Eddy Correlation Technique Research Articles

Salt marsh-atmosphere CO2 exchanges in Patos Lagoon Estuary, Southern Brazil

Blue carbon ecosystems are recognized as carbon sinks and therefore for their potential for climate mitigation. While carbon stocks and burial rates have been quantified and estimated regionally and globally, there are still many knowledge gaps on carbon fluxes exchanged particularly at the interface vegetation-atmosphere. In this study we measured the atmospheric CO2 concentrations in a salt marsh located in the Patos Lagoon Estuary, southern Brazil. Eddy correlation techniques were applied to account for the CO2 exchange fluxes between the vegetation and the atmosphere. Our dataset refers to two sampling periods spanning from July up to November 2016 and from January to April 2017. By using time series analysis techniques including wavelet and cross-wavelet analysis, our results show the natural cycles of the CO2 exchanges variability and the relationship of these cycles with other environmental variables. We also present the amplitudes of the salt marsh-atmosphere CO2 fluxes’ diurnal cycle for both study periods and demonstrate that the CO2 fluxes are modulated by the passage of transient atmospheric systems and by the level variation of surrounding waters. During daytime, our site was as a CO2 sink. Fluxes were measured as -6.71 ± 5.55 μmol m-2 s-1 and -7.95 ± 6.44 μmol m-2 s-1 for the winter-spring and summer-fall periods, respectively. During nighttime, the CO2 fluxes were reversed and our site behaved as a CO2 source. Beside the seasonal changes in sunlight and air temperature, differences between the two periods were marked by the level of marsh inundation, winds and plant biomass (higher in summer). The net CO2 balance showed the predominance of the photosynthetic activity over community respiration, indicating the role of the salt marsh as a CO2 sink. When considering the yearly-averaged net fluxes integrated to the whole area of the Patos Lagoon Estuary marshes, the total CO2 sink was estimated as -87.6 Mg C yr-1. This paper is the first to measure and study the vegetation-atmosphere CO2 fluxes of a salt marsh environment of Brazil. The results will contribute to the knowledge on the global carbon budget and for marsh conservation and management plans, including climate change policies.

Monitoring and Simulation of CO2 Exchange of Coastal Wetland Ecosystem in the Liaohe Delta Wetland, China

Eddy correlation technique was used to obtain the carbon fluxes (2012 and 2013) in the coastal wetlands of the Liaohe Delta. The carbon flux daily data were used to validate the biogeochemical models that are parameterized by data such as meteorology and water level (Wetland-DNDC). The observation results showed that the trend of carbon flux in the Liaohe Delta is absorption in growing season and weak discharge in non-growing season. The model results showed significant linear correlation with the observation, the simulation result was relatively correct. The results showed that the trend of Reed output is consistent with that of simulation. The annual carbon sequestration capacity was estimated to be 1502.24g-CO2-m−2 on an average in the recent 30 years, with a fluctuating increase trend. Further analysis explained the correlation between environmental factors such as temperature, precipitation and CO2 content and the annual carbon sequestration amount. Correlation between CO2 content and annual absorption was followed by temperature, with no correlation with precipitation. The carbon sequestration capacity of the Liaohe Delta increases with change of climate.

Development of observation-based parameterizations of standard deviations of wind velocity fluctuations over an Indian region

The present study utilizes the turbulence observations over an Indian region, Ranchi, to study the diurnal and seasonal characteristics of mean and turbulent parameters in the atmospheric surface layer under different wind speed and stability regimes. Data for the year 2009 are chosen to compute mean and turbulent statistics using the eddy correlation technique and are studied within the framework of Monin–Obukhov similarity theory. The analysis of the observational behaviour of the standard deviation of wind velocity fluctuations (σi, i = u, v, w) normalized by friction velocity (u*) suggests that these parameters remain independent of stability of layer in near-neutral to moderately stable/unstable conditions. However, they are observed to increase following the classical 1/3 power law with increasing stability/instability in moderate to strong stable/unstable conditions. Further, an attempt has been made to develop possible parameterizations of these coefficients in terms of Monin–Obukhov stability parameter in low and moderate to strong wind regimes for four different seasons. The proposed observation-based functional forms the σi as functions of the stability exhibit different behaviours depending on the optimal values of unknown coefficients obtained for different seasons. However, within the limit of the uncertainty in the observed input parameters, the values of the season-dependent set of coefficients do not affect the overall statistical agreement between predicted and observed values of the normalized standard deviation of wind velocity fluctuations.

Ozone risk assessment for an Alpine larch forest in two vegetative seasons with different approaches: comparison of POD1 and AOT40.

The upper vegetation belts like larch forests are supposed to be under great pressure because of climate change in the next decades. For this reason, the evaluation of the risks due to abiotic stressors like ozone is a key step. Two different approaches were used here: mapping AOT40 index by means of passive samplers and direct measurements of ozone deposition.Measurements of ozone fluxes using the eddy-correlation technique were carried out for the first time over a larch forest in Paspardo (I) at 1750m a.s.l. Two field campaigns were run: the first one in 2010 from July to October and the second one in the following year from June to September. Vertical exchange of ozone, energy, and momentum were measured on a tower platform at 26m above ground level to study fluxes dynamics over this ecosystem. Since the tower was located on a gentle slope, an "ad hoc" methodology was developed to minimize the effects of the terrain inclination. The larch forest uptake was estimated by means of a two-layer model to separate the understorey uptake from the larch one. Even if the total ozone fluxes were generally high, up to 30-40nmolO3m-2s-1 in both years, the stomatal uptake by the larch forest was relatively low (around 15% of the total deposition).Ozone risk was assessed considering the POD1 received by the larch forest and the exposure index AOT40 estimated with both local data and data from the map obtained by the passive samplers monitoring.

Mixing efficiency in the thermocline of lakes observed from eddy correlation flux measurements

AbstractVertical mixing in the thermocline of lakes is poorly understood and most of the current knowledge is based on ex situ methods like laboratory measurements and simulations. Here we used the eddy correlation technique (EC) to directly measure oxygen and buoyancy fluxes in the thermocline of two lakes (Lake Scharmützelsee and Lake Arendsee in 2012 and 2013, respectively). Additionally, sets of temperature microstructure profiles (SCAMP) were measured during the EC deployments. We used these data to quantify the mixing efficiency as well as the turbulent diffusivity. The derived turbulent diffusivities from EC for the Prandtl number of DO were one order of magnitude higher than predicted by commonly applied parameterization, while the diffusivities for the Prandtl number of heat confirmed the parameterization. The results from EC and SCAMP showed strong differences which we attribute to the fact that SCAMP measurements reflect snapshots of the instantaneous turbulence field while EC provides a temporal average of the prevailing turbulence. Finally, we discuss problems of the EC and the inertial dissipation method in a strongly stratified environment and propose how they could be improved to resolve the full temporal variability of mixing in thermoclines.

Correction of a 1 km daily rainfall dataset for modelling forest ecosystem processes in Italy

AbstractThe availability of daily meteorological data over extended wide areas is a common requirement to model ecosystem processes on a regional scale. The meteorological dataset E‐OBS (European Observations) has been downscaled in 2012 to a 1 km spatial resolution in Italy. The assessment of this dataset against ground measurements revealed a marked under‐estimation of rainfall, which is a major driving factor for vegetation growth in water‐limited Mediterranean environments. The objective of the present study is to correct the downscaled rainfall dataset through the use of additional ground meteorological data collected over a 30 year period (1981–2010). The corrected dataset was evaluated using the ecosystem process model BIOME‐BGC (BioGeochemical Cycles). After identifying the most under‐estimated areas, seven sites that were representative of different forest ecosystems were selected; for each of these sites, gross primary production (GPP) was simulated using the E‐OBS original and corrected rainfall datasets. The test was completed for one site in central Italy (IT‐Ro2), assessing the outputs of the BIOME‐BGC data versus the daily GPP data obtained by the eddy correlation technique. In all cases, the use of the corrected dataset produced more realistic GPP simulations; for IT‐Ro2, the mean bias error was 0.18 instead of 2.33 g C m−2 day−1. The beneficial effect of the correction depended on the aridity of the examined site, that is, it was the maximum in the driest Mediterranean ecosystems and almost null in the most humid environments. It can therefore be concluded that the corrected dataset is suitable to drive the long‐term simulation of forest ecosystem processes in all Italian eco‐climatic conditions.

Estimating daily forest carbon fluxes using a combination of ground and remotely sensed data

AbstractSeveral studies have demonstrated that Monteith's approach can efficiently predict forest gross primary production (GPP), while the modeling of net ecosystem production (NEP) is more critical, requiring the additional simulation of forest respirations. The NEP of different forest ecosystems in Italy was currently simulated by the use of a remote sensing driven parametric model (modified C‐Fix) and a biogeochemical model (BIOME‐BGC). The outputs of the two models, which simulate forests in quasi‐equilibrium conditions, are combined to estimate the carbon fluxes of actual conditions using information regarding the existing woody biomass. The estimates derived from the methodology have been tested against daily reference GPP and NEP data collected through the eddy correlation technique at five study sites in Italy. The first test concerned the theoretical validity of the simulation approach at both annual and daily time scales and was performed using optimal model drivers (i.e., collected or calibrated over the site measurements). Next, the test was repeated to assess the operational applicability of the methodology, which was driven by spatially extended data sets (i.e., data derived from existing wall‐to‐wall digital maps). A good estimation accuracy was generally obtained for GPP and NEP when using optimal model drivers. The use of spatially extended data sets worsens the accuracy to a varying degree, which is properly characterized. The model drivers with the most influence on the flux modeling strategy are, in increasing order of importance, forest type, soil features, meteorology, and forest woody biomass (growing stock volume).

The response of ecosystem carbon fluxes to LAI and environmental drivers in a maize crop grown in two contrasting seasons.

The eddy correlation technique was used to investigate the influence of biophysical variables and crop phenological phases on the behaviour of ecosystem carbon fluxes of a maize crop, in two contrasting growing seasons. In 2009, the reduced water supply during the early growing stage limited leaf area expansion, thus negatively affecting canopy photosynthesis. The variability of gross primary production (GPP) and ecosystem respiration (R eco) was mainly explained by seasonal variation of leaf area index (LAI). The seasonal variation of R eco was positively influenced by soil temperatures (T soil) in 2008 but not in 2009. In 2008, a contribution of both autotrophic and heterotrophic components to total R eco could be hypothesized, while during 2009, autotrophic respiration is supposed to be the most important component. Crop phenological phases affected the response of ecosystem fluxes to biophysical drivers.

Cold-water coral reefs and adjacent sponge grounds: hotspots of benthic respiration and organic carbon cycling in the deep sea

Cold-water coral reefs and adjacent sponge grounds are distributed widely in the deep ocean, where only a small fraction of the surface productivity reaches the seafloor as detritus. It remains elusive how these hotspots of biodiversity can thrive in such a food-limited environment, as data on energy flow and organic carbon utilization are critically lacking. Here we report in situ community respiration rates for cold-water coral and sponge ecosystems obtained by the non-invasive aquatic Eddy Correlation technique. Oxygen uptake rates over coral reefs and adjacent sponge grounds in the Traena Coral Field (Norway) were 9-20 times higher than those of the surrounding soft sediments. These high respiration rates indicate strong organic matter consumption, and hence suggest a local focusing onto these ecosystems of the downward flux of organic matter that is exported from the surface ocean. Overall, our results show that coral reefs and adjacent sponge grounds are hotspots of carbon processing in the food-limited deep ocean, and that these deep-sea ecosystems play a more prominent role in marine biogeochemical cycles than previously recognized.

Benthic O2 uptake of two cold-water coral communities estimated with the non-invasive eddy correlation technique

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 525:97-104 (2015) - DOI: https://doi.org/10.3354/meps11211 Benthic O2 uptake of two cold-water coral communities estimated with the non-invasive eddy correlation technique Lorenzo Rovelli1,*, Karl M. Attard2,3, Lee D. Bryant4,8, Sascha Flögel4, Henrik Stahl1,9, J. Murray Roberts1,5,6, Peter Linke4, Ronnie N. Glud1,2,3,7 1Scottish Association for Marine Sciences, Scottish Marine Institute, Oban PA37 1QA, UK 2Nordic Centre for Earth Evolution (NordCEE), University of Southern Denmark, 5230 Odense M, Denmark 3Greenland Climate Research Centre, Greenland Institute of Natural Resources, 3900 Nuuk, Greenland 4GEOMAR, Helmholtz Centre for Ocean Research Kiel, 24148 Kiel, Germany 5Centre for Marine Biodiversity and Biotechnology, School of Life Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK 6Center for Marine Science, University of North Carolina Wilmington, Wilmington, NC 28409, USA 7Arctic Research Centre, University of Århus, 8000 Århus C, Denmark 8Present address: Department of Architecture and Civil Engineering, University of Bath, Bath BA2 7AY, UK 9Present address: Zayed University, Dubai Academic City, Dubai, United Arab Emirates *Corresponding author: lorenzo.rovelli@sams.ac.uk ABSTRACT: The community respiration of 2 tidally dominated cold-water coral (CWC) sites was estimated using the non-invasive eddy correlation (EC) technique. The first site, Mingulay Reef Complex, was a rock ridge located in the Sea of Hebrides off Scotland at a depth of 128 m and the second site, Stjernsund, was a channel-like sound in Northern Norway at a depth of 220 m. Both sites were characterized by the presence of live mounds of the reef framework-forming scleractinian Lophelia pertusa and reef-associated fauna such as sponges, crustaceans and other corals. The measured O2 uptake at the 2 sites varied between 5 and 46 mmol m–2 d–1, mainly depending on the ambient flow characteristics. The average uptake rate estimated from the ~24 h long deployments amounted to 27.8 ± 2.3 mmol m–2 d–1 at Mingulay and 24.8 ± 2.6 mmol m–2 d–1 at Stjernsund (mean ± SE). These rates are 4 to 5 times higher than the global mean for soft sediment communities at comparable depths. The measurements document the importance of CWC communities for local and regional carbon cycling and demonstrate that the EC technique is a valuable tool for assessing rates of benthic O2 uptake in such complex and dynamic settings. KEY WORDS: Eddy correlation · Cold-water coral · Community oxygen exchange · Mingulay Reef Complex · Stjernsund Full text in pdf format PreviousNextCite this article as: Rovelli L, Attard KM, Bryant LD, Flögel S and others (2015) Benthic O2 uptake of two cold-water coral communities estimated with the non-invasive eddy correlation technique. Mar Ecol Prog Ser 525:97-104. https://doi.org/10.3354/meps11211 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 525. Online publication date: April 09, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

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 <0.4 s facilitates accurate EC fluxes estimates in turbulence regimes corresponding to horizontal velocities < 11 cm s−1; and 3) the smallest possible distance (<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.

Aquatic eddy correlation: quantifying the artificial flux caused by stirring-sensitive O2 sensors.

In the last decade, the aquatic eddy correlation (EC) technique has proven to be a powerful approach for non-invasive measurements of oxygen fluxes across the sediment water interface. Fundamental to the EC approach is the correlation of turbulent velocity and oxygen concentration fluctuations measured with high frequencies in the same sampling volume. Oxygen concentrations are commonly measured with fast responding electrochemical microsensors. However, due to their own oxygen consumption, electrochemical microsensors are sensitive to changes of the diffusive boundary layer surrounding the probe and thus to changes in the ambient flow velocity. The so-called stirring sensitivity of microsensors constitutes an inherent correlation of flow velocity and oxygen sensing and thus an artificial flux which can confound the benthic flux determination. To assess the artificial flux we measured the correlation between the turbulent flow velocity and the signal of oxygen microsensors in a sealed annular flume without any oxygen sinks and sources. Experiments revealed significant correlations, even for sensors designed to have low stirring sensitivities of ~0.7%. The artificial fluxes depended on ambient flow conditions and, counter intuitively, increased at higher velocities because of the nonlinear contribution of turbulent velocity fluctuations. The measured artificial fluxes ranged from 2 - 70 mmol m-2 d-1 for weak and very strong turbulent flow, respectively. Further, the stirring sensitivity depended on the sensor orientation towards the flow. For a sensor orientation typically used in field studies, the artificial flux could be predicted using a simplified mathematical model. Optical microsensors (optodes) that should not exhibit a stirring sensitivity were tested in parallel and did not show any significant correlation between O2 signals and turbulent flow. In conclusion, EC data obtained with electrochemical sensors can be affected by artificial flux and we recommend using optical microsensors in future EC-studies.

Short-term and seasonal variability of oxygen fluxes at the sediment–water interface in a riverine lake

In situ measurements of sediment-water oxygen fluxes conducted in a riverine lake during different seasons were analyzed with the aim of quantifying the combined effects of hydrodynamic forcing and seasonal changes in temperature on sediment oxygen uptake rate. Oxygen fluxes measured using the eddy correlation (EC) technique varied widely between −6.4 and −84 mmol m−2 day−1, while variations observed on hourly time scales were of comparable magnitude to seasonal variations. Oxygen fluxes were most strongly correlated to current speed in the benthic boundary layer and water depth, which both co-varied with discharge, temperature, and oxygen concentration. A direct correlation of measured fluxes with temperature and corresponding seasonal flux variations could not be observed. To explore the potential effect of temperature on oxygen fluxes, we applied a simplified analytical model, which couples the effect of hydrodynamic forcing with a temperature-dependent oxygen consumption rate within the sediment. The results suggest that the flux is a non-linear function of both variables and both can have comparable effects on the magnitude of the oxygen fluxes. The model confirms our observation that short-term variations of oxygen fluxes in response to hydrodynamic forcing can mask longer-term seasonal variations driven by temperature. The model further indicates that the magnitude and form of the temperature dependence of oxygen uptake and mineralization rates in freshwater sediments obtained from laboratory incubations can be strongly affected by flow conditions during incubations. We conclude that predictions of oxygen uptake and mineralization rates under changing climatic conditions should also take potential changes of flow conditions into account.

Multiple timescale processes drive ecosystem metabolism in eelgrass (Zostera marina) meadows

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 507:1-13 (2014) - DOI: https://doi.org/10.3354/meps10843 FEATURE ARTICLE Multiple timescale processes drive ecosystem metabolism in eelgrass (Zostera marina) meadows Jennie E. Rheuban1,2,*, Peter Berg1, Karen J. McGlathery1 1Department of Environmental Sciences, University of Virginia, Charlottesville, Virginia, USA 2Present address: Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA *Corresponding author: jrheuban@whoi.edu ABSTRACT: The oxygen flux between benthic ecosystems and the overlying water column is a measure of metabolic status and a commonly used proxy for carbon cycling. In this study, oxygen flux was measured seasonally using the eddy correlation technique in a restored eelgrass (Zostera marina L.) meadow in the Virginia coastal bays (USA). In 5 intensive field campaigns, we covered seasonal variation in oxygen metabolism and biomass with overlap in late summer to observe interannual variability. The high-resolution measurements allowed identification of the drivers of metabolism at multiple timescales: minute to hourly, daily, and monthly to seasonally. There was a strong correlation between nighttime hourly fluxes and current velocity that varied seasonally with seagrass shoot density and temperature. No similar relationship was observed during the day. A hysteresis effect in oxygen flux throughout the day was observed during October and August that was most likely due to increased respiration (R) in the afternoon. In October, net community production was 90% lower in the afternoon than in the morning at the same irradiance. From this hysteresis, we calculated that daytime R may be up to 2.5-fold larger than nighttime R. The magnitudes of daily gross primary production (GPP) and R were well correlated throughout the year with close to a 1:1 ratio that reflected a tight coupling between GPP and R on daily to seasonal timescales. Our results document the dynamic nature of oxygen fluxes that, when integrated over time, translate into highly variable rates of ecosystem metabolism over daily to seasonal timescales. This variation must be incorporated to accurately determine trophic status. KEY WORDS: Zostera marina L. · Net ecosystem metabolism · Oxygen · Production · Respiration Full text in pdf format Information about this Feature Article NextCite this article as: Rheuban JE, Berg P, McGlathery KJ (2014) Multiple timescale processes drive ecosystem metabolism in eelgrass (Zostera marina) meadows. Mar Ecol Prog Ser 507:1-13. https://doi.org/10.3354/meps10843 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 507. Online publication date: July 17, 2014 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2014 Inter-Research.

Ecosystem metabolism along a colonization gradient of eelgrass (Zostera marina) measured by eddy correlation

The Virginia coastal bays experienced local extinction of eelgrass (Zostera marina) during the early 1930s, and restoration beginning in 2001 has generated an ecosystem state change from bare to vegetated sediments. Oxygen fluxes were measured seasonally using the eddy correlation technique at three sites representing different stages of seagrass colonization: unvegetated (bare), 5 yr, and 11 yr since seeding. Derived seasonal ecosystem respiration (R) and gross primary production (GPP) increased up to 10‐fold and 25‐fold, respectively, with meadow age. Although hourly oxygen (O2) fluxes were highly correlated with light at the vegetated sites, no identifiable trends with light were observed at the bare site. The light compensation point where O2 production and respiration are in balance increased from 46 µmol photons m−2 s−1 to 257 µmol photons m−2 s−1 and 63 µmol photons m−2 s−1 to 472 µmol photons m−2 s−1 at the 5 yr and 11 yr seagrass sites, respectively, with increasing seasonal temperatures from 12.3°C to 27.9°C and 9.3°C to 30.5°C, respectively. This suggests that more light, and thus more O2 production, is required to offset increasing respiration with both temperature and meadow age. Photosynthesis—irradiance curves generated from hourly O2 fluxes throughout the seasons were used to estimate annual net ecosystem metabolism (NEM). Annual NEM rates at the bare, 5 yr, and 11 yr sites were −7.6, 8.6, and −7.0 mol O2 m−2 yr−1, respectively. Although the system went through a period of net autotrophy during early stages of colonization, the ecosystem state change from unvegetated sediments to dense seagrass meadows changed the magnitude of both GPP and R, but not the overall metabolic balance of the system.

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.

Direct measurements of wind stress over the surf zone

Field data of the wind stress over surf zone waves are presented from an open ocean beach on the East Australian Coast. Two ultrasonic anemometers were deployed at nominal heights of 5 and 10 m above the water surface in the intertidal and inner surf zones, with concurrent measurements of water levels and offshore wave parameters. Following a rigorous quality control procedure, the wind stress was determined by the eddy correlation technique. A constant stress layer was observed between 5 and 10 m elevation. Considering near-neutral conditions only, the wind drag coefficients were found to systematically change with the wind angle of approach relative to the shoreline and are much smaller for longshore wind than during onshore wind. The concept of an apparent wave steepness changing with wind direction is suggested to explain this behavior. The drag coefficients over the surf zone during onshore wind and near-neutral conditions were determined to be almost twice the values expected at the same wind speed and open ocean conditions. The observed Charnock coefficient was similarly an order of magnitude larger than open ocean values. A wave celerity of the order of that expected in the inner surf zone is required to explain the observed large roughness and drag coefficients using existing wave age-dependent parameterizations. This suggests that the slower wave celerity in the surf zone is an important contributor to the increased wind stress, in addition to the sawtooth wave shape. Key Points A new field data set of the wind stress over surf zone waves is presented Concept of an apparent wave steepness is proposed to explain drag observations Surf zone drag coefficients were almost twice the values outside the surf

Evapotranspiration from an Olive Orchard using Remote Sensing-Based Dual Crop Coefficient Approach

A remote sensing-based approach to estimate actual evapotranspiration (ET) was tested in an area covered by olive trees and characterized by Mediterranean climate. The methodology is a modified version of the standard FAO-56 dual crop coefficient procedure, in which the crop potential transpiration, T p, is obtained by directly applying the Penman-Monteith (PM) equation with actual canopy characteristics (i.e., leaf area index, albedo and canopy height) derived from optical remote sensing data. Due to the minimum requirement of in-situ ancillary inputs, the methodology is suitable also for applications on large areas where the use of tabled crop coefficient values become problematic, due to the need of corrections for specific crop parameters, i.e., percentage of ground cover, crop height, phenological cycles, etc. The methodology was applied using seven airborne remote sensing images acquired during spring-autumn 2008. The estimates based on PM approach always outperforms the ones obtained using simple crop coefficient constant values. Additionally, the comparison of simulated daily evapotranspiration and transpiration with the values observed by eddy correlation and sap flow techniques, respectively, shows a substantial agreement during both dry and wet days with an accuracy in the order of 0.5 and 0.3 mm d−1, respectively. The obtained results suggest the capability of the proposed approach to correctly partition evaporation and transpiration components during both the irrigation season and rainy period also under conditions of significant reduction of actual ET from the potential one.

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.