Eddy Correlation Measurements Research Articles

Hydrodynamics, Diagenesis and Hypoxia Variably Drive Benthic Oxygen Flux in a River‐Reservoir System

AbstractBenthic oxygen flux with complex spatiotemporal variations is essential for the global budget of carbon dioxide and the regional security of water quality and ecology, but its dominant driver under different circumstances has yet to be identified. In this study, a parametric scheme of oxygen flux was proposed and validated with aquatic eddy correlation measurements and then coupled with a diagenesis model and a water environment model. The coupled model was applied to a river‐reservoir with significant environmental gradients in hydrodynamics, diagenesis, and hypoxia, which are three factors that competitively drive the variation in benthic oxygen flux. The results indicate that hydrodynamics dominate the flux in the riverine and thalweg areas, diagenesis is the dominant driver of the lacustrine and bank areas, and hypoxia shows dominance only in the hypolimnetic anoxic area. In general, diagenesis is the dominant driver of oxygen flux in river‐reservoirs, followed by hydrodynamics, both of which are more prominent than hypoxia. If the operated reservoir experiences a wet year, the dominance of hydrodynamics tends to increase, while that of diagenesis and hypoxia decreases. The three divers exhibit similar but more stable dominance in riverine systems than in reservoirs, while diagenesis becomes the exclusive driver of oxygen fluxes in lacustrine systems.

Eddy correlation measurements to visualize CO2 and water vapor concentrations and fluxes

Eddy correlation measures gas exchange between canopy and the overlying atmosphere by evaluating the correlation between fluctuations in the gas-of-interest’s mixing ratio and the vertical wind velocity and considered the most accurate approach for measuring gas fluxes, mostly carbon dioxide and water vapor under ideal homogeneous conditions. It has been used in micrometeorology for decades to quantify mass and energy transfer between urban, natural and agricultural ecosystems and the atmosphere. We assessed its application under various—homogeneous and non-homogeneous—conditions. Our study indicates that fluxes of CO2 and H2O correlate well with plants activity only when turbulent conditions are present, in open fields. At such conditions, direct measurements of concentration of those gases are not an accurate indicator for plants activity. On the other hand, in closed systems (e.g. greenhouses)- fluxes as measured by an eddy correlation system can't accurately be related to the state of the vegetation, but the fluctuations in the concentrations of CO2 and H2O directly correlate to the actual plants activity. Adapting conditions in greenhouses to limiting factors as temperature, increases CO2 sequestration by plants, and may increase productivity

Eddy correlation measurements of benthic oxygen fluxes in a stratified and operated reservoir

Benthic oxygen fluxes at the sediment-water interface (SWI) regulate the process and even direction of biochemical interfacial reactions; thus, they are crucial to the water quality. However, many medium- and large-sized reservoirs suffer from thermal stratification and human operations, lead to a significant impact on SWI oxygen exchanges. A 3-month-long noninvasive measurement of SWI oxygen fluxes was conducted in the Daheiting Reservoir using the eddy correlation method. Its result shows that reservoir stratification markedly affects the benthic boundary layer environment factors such as temperature, dissolved oxygen (DO) concentration, and hydrodynamics. In contrast, reservoir operations determine the benthic flow directions and its correlation to velocities in the mixed stage. A further discussion suggests that benthic temperatures mainly participate in the thermal structure rather than directly affecting the benthic fluxes. Conversely, it is DO concentrations coupled with hydrodynamics that control SWI oxygen fluxes. The flux is inhibited by DO shortage in anoxic conditions, but it is mainly controlled by hydrodynamics with redundant DO. Likewise, the hydrodynamics affects the fluxes by changing the diffusive boundary layer thickness, but this effect is also restrained by the DO field. These results indicate that a long-term and in-situ measurement of SWI oxygen fluxes in drinking reservoirs is of considerable significance to forecast water quality disasters, optimize operation plans, and ensure water security.

A Multi-Function Sensor for Eddy Correlation Measurements of Benthic Flux.

Eddy Correlation (EC) is a technique that can be used to measure transport of substances in aquatic ecosystems between bottom sediments and the overlying water (i.e. benthic fluxes). Based on high-speed, simultaneous, and co-located velocity and concentration measurements, EC has been successfully used in a variety of freshwater and marine settings to determine benthic fluxes of dissolved oxygen. Application to a larger range of compounds is limited, however, by the lack of suitable chemical sensors. Here, we describe FACT, a novel, high-speed, multi-function sensor created to expand the range of benthic fluxes that can be measured with EC. An optical fiber spectrofluorometer with a proximally located conductivity cell and thermistor, FACT enables benthic flux measurements of fluorescing compounds, such as fluorescent dissolved organic matter, as well as of heat and salinity which can be used as tracers for submarine groundwater discharge. The high bandwidth and open-beam geometry of the fluorescence sensor are particularly beneficial for EC measurements. FACT was integrated with a velocity sensor into a full EC system capable of simultaneous benthic flux measurements of fluorescing compounds, heat, and salinity. Tested in a laboratory tank, fluxes measured by all three sensors were found to track each other as well as compare favorably with expected values. Furthermore, the ability to measure fluxes of multiple substances both extends the applicability of EC to a wider range of natural sites, and can provide insight into issues of sensing volume and time responses as they affect the application of EC to natural waters.

Energy and water budgets of asphalt concrete pavement under simulated rain events

Urban areas are subject to high human pressure and forthcoming enhanced hydrologic and climatic risks due to both city development and climate change. An asphalt concrete parking lot was instrumented in Nantes, France, to quantify the energy and hydrological responses of the surface to simulated rainfalls. The surface fluxes (precipitation, evaporation, radiation exchanges, sensible heat convection and conduction, runoff) were measured in situ and used to close the water budget with residual closure errors lower than 10%, depending on the surface evaporation retrieval method. The latent heat flux estimated from scintillometry measurements provided a better water budget closure than the direct eddy-correlation measurements. Runoff was the primary component of the water budget and represented around 80% of the total precipitation, compared to 17% for surface evaporation. The scintillometry method provided water evaporation time series at a 1-min time scale during the experiment. These series were used to characterize the rapid changes in the hydrological and energetic budgets of the asphalt surface after a precipitation event. During the drying phase the surface evaporation was significantly active, yielding 80% of the turbulent fluxes with a Bowen ratio of 0.25.

Directional Shear in the Nocturnal Atmospheric Surface Layer

We examine the potential importance of wind-directional shear in the surface layer of the stable nocturnal boundary layer by analyzing two tower datasets with eddy-correlation measurements at multiple levels. Directional shear is a major contributor to the total vector shear for weak winds due primarily to frequent shallow drainage flows at one site and due primarily to non-stationary modes at the second site. For weak winds, the turbulence intensity is more related to the wind-directional shear than to the wind speed or stratification, at least for these two datasets.

A Variational Method for Estimating Surface Turbulent Heat Fluxes with a Consideration of Energy Storage

Abstract Two approaches are proposed to introduce the surface energy storage into the cost function in a variational method for improving the estimates of surface turbulent heat fluxes. In the first approach, each of the energy storage terms is directly calculated based on available observations, and in the second approach, the total energy storage is fitted by the piecewise linear regression function. The heat flux estimates are validated with the eddy correlation (EC) measurements at two carefully selected stations with different land covers and weather conditions in northwestern China and east of the Tibetan Plateau, respectively. In contrast to the variational method without considering the energy storage in the cost function, two new approaches have improved the heat flux estimates, with the first approach being slightly better, especially around midday and/or under strong unstable conditions. It is also reasonable that the calculated/fitted energy storage with the measurements in the previous time period can be transferred for the heat flux estimates in the later time period. Furthermore, the heat flux estimates with both approaches are less sensitive to the errors in the profiles of temperature, humidity, and wind, as well as energy storage, so they may be more reliable.

A Diagnostic Approach towards the Causes of Energy Balance Closure Problem

Hydrometeorological models are often evaluated and optimized on the basis of micrometeorological measurements. However, it has been known for more than three decades that surface measurements of sensible and latent heat energy (LE) are systematically underestimated. We studied this problem using six years of eddy-correlation measurements for four fields (corn, soybean, and prairie) in central Iowa, USA. We recorded major components of the energy equation (i.e. net radiation, sensible heat flux, LE, and soil heat flux, photosynthesis), and indirectly estimated most of the minor components of energy balance (namely storage in the soil, canopy and air). Storage in the canopy was related to leaf area index (LAI) acquired from Moderate Resolution Imaging Spectrometer (MODIS). In this paper, a diagnostic approach is investigated where systematic error is identified first. Three dimensional (3D) plots of the residual of energy equation vs. potential variables indicated the imbalance was largest mainly during the cold non-growing season when the soil was dry. Correlations between energy balance residual (EBR) and energy components showed that soil storage was not precisely estimated. Finally, an a-posteriori analysis (constrained linear multiple regression (CMLR)) was conducted to quantify the contribution of major/minor components of the energy equation towards EBR. The result highlights that the contribution of pertinent components of energy to EBR is mainly controlled by prevailing monthly hydrometeorological conditions; however, precise quantification of causes of imbalance is site-specific. A comparison between the a-posteriori analysis technique and the Bowen-ratio method demonstrates that the Bowen-ratio basically presumes a higher level of underestimation in LE. The results obtained in this study suggest that a-posteriori analysis may offer a superior methodology to correct measured eddy-correlation measurements. Furthermore, the overall trends in the correction of LE measurements suggest that there is a potential for rough monthly corrections of LE, irrespective of the type of crop.

Coastal Zone Surface Stress with Stable Stratification

AbstractSummertime eddy correlation measurements from an offshore tower are analyzed to investigate the dependence of the friction velocity for stable conditions on the mean wind speed V, air–sea difference of virtual potential temperature δθυ, and nonstationary submeso motions. The quantity δθυ sometimes exceeds 3°C, usually because of the advection of warm air from land over cooler water at this site. Thin stable boundary layers result. Unexpectedly, does not depend systematically on the stratification δθυ even for weak winds. For weak winds, increases systematically with increasing submeso variations of the wind. The relationship for a given V is greater in nonstationary conditions. Additionally, this study examines as a function of wind direction. The relationship appears to be affected by swell direction for weak winds and advection from land for short fetches.

Integrating soil moisture retrieved from L-band microwave radiation into an energy balance model to improve evapotranspiration estimation on the irrigated oases of arid regions in northwest China

Irrigated oases are the main water consumers in arid and semi-arid regions. As plant evapotranspiration (ET) in these regions mainly depends on irrigated water, accurate quantification of evapotranspiration (ET) on the irrigated oases is crucial for allocation and management of irrigation water resources. In this study, we integrated the soil moisture retrieved from Polarimetric L-band Multibeam Radiometer (PLMR) into the Surface Energy Balance System (SEBS) model for improving ET estimates under water stress conditions. The study area is the irrigated oasis in the middle reaches of the Heihe River where airborne and satellite-borne remotely sensed data as well as in situ observations are available through the Heihe Watershed Allied Telemetry Experimental Research (HiWATER). The main goal of this experiment is to monitor the energy-water exchange between near-surface atmosphere and land surface, and to assess its influencing factors within the oasis–desert ecosystem. The soil moisture data were retrieved using the L-band Microwave Emission of the Biosphere (L-MEB) model fed with the airborne dual-polarized and multi-angular viewing of PLMR. The comparison of soil moisture retrieval from PLMR data with the soil moisture measured by a wireless sensor network (WSN) showed good consistency, with an absolute mean error (ME) <0.004cm3cm−3 and a root mean square error (RMSE) value <0.05cm3cm−3. Further, the actual daily evapotranspiration was estimated using the soil moisture integrated (SM-integrated) SEBS algorithm fed with the Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) images and soil moisture data retrieved from PLMR data. The sensible heat fluxes and daily evapotranspiration (ETdaily) obtained by the SM-integrated SEBS and the original SEBS were compared with the eddy correlation (EC) measurements collected from HiWATER experiment. The results indicate an obvious improvement when soil moisture information is integrated into the SEBS. This method overcomes the weakness of remote sensing based (RS-based) surface energy balance models of overestimating evapotranspiration particularly in semi-arid and arid regions. It shows a prospect that the combination of optical and microwave remote sensing can further improve the RS-based ET estimation.

An Assessment of the Precision and Confidence of Aquatic Eddy Correlation Measurements

AbstractThe quantification of benthic fluxes with the aquatic eddy correlation (EC) technique is based on simultaneous measurement of the current velocity and a targeted bottom water parameter (e.g., O2, temperature). High-frequency measurements (64 Hz) are performed at a single point above the seafloor using an acoustic Doppler velocimeter (ADV) and a fast-responding sensor. The advantages of aquatic EC technique are that 1) it is noninvasive, 2) it integrates fluxes over a large area, and 3) it accounts for in situ hydrodynamics. The aquatic EC has gained acceptance as a powerful technique; however, an accurate assessment of the errors introduced by the spatial alignment of velocity and water constituent measurements and by their different response times is still needed.Here, this paper discusses uncertainties and biases in the data treatment based on oxygen EC flux measurements in a large-scale flume facility with well-constrained hydrodynamics. These observations are used to review data processing procedures and to recommend improved deployment methods, thus improving the precision, reliability, and confidence of EC measurements. Specifically, this study demonstrates that 1) the alignment of the time series based on maximum cross correlation improved the precision of EC flux estimations; 2) an oxygen sensor with a response time of &lt;0.4 s facilitates accurate EC fluxes estimates in turbulence regimes corresponding to horizontal velocities &lt; 11 cm s−1; and 3) the smallest possible distance (&lt;1 cm) between the oxygen sensor and the ADV’s sampling volume is important for accurate EC flux estimates, especially when the flow direction is perpendicular to the sensor’s orientation.

Deployment of a sequential two-photon laser-induced fluorescence sensor for the detection of gaseous elemental mercury at ambient levels: fast, specific, ultrasensitive detection with parts-per-quadrillion sensitivity

Abstract. The operation of a laser-based sensor for gas-phase elemental mercury, Hg(0), is described. It utilizes sequential two-photon laser excitation with detection of blue-shifted laser-induced fluorescence (LIF) to provide a highly specific detection scheme that precludes detection of anything other than atomic mercury. It has high sensitivity, fast temporal resolution, and can be deployed for in situ measurements in the open atmosphere with essentially no perturbation of the environment. An ambient sample can also be pulled through a fluorescence cell, allowing for standard addition calibrations of the concentration. No type of preconcentration is required and there appears to be no significant interferences from other atmospheric constituents, including gas-phase oxidized mercury species. As a consequence, it is not necessary to remove oxidized mercury, commonly referred to as reactive gaseous mercury (RGM), from the air sample. The instrument has been deployed as part of an instrument intercomparison and compares well with conventional instrumentation that utilizes preconcentration on gold followed by analysis using cold-vapor atomic fluorescence spectroscopy (CVAFS). Currently, the achievable detection sensitivity is ~ 15 pg m−3 (~ 5 × 104 atoms cm−3, ~ 2 ppq) at a sampling rate of 0.1 Hz, i.e., averaging 100 shots with a 10 Hz laser system. Preliminary results are described for a 50 Hz instrument that utilizes a modified excitation sequence and has monitored ambient elemental mercury with an effective sampling rate of 10 Hz. Additional work is required to produce the precision necessary to perform eddy correlation measurements. Addition of a pyrolysis channel should allow for the measurement of total gaseous mercury (TGM) and hence RGM (by difference) with good sensitivity and time resolution.

Exploiting satellite earth observation to quantify current global oceanic DMS flux and its future climate sensitivity

We used coincident Envisat RA2 and AATSR temperature and wind speed data from 2008/2009 to calculate the global net sea-air flux of dimethyl sulfide (DMS), which we estimate to be 19.6 Tg S a−1. Our monthly flux calculations are compared to open ocean eddy correlation measurements of DMS flux from 10 recent cruises, with a root mean square difference of 3.1 μmol m−2 day−1. In a sensitivity analysis, we varied temperature, salinity, surface wind speed, and aqueous DMS concentration, using fixed global changes as well as CMIP5 model output. The range of DMS flux in future climate scenarios is discussed. The CMIP5 model predicts a reduction in surface wind speed and we estimate that this will decrease the global annual sea-air flux of DMS by 22% over 25 years. Concurrent changes in temperature, salinity, and DMS concentration increase the global flux by much smaller amounts. The net effect of all CMIP5 modelled 25 year predictions was a 19% reduction in global DMS flux. 25 year DMS concentration changes had significant regional effects, some positive (Southern Ocean, North Atlantic, Northwest Pacific) and some negative (isolated regions along the Equator and in the Indian Ocean). Using satellite-detected coverage of coccolithophore blooms, our estimate of their contribution to North Atlantic DMS emissions suggests that the coccolithophores contribute only a small percentage of the North Atlantic annual flux estimate, but may be more important in the summertime and in the northeast Atlantic.

Dynamics of the benthic boundary layer and seafloor contributions to oxygen depletion on the Oregon inner shelf

Measurement of in situ O2 consumption and production within permeable sediments, such as those found over the Oregon–Washington inner shelf, has traditionally been done using methods that isolate the sediments from the dynamic influences of currents and wave motions. Modified from atmospheric research, the non-invasive eddy correlation technique can be used to characterize benthic boundary layer dynamics and measure O2 flux across the sediment–water interface without excluding the natural hydrodynamic flow. In 2009, eddy correlation measurements were made in 5 discrete months with varying conditions at a 30m site off Yaquina Head, Newport, OR. The O2 flux was found to be primarily into the bed (−18±3mmolm−2d−1; mean±SE, n=137 15-min bursts) but was sensitive to non-steady state changes in O2 concentrations caused by the differential advection of water masses with variable mean O2 concentrations. Important contributions to O2 eddy fluxes at surface wave frequencies were seen in eddy correlation cospectra and these are interpreted as being indicative of consumption enhanced by advective transport of O2 into the bed. The sediments were deposits of fine sand with permeabilities of 1.3–4.7×10−11m2 and wave-generated ripples. Sediment pigment and organic carbon concentrations were low (chlorophyll-α: 0.02–0.45μgg−1, phaeophytin-α: 0.38–1.38μgg−1 and organic carbon: 0.05–0.39% dry wt in discrete depth intervals from cores collected between March and October), but it was evident that during the summer fresh pigments were trapped in the sand and rapidly mixed over the uppermost 0–13cm. From these results it is inferred that physical forcing associated largely with waves and currents may accentuate the role of sediment-covered inner shelf habitats as a regional O2 sink compared to the middle shelf. In effect, the action of waves and currents in the benthic boundary layer enables aerobic respiration that counterbalances the oxygenation of the water column by primary production and mixing in the surface layer.

Biological‐ and physical‐induced oxygen dynamics in melting sea ice of the Fram Strait

We investigated the production, consumption, and exchange of O2 in melting sea ice to assess the biological‐ and physical‐induced O2 turnover. The underside of the ice was covered with 5–20 cm3 large, buoyant algal aggregates. Their gross primary production amounted to 0.49 mmol C m−2 d−1, which was 4.5 times higher than the primary production of sea ice—encrusted microalgae (0.11 mmol C m−2 d−1). The phototrophic biomass of the aggregates (2.94 mg chlorophyll a m−2) was six times higher than that encountered in the sea ice itself. Taxono‐specific investigations strongly suggest that the aggregates were formed from agglutinated algae released from the melting ice. At the prevailing light conditions, the sea ice—encrusted communities were almost at metabolic balance, while the aggregates were net heterotrophic. Together, the two communities were responsible for an overall O2 consumption of 0.32 mmol m−2 d−1. The sea ice—associated communities thereby represent a southward‐drifting carbon source that is being exhausted by sea ice—affiliated food webs. The sea ice volume decreased rapidly, releasing meltwater at a rate 25 L m−2 d−1, but no surface melt ponds were formed. Aquatic eddy correlation (EC) measurements on the underside of the ice revealed a light‐dependent O2 exchange rate. However, the integrated signal resolved a net O2 uptake of 7.70 mmol m−2 d−1. The net O2 exchange was therefore dominated by the production of O2‐depleted meltwater rather than biological activity. The EC technique represents a new non‐invasive tool for O2 studies in sea ice communities.

Advances in Air–Sea $$\hbox {CO}_2$$ CO 2 Flux Measurement by Eddy Correlation

Eddy-correlation measurements of the oceanic \(\hbox {CO}_2\) flux are useful for the development and validation of air–sea gas exchange models and for analysis of the marine carbon cycle. Results from more than a decade of published work and from two recent field programs illustrate the principal interferences from water vapour and motion, demonstrating experimental approaches for improving measurement precision and accuracy. Water vapour cross-sensitivity is the greatest source of error for \(\hbox {CO}_2\) flux measurements using infrared gas analyzers, often leading to a ten-fold bias in the measured \(\hbox {CO}_2\) flux. Much of this error is not related to optical contamination, as previously supposed. While various correction schemes have been demonstrated, the use of an air dryer and closed-path analyzer is the most effective way to eliminate this interference. This approach also obviates density corrections described by Webb et al. (Q J R Meteorol 106:85–100, 1980). Signal lag and frequency response are a concern with closed-path systems, but periodic gas pulses at the inlet tip provide for precise determination of lag time and frequency attenuation. Flux attenuation corrections are shown to be \(<\)5 % for a cavity ring-down analyzer (CRDS) and dryer with a 60-m inlet line. The estimated flux detection limit for the CRDS analyzer and dryer is a factor of ten better than for IRGAs sampling moist air. While ship-motion interference is apparent with all analyzers tested in this study, decorrelation or regression methods are effective in removing most of this bias from IRGA measurements and may also be applicable to the CRDS.

Autonomous Ocean Turbulence Measurements Using Shear Probes on a Moored Instrument

Abstract An internally recording, autonomous instrument has been tested for measurements of ocean turbulence from a mooring line. Measurements were made at a single level in the water column, but for an extended period of time, at a predetermined duty cycle. The instrument is designed to measure, independently, in two different parts of the turbulence wavenumber spectrum: eddy correlation measurements in the inertial subrange and small-scale shear and temperature gradient measurements in the dissipation subrange using shear probes and fast-response thermistors. For the deployment reported here, the instrument is located in the wave-affected layer, and only the dissipation subrange from the shear probes can be confidently utilized for turbulence measurements. The velocity spectra in the inertial subrange are severely contaminated by platform motion and noise, and the dissipation range of the temperature gradient spectrum is not satisfactorily resolved. The shear spectra are found to be relatively free of contamination in the 1–20-Hz frequency range and are used for dissipation rate calculations. The quality of the measurements is constrained by the angle of attack and the magnitude of mean flow relative to the wave oscillatory velocities. Dissipation rates are consistent with a scaling expected from breaking long waves, when background shear is weak, and are elevated when the gradient Richardson number is small, consistent with additional turbulence production by shear. While limited to a single depth, the instrument makes it possible to collect time series for 3 weeks continuously or for 3 months at a 25% duty cycle.

Eddy‐correlation measurements of benthic fluxes under complex flow conditions: Effects of coordinate transformations and averaging time scales

Eddy‐correlation measurements of sediment oxygen uptake rates in aquatic systems are increasingly used to obtain areal‐averaged fluxes with a high temporal resolution. Here we discuss the effects of coordinate rotation and averaging time scale for Reynolds decomposition on flux estimates. Using 119 hours of continuous eddy‐correlation measurements of sediment oxygen fluxes in an impounded river, we demonstrate that rotation of measured current velocities into streamline coordinates can be a crucial and necessary step in data processing under complex flow conditions in non‐flat environments with complex topography. We found that under these conditions neither time series detrending nor coordinate rotation can remove low‐frequency velocity variations completely. These variations result in spurious flux contributions and in a pronounced dependence of the derived fluxes on averaging time scales. Application of the planar fit transformation was found to provide an alternative means for transforming measured velocities into streamline coordinates for longer‐term observations. The observed sensitivity of estimated fluxes to coordinate transformation and selection of averaging time scale is discussed in the context of the theoretical concepts underlying eddy‐correlation measurements and a set of recommendations for planning and analyses of flux measurements are derived.

The effects of spatial and temporal variability at the sediment surface on aquatic eddy correlation flux measurements

The eddy correlation technique has become a widely used approach to measure ecosystem scale fluxes in aquatic benthic environments; however, there are few theoretical studies designed to guide deployment planning, the characterization of flux variability, or how to correctly interpret measured fluxes. Here, a three‐dimensional numerical model for the turbulent transport and mixing in the near‐bottom water was used to examine how well variations in vertical flux due to heterogeneities in the benthic community are integrated in eddy correlation measurements. The results showed that an appropriate choice of measuring height above the sediment surface is crucial to obtain an accurate average of the vertical flux from heterogeneous benthic communities. Through regression analysis of modeling results, a set of simple analytical expressions were derived for this threshold measuring height as a function of two parameters: heterogeneity patch size and sediment surface roughness. The same model was also used to examine the time lag between when a change in vertical flux occurs at the sediment surface and when it reaches the measuring point. The results showed that this response time varies by several orders of magnitude depending on flow velocity, sediment surface roughness, and measuring height. Simple analytical expressions were also derived for the response time. Overall, the findings indicate that both the spatial variability in benthic communities and the response to temporal changes are important factors that should be considered in planning eddy correlation measurements and interpreting eddy correlation fluxes.

Eddy correlation measurements of oxygen fluxes in permeable sediments exposed to varying current flow and light

Based on noninvasive eddy correlation measurements at a marine and a freshwater site, this study documents the control that current flow and light have on sediment‐water oxygen fluxes in permeable sediments. The marine sediment was exposed to tidal‐driven current and light, and the oxygen flux varied from night to day between −29 and 78 mmol m−2 d−1. A fitting model, assuming a linear increase in oxygen respiration with current flow, and a photosynthesis‐irradiance curve for light‐controlled production reproduced measured fluxes well (R2 = 0.992) and revealed a 4‐fold increase in oxygen uptake when current velocity increased from ∼ 0 to 20 cm s−1. Application of the model to a week‐long measured record of current velocity and light showed that net ecosystem metabolism varied substantially among days, between −27 and 31 mmol m−2 d−1, due to variations in light and current flow. This variation is likely typical of many shallow‐water systems and highlights the need for long‐term flux integrations to determine system metabolism accurately. At the freshwater river site, the sediment‐water oxygen flux ranged from −360 to 137 mmol m−2 d−1. A direct comparison during nighttime with concurrent benthic chamber incubations revealed a 4.1 times larger eddy flux than that obtained with chambers. The current velocity during this comparison was 31 cm s−1, and the large discrepancy was likely caused by poor imitation by the chambers of the natural pore‐water flushing at this high current velocity. These results emphasize the need for more noninvasive oxygen flux measurements in permeable sediments to accurately assess their role in local and global carbon budgets.