Eddy Correlation Technique Research Articles

In Situ Coral Reef Oxygen Metabolism: An Eddy Correlation Study

Quantitative studies of coral reefs are challenged by the three-dimensional hard structure of reefs and the high spatial variability and temporal dynamics of their metabolism. We used the non-invasive eddy correlation technique to examine respiration and photosynthesis rates, through O2 fluxes, from reef crests and reef slopes in the Florida Keys, USA. We assessed how the photosynthesis and respiration of different reef habitats is controlled by light and hydrodynamics. Numerous fluxes (over a 0.25 h period) were as high as 4500 mmol O2 m−2 d−1, which can only be explained by efficient light utilization by the phototrophic community and the complex canopy structure of the reef, having a many-fold larger surface area than its horizontal projection. Over diel cycles, the reef crest was net autotrophic, whereas on the reef slope oxygen production and respiration were balanced. The autotrophic nature of the shallow reef crests implies that the export of organics is an important source of primary production for the larger area. Net oxygen production on the reef crest was proportional to the light intensity, up to 1750 µmol photons m−2 s−1 and decreased thereafter as respiration was stimulated by high current velocities coincident with peak light levels. Nighttime respiration rates were also stimulated by the current velocity, through enhanced ventilation of the porous framework of the reef. Respiration rates were the highest directly after sunset, and then decreased during the night suggesting that highly labile photosynthates produced during the day fueled early-night respiration. The reef framework was also important to the acquisition of nutrients as the ambient nitrogen stock in the water had sufficient capacity to support these high production rates across the entire reef width. These direct measurements of complex reefs systems yielded high metabolic rates and dynamics that can only be determined through in situ, high temporal resolution measurements.

Drastic variation in the surface boundary layer parameters over Cochin during the annular solar eclipse: Analysis using sonic anemometer data

Atmospheric surface boundary layer parameters vary anomalously in response to the occurrence of annular solar eclipse on 15th January 2010 over Cochin. It was the longest annular solar eclipse occurred over South India with high intensity. As it occurred during the noon hours, it is considered to be much more significant because of its effects in all the regions of atmosphere including ionosphere. Since the insolation is the main driving factor responsible for the anomalous changes occurred in the surface layer due to annular solar eclipse, occurred on 15th January 2010, that played very important role in understanding dynamics of the atmosphere during the eclipse period because of its coincidence with the noon time. The Sonic anemometer is able to give data of zonal, meridional and vertical wind as well as the air temperature at a temporal resolution of 1s. Different surface boundary layer parameters and turbulent fluxes were computed by the application of eddy correlation technique using the high resolution station data. The surface boundary layer parameters that are computed using the sonic anemometer data during the period are momentum flux, sensible heat flux, turbulent kinetic energy, frictional velocity (u⁎), variance of temperature, variances of u, v and w wind. In order to compare the results, a control run has been done using the data of previous day as well as next day. It is noted that over the specified time period of annular solar eclipse, all the above stated surface boundary layer parameters vary anomalously when compared with the control run. From the observations we could note that momentum flux was 0.1Nm−2 instead of the mean value 0.2Nm-2 when there was eclipse. Sensible heat flux anomalously decreases to 50Nm−2 instead of the mean value 200Nm−2 at the time of solar eclipse. The turbulent kinetic energy decreases to 0.2m2s−2 from the mean value 1m2s−2. The frictional velocity value decreases to 0.05ms−1 instead of the mean value 0.2ms−1. The present study aimed at understanding the dynamics of surface layer in response to the annular solar eclipse over a tropical coastal station, occurred during the noon hours. Key words: annular solar eclipse, surface boundary layer, sonic anemometer

Wet and dry deposition of particles associated metals (Cd, Pb, Zn, Ni, Hg) in a rural wetland site, Marais Vernier, France

Atmospheric transport plays an important role in the transfer of pollutants to ecosystems. During such transport, the atmospheric aerosol is one of the key factors in terms of deposition. Over 18 months, we studied the concentrations of Zn, Pb, Ni, Cd, Hg in air and precipitations in a wetland site in the north west of France. The objective of this study was to characterise the quantities transferred by dry and wet deposition. An eddy correlation technique was used to measure dry deposition velocities and determine the dry deposition fluxes from atmospheric concentration. In this site, wet deposition is almost always preponderant, except for Pb and Cd over several months. Measurements in the air and in rainwater are low and show that the site is protected from important anthropogenic influences, with levels that can be considered as background levels. In terms of deposition fluxes, the configuration of the site and the meteorological conditions (low wind speeds, low thermal fluxes) lead to low dry depositions. Wet depositions, although more important are also relatively low compared to the literature, directly linked to the pluviometry during the study period.

Characterisation of PM2.5 concentrations and turbulent fluxes on a island of the Venice lagoon using high temporal resolution measurements

This work presents an analysis of PM2.5 concentrations and vertical turbulent fluxes on an island of the Venice lagoon. Data were collected during three measurement campaigns in spring, summer and winter periods. Measurements were taken with a high-resolution optical PM2.5 detector, coupled with a micrometeorological station that allowed the evaluation of the vertical turbulent fluxes of PM2.5 using the eddy-correlation technique. The main objective of this paper is to analyse the daily and seasonal pattern in PM2.5 concentrations and fluxes and to discuss their correlation with the main meteorological and micrometeorological parameters using high temporal resolution measurements. Observed data showed a seasonal pattern in turbulent fluxes with daytime average positive value during winter and negative during summer. Deposition velocities, ranged from –60 to 20 mm/s, appeared to be mainly influenced by atmospheric stability. There were larger emissions in cases of high wind velocities blowing from water sector indicating a significant potential contribution of sea spray to PM2.5 fluxes. The local atmospheric circulation, due to the orography of the area, was characterised by diurnal winds coming from the Adriatic Sea and nocturnal wind coming from the Alps. This circulation influenced deposition velocity creating an increase of negative fluxes in the morning at the starting of the sea breeze. A diurnal pattern in concentration has been observed and it is similar for all three measurement campaigns, with higher concentrations in nocturnal periods. The daily pattern was investigated in terms of its correlation with meteorological and micro-meteorological parameters, and was found highly correlated with the diurnal pattern of boundary layer height (BLH) and with relative humidity.

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

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 O2 eddy correlation systems were used to examine O2 exchange rates driven by biological and physical processes. The study was conducted below 0.7 m thick sea-ice in mid-March 2010 in a southwest Greenland fjord and revealed low rates of ice melt at a maximum of 0.80 mm d−1. The O2 flux associated with release of O2 depleted melt water was less than 13 % of the average daily O2 respiration rate. Ice melt and insufficient vertical turbulent mixing due to low current velocities caused periodic stratification immediately below the ice. This prevented the determination of fluxes 61 % of the deployment time. These time intervals were identified by examining the velocity and the linearity and stability of the cumulative flux. The examination of unstratified conditions through vertical velocity and O2 spectra and their cospectra revealed characteristic fingerprints of well-developed turbulence. From the measured O2 fluxes a photosynthesis/irradiance curve was established by least-squares fitting. This relation showed that light limitation of net photosynthesis began at 4.2 μmol photons m−2 s−1, and that algal communities were well-adapted to low-light conditions as they were light saturated for 75 % of the day during this early spring period. However, the sea-ice associated microbial and algal community was net heterotrophic with a daily gross primary production of 0.69 mmol O2 m−2 d−1 and a respiration rate of −2.13 mmol O2 m−2 d−1 leading to a net ecosystem metabolism of −1.45 mmol O2 m−2 d−1. This application of the eddy correlation technique produced high temporal resolution O2 fluxes and ice melt rates that were measured without disturbing the in situ environmental conditions while integrating over an area of approximately 50 m2 which incorporated the highly variable activity and spatial distributions of sea-ice communities.

Oxygen optodes as fast sensors for eddy correlation measurements in aquatic systems

The aquatic eddy‐correlation technique can be used to noninvasively determine the oxygen exchange across the sediment‐water interface by analyzing the covariance of vertical flow velocity and oxygen concentration in a small measuring volume above the sea bed. The method requires fast sensors that can follow the rapid changes in flow and the oxygen transported by this flow to calculate the momentary advective flux driven by turbulent motions. In this article, we demonstrate that fast optical oxygen sensors, known as optodes, represent a good alternative to the traditional Clark‐type electrochemical microelectrodes for such measurements. Optodes have the advantage over microelectrodes of being insensitive to flow, less susceptible to signal drift, more durable under typical field conditions, less expensive, and repairable. Comparisons of the response times of optodes and microelectrodes to rapid oxygen changes showed that optimized optodes had the same response time (162 ± 66 ms) as the microelectrodes (160 ± 57 ms) and were fast enough to capture the oxygen fluctuations that are relevant for the eddy correlation flux calculations. Side by side comparisons of benthic O2 flux data collected with microelectrode‐based eddy correlation instruments and optode‐based eddy correlation instruments in freshwater and marine environments showed no significant differences between the measured fluxes. The optodes allow the development of more user‐friendly eddy correlation instruments that combine the advantages of non‐invasive measurements and integration of fluxes over a large footprint area, using a relatively rugged and less expensive sensor.

Fluxes of Polychlorinated Biphenyls Volatilizing from the Hudson River, New York Measured Using Micrometeorological Approaches

This study represents the first time that a micrometeorological technique, using turbulent transport measurements, has been used to determine the direction and magnitude of air-water exchange of polychlorinated biphenyls (PCBs). The study was conducted during July 2008 on the Hudson River estuary near the Tappan Zee Bridge, which is the site of some of the most serious PCB contamination in the world. Gas-phase ΣPCB concentrations measured at two heights above the water column averaged 1.1 ng m(-3), and concentrations were usually lower in the upper air sample, indicating net transport of PCBs from the water column to the air. Volatilization PCB fluxes were calculated using the modified Thornthwaite-Holzman equation. Values of friction velocity and atmospheric stability were calculated using the Aerodynamic Gradient and Eddy Correlation techniques. The PCB fluxes were corrected for changes in atmospheric stability using the atmospheric stability factor of water vapor (ϕ(w)) calculated from empirical formulations which ranged from 1.0 to 3.2 (neutral to stable atmospheric boundary layer conditions). Vertical ΣPCB fluxes ranged from +0.5 μg m(-2) d (-1) to +13 μg m(-2) d (-1). Mono- through tri-homologues accounted for about half of ΣPCB fluxes, with tetra- through hexa-homologue accounting for the other half. This work demonstrates the utility of a micrometeorological approach to measuring the air-water exchange of organic contaminants.

Nitrate and oxygen flux across the sediment‐water interface observed by eddy correlation measurements on the open continental shelf

Chemical fluxes into and out of permeable seafloor sediments, such as those found on much of the continental shelf, are difficult to measure using conventional techniques. To overcome this difficulty, the eddy correlation method was adapted by oceanographers to determine oxygen flux across the sediment‐water interface. In this article, we demonstrate that the eddy correlation method can also be used to measure nitrate flux across the sediment‐water interface. A modified ISUS (In Situ Ultraviolet Spectrophotometer) optical nitrate sensor was used to measure dissolved nitrate at a frequency of 1.8 Hz. These observations were combined with vertical velocity measurements to calculate nitrate fluxes. Oxygen fluxes were also determined using a fast oxygen electrode system. The results for nitrate and oxygen flux compare well with simultaneous benthic flux chamber measurements made at a muddy sediment site at 95 m depth in Monterey Bay on the central California coast. The nitrate fluxes determined by the eddy correlation technique were considerably higher than benthic flux chamber values at sandy sediment sites at similar depths to the north and south of Monterey Bay. This difference is expected in permeable sediments where lateral flow through sediments will ventilate chamber water.

Simple, robust eddy correlation amplifier for aquatic dissolved oxygen and hydrogen sulfide flux measurements

The aquatic application of the eddy correlation (EC) technique is growing more popular and is gradually becoming a standard method for resolving benthic O2 fluxes. By including the effects of the local hydrodynamics, the EC technique provides greater insight into the nature of benthic O2 exchange than traditional methods (i.e., benthic chambers and lander microprofilers). The growing popularity of the EC technique has led to a greater demand for easily accessible and robust EC instrumentation. Currently, the EC instrumentation is limited to two commercially available systems that are still in the development stage. Here, we present a robust, open source EC picoamplifier that is simple in design and can be easily adapted to both new and existing acoustic Doppler velocimeters (ADV). The picoamplifier has a response time of < 0.1 ms and features galvanic isolation that ensures very low noise contamination of the signal. It can be adjusted to accommodate varying ranges of microelectrode sensitivity as well as other types of amperometric microelectrodes. We show that the extracted flux values are not sensitive to reduced microelectrode operational ranges (i.e., lower resolution) and that no signal loss results from using either a 16‐ or 14‐bit analog‐to‐digital converter. Finally, we demonstrate the capabilities of the picoamplifier with field studies measuring both dissolved O2 and H2S EC fluxes. The picoamplifier presented here consistently acquires high‐quality EC data and provides a simple solution for those who wish to obtain EC instrumentation. The schematic of the amplifier's circuitry is given in the Web Appendix.

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

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

Five methods to interpret field measurements of energy fluxes over a micro-sprinkler-irrigated mango orchard

Energy balance measurements were carried out in a mango orchard during two growing seasons in the semi-arid region of Brazil. The actual evapotranspiration (ET) was acquired by eddy correlation (EC) and Bowen ratio energy balance (BR) techniques. The daily energy balance closure in the EC measurements showed an average gap of 12%, with a root mean square error (RMSE) of 1.7 MJ m−2 d−1. Three different correction procedures were tested for closing the energy balance from the EC system: (1) the surface energy balance residual method (RES), (2) the Bowen ratio determined from the EC fluxes, the combination approach (EC_BR), and (3) a new regression energy balance closure technique (REG). All closing energy balance methods presented good correlation with the direct EC measurements, but the trends were not similar. The latent heat fluxes estimated by the BR method—λE BR—were higher than those from the direct EC measurements—λE EC. When using the RES method, the half-hour λE EC measurements represented around 88% of the λE RES values, as the uncertainties of net radiation—R n—and soil heat fluxes—G—are propagated into the RES method. The latent heat flux derived from the combination approach—λE EC_BR—also brings these uncertainties, being the agreements comparable with those for RES method. It was therefore concluded that a single correction method for EC measurements considering only the latent and sensible heat fluxes does not exist. A new way to solve the lack of energy balance closure from EC techniques was tested by means of a curve fitting, the REG method. Considering the REG corrections applied to the energy balance components involving all periods of the day and the average conditions of the two growing seasons, half-hour values of λE EC were overmeasured by 18%, H EC was undermeasured by 17%, and G values required a correction of 466%. The REG method appeared promising because it considers different weights for all energy balance components in the optimization process. Taking the REG results for the drier second growing season as a reference, it was concluded that seasonal ET values by the other methods in mango orchard ranged from 7 to 28% higher, showing that turbulent flux measurements lack accuracy for executing on-farm water-saving programmes and calibrating transient soil water flow models.

Dissolved oxygen fluxes and ecosystem metabolism in an eelgrass (Zostera marina) meadow measured with the eddy correlation technique

Dissolved oxygen (DO) fluxes were measured by eddy correlation to estimate net ecosystem metabolism (NEM) during summer in a restored eelgrass (Zostera marina) meadow and a nearby, unvegetated sediment. This technique measures benthic fluxes under true in situ light and hydrodynamic conditions, integrates over a large area (typically > 100 m2), and captures short‐term variations. DO fluxes measured through eight 24‐h periods showed pronounced temporal variation driven by light and local hydrodynamics on multiple scales: hour‐to‐hour, within each daily cycle, and between deployments. The magnitude of variation between hours during single deployments equaled that between deployments, indicating that short‐term variation must be included for metabolism estimates to be accurate. DO flux variability was significantly correlated to mean current velocity for the seagrass site and to significant wave height for the unvegetated site. Fluxes measured in low‐flow conditions analogous to many chamber and core incubations underestimated those measured in higher‐flow conditions typical of in situ conditions by a factor of 2–6. Rates of gross primary production (GPP), respiration (R), and NEM varied substantially between individual deployments, reflecting variations in light and hydrodynamic conditions, and daily values of GPP and R for individual deployments were tightly linked. Average daily NEM of the seagrass site was higher than that of the unvegetated site; the seagrass site was in metabolic balance, and the unvegetated site showed a tendency toward net heterotrophy during this midsummer period.

Integration of ground and satellite data to model Mediterranean forest processes

The current work presents the testing of a modeling strategy that has been recently developed to simulate the gross and net carbon fluxes of Mediterranean forest ecosystems. The strategy is based on the use of a NDVI-driven parametric model, C-Fix, and of a biogeochemical model, BIOME-BGC, whose outputs are combined to simulate the behavior of forest ecosystems at different development stages. The performances of the modeling strategy are evaluated in three Italian study sites (San Rossore, Lecceto and Pianosa), where carbon fluxes are being measured through the eddy correlation technique. These sites are characterized by variable Mediterranean climates and are covered by different types of forest vegetation (pine wood, Holm oak forest and Macchia, respectively). The results of the tests indicate that the modeling strategy is generally capable of reproducing monthly GPP and NEE patterns in all three study sites. The highest accuracy is obtained in the most mature, homogenous pine wood of San Rossore, while the worst results are found in the Lecceto forest, where there are the most heterogeneous terrain, soil and vegetation conditions. The main error sources are identified in the inaccurate definition of the model inputs, particularly those regulating the site water budgets, which exert a strong control on forest productivity during the Mediterranean summer dry season. In general, the incorporation of NDVI-derived fAPAR estimates corrects for most of these errors and renders the forest flux simulations more stable and accurate.

Benthic O2 exchange across hard-bottom substrates quantified by eddy correlation in a sub-Arctic fjord

Oxygen exchange across the seabed is a key measure for quantifying benthic carbon turnover. However, technical limitations have restricted such mea- surements to sedimentary areas. Few assessments on hard-bottom substrates exist although such biotopes are frequent along many coastlines. In spring 2009, we used the non-invasive eddy correlation technique to quantify the in situ O2 exchange rate across 3 typical substrates in West Greenland: (1) in soft sediments, the O2 exchange in darkness averaged 2.0 mmol m -2 h -1 . The rate ex- ceeded the O2 exchange quantified from microprofile measurements by 80%, but was not significantly differ- ent from parallel core incubations. (2) Measurements across consolidated sand and stones revealed a high activity with average O2 exchange similar to that of sedimentary areas. Despite being net heterotrophic, the benthic primary production on average amounted to 27 mmol O2 m -2 d -1 , which per unit area is similar to the pelagic gross production of the central fjord. Carbon fixed by benthic diatoms and coralline red algae was efficiently recycled by microbes or grazed by the abun- dant epifauna. (3) A vertical cliff covered with sea cucumbers also revealed high O2 consumption rates (1.2 mmol m -2 h -1 ). The O2 exchange derived from the eddy correlation approach exhibited pronounced short- term variability reflecting the interplay of several dynamic controls, most importantly short-term variations in local hydrodynamics. All together, measurements across hard-bottom substrates revealed active benthic communities that mediate high carbon turnover rates.

Application of Oxygen Eddy Correlation in Aquatic Systems

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

Dew Formation, Eddy-Correlation Latent Heat Fluxes, and the Surface Energy Imbalance at Cabauw During Stable Conditions

Observations collected between 2000 and 2008 at the Cabauw meteorological measurement platform in the Netherlands were conditionally sampled to select nights with stably stratified atmospheric conditions, clear skies and weak horizontal wind speeds (<3 m s−1). For these conditions the eddy-correlation latent heat fluxes are found to be negligibly small, while the conditionally sampled surface energy balance exhibits a maximum residual. However, inspection of the specific humidities for these conditions reveals systematic drying trends that are a maximum at the lowest measurement level above the surface. These drying trends occur for any prevailing wind direction. Latent heat fluxes are calculated from the humidity budget equation and from a Penman-Monteith dewfall model, with the results suggesting that during clear, stable nights the observed latent heat fluxes as obtained from the eddy-correlation technique are erroneously small.

Eddy Correlation Measurements of Ozone Fluxes over Coastal Waters West of Ireland

Measurements of ozone fluxes using the eddy‐correlation (EC) technique were carried out for the first time at the Mace Head atmospheric research station, on the west coast of Ireland between August‐October 2009. Vertical exchange of ozone was measured from a tower platform at 22 m above mean sea level to study fluxes over coastal waters excluding the tidal region. The results were averaged over 30 min and exhibited predominantly downward but also upward transport of ozone in the boundary layer. Data quality was found to be high based on inspection of cospectra and micrometeorological measurements. During the study period, a major physical influence on O3 fluxes was found to be wind speed. Measured fluxes were of the same magnitude as reported in previous open ocean studies ranging from approximately +0.2 to −0.5 μg m−2 s−1 (−0.017 μg m−2 s−1 on average, corresponding to a deposition velocity of 0.25 mm s−1 or a surface resistance of 4.13 s mm−1). These results are considered to represent ozone fluxes over shallow coastal waters west of Ireland for conditions during summer and fall not affected by phytoplankton blooms.

The Sky Arrow ERA, an innovative airborne platform to monitor mass, momentum and energy exchange of ecosystems

Substantial worldwide efforts are underway aimed at identifying the spatial and temporal distribution of the global sources and sinks of atmospheric carbon dioxide (CO2). The sink/source strength of vegetated surfaces at ground sites can now be estimated with reasonable accuracy and micrometeorological techniques are now well established, while difficulties exist in up scaling these figures to the regional and global scales. Airborne measurement of mass, momentum, and energy fluxes for boundary layer research has been available for decades requiring the use of large aircraft to carry instruments and dedicated support facilities. The advent of compact, lowpower instruments and high speed, high-capacity digital data acquisition systems has recently allowed small research aircraft to perform such measurements with high accuracy. This paper first describes the Sky Arrow ERA (Environmental Research Aircraft), a small research aircraft that has been recently developed in Italy, in the frame of an international scientific collaboration. This aircraft can be operated to measure fluxes of mass, momentum and energy while flying at low altitude and reduced ground speed. The fluxes are computed with the airborne eddy correlation technique. The basic theory at the basis of the flux measurement technique is also described in the paper, and two application examples are discussed to illustrate the quality and the accuracy of the measurements that can be made using this research platform. Potential applications of those data to parametrize land surface schemes, validate simulation models and provide extensive and reliable ground truthing for satellite remote sensing applications are highlighted.

Measuring seagrass photosynthesis: methods and applications

This review originates from a keynote lecture given at the recent 8th Group for Aquatic Productivity (GAP) workshop held in Eilat, Israel. Here we examine the most important methodolo- gies for photosynthetic measurements in seagrasses and evaluate their applications, advantages and disadvantages, and also point out the most relevant results. The most commonly used methodologies are based on oxygen (O2) evolution and chlorophyll fluorescence measurements. O2-based method- ologies allowed for the first approaches to evaluate seagrass productivity, whereas chlorophyll a flu- orescence has more recently become the choice method for in situ experiments, particularly in eval- uating photosynthetic responses to light and assessing stress responses. New methodologies have also emerged, such as O2 optodes, underwater CO2 flux measurements, geo-acoustic inversion and the eddy correlation technique. However, these new methods still need calibration and validation. Our analysis of the literature also reveals several significant gaps in relevant topics concerning sea- grass photosynthesis, namely the complete absence of studies on deep-growing populations that photosynthesise under extreme low light conditions and the uncertainties about the true degree of seagrass carbon limitation, which limits our ability to predict responses to global changes.

Carbon dioxide and water vapour exchange in a tropical dry forest as influenced by the North American Monsoon System (NAMS)

To better understand the effects and relationship between precipitation, net ecosystem carbon dioxide (NEE) and water vapor exchange (ET), we report a study conducted in the tropical dry forest (TDF) in the northwest of Mexico. Ecosystem gas exchange was measured using the eddy correlation technique during the presence of North American Monsoon System (NAMS) in 2006. Patterns in NEE and ET were different in wet and dry periods. Three markedly defined periods were found during the six-month study period. A pre-monsoon period, where gas exchange was close to zero. A monsoon period, divided in two stages: 1) early monsoon: a strong increase in the respiratory rate marked by a peak of positive values, with a maximum of 22 g CO 2 m −2 day −1, and, 2) late monsoon: an assimilation period occurred in the peak of the monsoon period, with sustained values around −20 g CO 2 m −2 day −1. The final was a post-monsoon period, where ecosystems returned to dormancy. NEE and ET trends in the TDF were similar to other seasonally dry ecosystems influenced by the NAMS. During the study period the TDF of Northwest Mexico acted as a sink capturing 374 g CO 2 m 2 with an ecosystem water use efficiency (-NEE/ET) comparable to other ecosystems in the region. Mechanistic information about biological and environmental variables controlling gas exchange dynamics is still necessary to predict how seasonally dry ecosystems would respond to climate change.