes in general were lower than 2OEC fluxes, and the nighttimefluxes recorded by the two instruments were statistically different. We attribute this to the elimination of uncertainties associated with the timeshift correction. The deployments at ∼ 10 m water depth revealed high day- and nighttime O2 fluxes despite the relatively loworganic content of the coarse sediment and overlying water. High light utilisation efficiency of the microphytobenthos and bottom currents increasingpore water exchange facilitated the high benthic production and coupled respiration. 3OEC measurements after sunset documented a gradual transfer ofnegative flux signals from the small turbulence generated at the sediment–water interface to the larger wave-dominated eddies of the overlying watercolumn that still carried a positive flux signal, suggesting concurrent fluxes in opposite directions depending on eddy size and a memory effect oflarge eddies. The results demonstrate that the 3OEC can improve the precision of benthic flux measurements, including measurements in environmentsconsidered challenging for the eddy covariance technique, and thereby produce novel insights into the mechanisms that control flux. We consider thefluxes produced by this instrument for the permeable reef sands the most realistic achievable with present-day technology.
more »
« less
A high‐resolution submersible oxygen optode system for aquatic eddy covariance
The aquatic eddy covariance technique is increasingly used to determine oxygen (O2) fluxes over benthic ecosystems. The technique uses O2measuring systems that have a high temporal and numerical resolution. In this study, we performed a series of lab and field tests to assess a new optical submersible O2meter designed for aquatic eddy covariance measurements and equipped with an existing ultra‐high speed optical fiber sensor. The meter has a 16‐bit digital‐to‐analog‐signal conversion that produces a 0–5 V output at a rate up to 40 Hz. The device was paired with an acoustic Doppler velocimeter. The combined meter and fiber‐optic O2sensor's response time was significantly faster in O2‐undersaturated water compared to in O2‐supersaturated water (0.087 vs. 0.12 s), but still sufficiently fast for aquatic eddy covariance measurements. The O2optode signal was not sensitive to variations in water flow or light exposure. However, the response time was affected by the direction of the flow. When the sensor tip was exposed to a flow from the back rather than the front, the response time increased by 37%. The meter's internal signal processing time was determined to be ~ 0.05 s, a delay that can be corrected for during postprocessing. In order for the built‐in temperature correction to be accurate, the meter should always be submerged with the fiber‐optic sensor. In multiple 21–47 h field tests, the system recorded consistently high‐quality, low‐noise O2flux data. Overall, the new meter is a powerful option for collecting robust aquatic eddy covariance data.
more » « less- NSF-PAR ID:
- 10444100
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Limnology and Oceanography: Methods
- Volume:
- 21
- Issue:
- 3
- ISSN:
- 1541-5856
- Page Range / eLocation ID:
- p. 152-163
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
more » « less
Abstract Seagrass meadows play an important role in “blue carbon” sequestration and storage, but their dynamic metabolism is not fully understood. In a dense
Zostera marina meadow, we measured benthic O2fluxes by aquatic eddy covariance, water column concentrations of O2, and partial pressures of CO2(pCO2) over 21 full days during peak growing season in April and June. Seagrass metabolism, derived from the O2flux, varied markedly between the 2 months as biomass accumulated and water temperature increased from 16°C to 28°C, triggering a twofold increase in respiration and a trophic shift of the seagrass meadow from being a carbon sink to a carbon source. Seagrass metabolism was the major driver of diurnal fluctuations in water column O2concentration and pCO2, ranging from 173 to 377μ mol L−1and 193 to 859 ppmv, respectively. This 4.5‐fold variation in pCO2was observed despite buffering by the carbonate system. Hysteresis in diurnal water column pCO2vs. O2concentration was attributed to storage of O2and CO2in seagrass tissue, air–water exchange of O2and CO2, and CO2storage in surface sediment. There was a ~ 1:1 mol‐to‐mol stoichiometric relationship between diurnal fluctuations in concentrations of O2and dissolved inorganic carbon. Our measurements showed no stimulation of photosynthesis at high CO2and low O2concentrations, even though CO2reached levels used in IPCC ocean acidification scenarios. This field study does not support the notion that seagrass meadows may be “winners” in future oceans with elevated CO2concentrations and more frequent temperature extremes. -
more » « less
Abstract Low‐power, open‐path gas sensors enable eddy covariance (EC) flux measurements in remote areas without line power. However, open‐path flux measurements are sensitive to fluctuations in air temperature, pressure, and humidity. Laser‐based, open‐path sensors with the needed sensitivity for trace gases like methane (CH4) and nitrous oxide (N2O) are impacted by additional spectroscopic effects. Corrections for these effects, especially those related to temperature fluctuations, often exceed the flux of gases, leading to large uncertainties in the associated fluxes. For example, the density and spectroscopic corrections arising from temperature fluctuations can be one or two orders of magnitude greater than background N2O fluxes. Consequently, measuring background fluxes with laser‐based, open‐path sensors is extremely challenging, particularly for N2O and gases with similar high‐precision requirements. We demonstrate a new laser‐based, open‐path N2O sensor and a general approach applicable to other gases that minimizes temperature‐related corrections for EC flux measurements. The method identifies absorption lines with spectroscopic effects in the opposite direction of density effects from temperature and, thus, density and spectroscopic effects nearly cancel one another. The new open‐path N2O sensor was tested at a corn (
Zea mays L.) field in Southwestern Michigan, United States. The sensor had an optimal precision of 0.1 ppbv at 10 Hz and power consumption of 50 W. Field trials showed that temperature‐related corrections were 6% of density corrections, reducing EC random errors by 20‐fold compared to previously examined lines. Measured open‐path N2O EC fluxes showed excellent agreement with those made with static chambers (m = 1.0 ± 0.3;r 2 = .96). More generally, we identified absorption lines for CO2and CH4 flux measurements that can reduce the temperature‐related corrections by 10–100 times compared to existing open‐path sensors. The proposed method provides a new direction for future open‐path sensors, facilitating the expansion of accurate EC flux measurements. -
The weak-wind boundary layer is characterized by turbulent and submeso-scale motions that break the assumptions necessary for using traditional eddy covariance observations such as horizontal homogeneity and stationarity, motivating the need for an observational system that allows spatially resolving measurements of atmospheric flows near the surface. Fiber-Optic Distributed Sensing (FODS) potentially opens the door to observing a wide-range of atmospheric processes on a spatially distributed basis and to date has been used to resolve the turbulent fields of air temperature and wind speed on scales of second and decimeters. Here we report on progress developing a FODS technique for observing spatially distributed wind direction. We affixed microstructures shaped as cones to actively-heated fiber-optic cables with opposing orientations to impose directionally-sensitive convective heat fluxes from the fiber-optic cable to the air, leading to a difference in sensed temperature that depends on the wind direction. We demonstrate the behavior of a range of microstructure parameters including aspect ratio, spacing, and size and develop a simple deterministic model to explain the temperature differences as a function of wind speed. The mechanism behind the directionally-sensitive heat loss is explored using Computational Fluid Dynamics simulations and infrared images of the cone-fiber system. While the results presented here are only relevant for observing wind direction along one dimension it is an important step towards the ultimate goal of a full three-dimensional, distributed flow sensor.more » « less
-
The weak-wind boundary layer is characterized by turbulent and submeso-scale motions that break the assumptions necessary for using traditional eddy covariance observations such as horizontal homogeneity and stationarity, motivating the need for an observational system that allows spatially resolving measurements of atmospheric flows near the surface. Fiber Optic Distributed Sensing (FODS) potentially opens the door to observing a wide-range of atmospheric processes on a spatially 5 distributed basis and to date has been used to resolve the turbulent fields of air temperature and wind speed on scales of second and decimeters. Here we report on progress developing a FODS technique for observing spatially distributed wind direction. We affixed microstructures shaped as cones to actively-heated fiber-optic cables with opposing orientations to impose directionally-sensitive convective heat fluxes from the fiber-optic cable to the air, leading to a difference in sensed temperature that depends on the wind direction. We demonstrate the behavior of arange of microstructure parameters including aspect ratio, 10 spacing, and size and develop a simple deterministic model to explain the temperature differences as a function of wind speed. The mechanism behind the directionally-sensitive heat loss is explored using Computational Fluid Dynamics simulations and infrared images of the cone-fiber system. While the results presented here are only relevant for observing wind direction along one dimension it is an important step towards the ultimate goal of a full three-dimensional, distributed flow sensor.more » « less
