Supersaturation and evasion of CO2 and CH4 in surface waters at Mer Bleue peatland,Canada

Carbon dioxide (CO₂) and methane (CH₄) concentrations and evasion rates were measured in surface waters draining Mer Bleue peatland (Ontario, Canada) between spring and autumn 2005. All sites exhibit a consistent pattern of supersaturation throughout the year, which is broadly related to hydrological and temperature changes between spring snowmelt and autumn freezing. Both measurements and estimates of CO₂ and CH₄ evasion from open water to the atmosphere suggest that parts of the catchment (including beaver dams) are significant degassing hot spots. We present data showing how vertical gaseous carbon fluxes compare with lateral carbon fluxes and make an initial estimate of the importance to the overall carbon budget of CO₂ and CH₄ evasion to the atmosphere from water surfaces at Mer Bleue. Copyright © 2007 John Wiley & Sons, Ltd.     

Variations in dissolved CO2 and CH4 in a first‐order stream and catchment: an investigation of soil–stream linkages

Spatial and seasonal variations in CO₂ and CH₄ concentrations in streamwater and adjacent soils were studied at three sites on Brocky Burn, a headwater stream draining a peatland catchment in upland Britain. Concentrations of both gases in the soil atmosphere were significantly higher in peat and riparian soils than in mineral soils. Peat and riparian soil CO₂ concentrations varied seasonally, showing a positive correlation with air and soil temperature. Streamwater CO₂ concentrations at the upper sampling site, which mostly drained deep peats, varied from 2·8 to 9·8 mg l^?1 (2·5 to 11·9 times atmospheric saturation) and decreased markedly downstream. Temperature‐related seasonal variations in peat and riparian soil CO₂ were reflected in the stream at the upper site, where 77% of biweekly variation was explained by an autoregressive model based on: (i) a negative log‐linear relationship with stream flow; (ii) a positive linear relationship with soil CO₂ concentrations in the shallow riparian wells; and (iii) a negative linear relationship with soil CO₂ concentrations in the shallow peat wells, with a significant 2‐week lag term. These relationships changed markedly downstream, with an apparent decrease in the soil–stream linkage and a switch to a positive relationship between stream flow and stream CO₂. Streamwater CH₄ concentrations also declined sharply downstream, but were much lower (<0·01 to 0·12 mg l^?1) than those of CO₂ and showed no seasonal variation, nor any relationship with soil atmospheric CH₄ concentrations. However, stream CH₄ was significantly correlated with stream flow at the upper site, which explained 57% of biweekly variations in dissolved concentrations. We conclude that stream CO₂ can be a useful integrative measure of whole catchment respiration, but only at sites where the soil–stream linkage is strong. Copyright © 2004 John Wiley & Sons, Ltd.     

Wetland nitrogen dynamics in an Adirondack forested watershed

Wetlands often form the transition zone between upland soils and watershed streams, however, stream–wetland interactions and hydrobiogeochemical processes are poorly understood. We measured changes in stream nitrogen (N) through one riparian wetland and one beaver meadow in the Archer Creek watershed in the Adirondack Mountains of New York State, USA from 1 March to 31 July 1996. In the riparian wetland we also measured changes in groundwater N. Groundwater N changed significantly from tension lysimeters at the edge of the peatland to piezometer nests within the peatland. Mean N concentrations at the peatland perimeter were 1·5, 0·5 and 18·6 µmol L^?1 for NH₄⁺, NO₃^? and DON (dissolved organic nitrogen), respectively, whereas peatland groundwater N concentration was 56·9, 1·5 and 31·6 µmol L^?1 for NH₄⁺, NO₃^? and DON, respectively. The mean concentrations of stream water N species at the inlet to the wetlands were 1·5, 10·1 and 16·9 µmol L^?1 for NH₄⁺, NO₃^? and DON, respectively and 1·6, 28·1 and 8·4 µmol L^?1 at the wetland outlet. Although groundwater total dissolved N (TDN) concentrations changed more than stream water TDN through the wetlands, hydrological cross‐sections for the peatland showed that wetland groundwater contributed minimally to stream flow during the study period. Therefore, surface water N chemistry was affected more by in‐stream N transformations than by groundwater N transformations because the in‐stream changes, although small, affected a much greater volume of water. Stream water N input–output budgets indicated that the riparian peatland retained 0·16 mol N ha^?1 day^?1 of total dissolved N and the beaver meadow retained 0·26 mol N ha^?1 day^?1 during the study period. Nitrate dominated surface water TDN flux from the wetlands during the spring whereas DON dominated during the summer. This study demonstrates that although groundwater N changed significantly in the riparian peatland, those changes were not reflected in the stream. Consequently, although in‐stream changes of N concentrations were less marked than those in groundwater, they had a greater effect on stream water chemistry—because wetland groundwater contributed minimally to stream flow. Copyright © 2004 John Wiley & Sons, Ltd.     

The solute budget of a forest catchment and solute fluxes within a Pinus radiata and a secondary native forest site,southern Chile

Solute concentrations and fluxes in rainfall, throughfall and stemflow in two forest types, and stream flow in a 90 ha catchment in southern Chile (39°44′S, 73°10′W) were measured. Bulk precipitation pH was 6·1 and conductivity was low. Cation concentrations in rainfall were low (0·58 mg Ca²⁺ l^?1, 0·13 mg K⁺ l^?1, 0·11 mg Mg²⁺ l^?1 and <0·08 mg NH₄–N l^?1), except for sodium (1·10 mg l^?1). Unexpected high levels of nitrate deposition in rainfall (mean concentration 0·38 mg NO₃–N l^?1, total flux 6·3 kg NO₃–N ha^?1) were measured. Concentrations of soluble phosphorous in bulk precipitation and stream flow were below detection limits (<0·09 mg l^?1) for all events. Stream‐flow pH was 6·3 and conductivity was 28·3 μs. Stream‐water chemistry was also dominated by sodium (2·70 mg l^?1) followed by Ca, Mg and K (1·31, 0·70 and 0·36 mg l^?1). The solute budget indicated a net loss of 3·8 kg Na⁺ ha^?1 year^?1, 5·4 kg Mg²⁺ ha^?1 year^?1, 1·5 kg Ca²⁺ ha^?1 year^?1 and 0·9 kg K⁺ ha^?1 year^?1, while 4·9 kg NO₃–N ha^?1 year^?1 was retained by the ecosystem. Stream water is not suitable for domestic use owing to high manganese and, especially, iron concentrations. Throughfall and stemflow chemistry at a pine stand (Pinus radiata D. Don) and a native forest site (Siempreverde type), both located within the catchment, were compared. Nitrate fluxes within both forest sites were similar (1·3 kg NO₃–N ha^?1 year^?1 as throughfall). Cation fluxes in net rainfall (throughfall plus stemflow) at the pine stand generally were higher (34·8 kg Na⁺ ha^?1 year^?1, 21·5 kg K⁺ ha^?1 year^?1, 5·1 kg Mg²⁺ ha^?1 year^?1) compared with the secondary native forest site (24·7 kg Na⁺ ha^?1 year^?1, 18·9 kg K⁺ ha^?1 year^?1 and 4·4 kg Mg²⁺ ha^?1 year^?1). However, calcium deposition beneath the native forest stand was higher (15·9 kg Ca²⁺ ha^?1 year^?1) compared with the pine stand (12·6 kg Ca²⁺ ha^?1 year^?1). Copyright © 2002 John Wiley & Sons, Ltd.     

Beaver dams along an agricultural stream in southern Ontario,Canada: their impact on riparian zone hydrology and nitrogen chemistry

The hydrology and nitrogen biogeochemistry of a riparian zone were compared before and after the construction of beaver dams along an agricultural stream in southern Ontario, Canada. The beaver dams increased surface flooding and raised the riparian water table by up to 1·0 m. Increased hydraulic gradients inland from the stream limited the entry of oxic nitrate‐rich subsurface water from adjacent cropland. Permeable riparian sediments overlying dense till remained saturated during the summer and autumn months, whereas before dam construction a large area of the riparian zone was unsaturated in these seasons each year. Beaver dam construction produced significant changes in riparian groundwater chemistry. Median dissolved oxygen concentrations were lower in riparian groundwater after dam construction (0·9–2·1 mg L^?1) than in the pre‐dam period (2·3–3·9 mg L^?1). Median NO₃‐N concentrations in autumn and spring were also lower in the post‐dam (0·03–0·07 mg L^?1) versus the pre‐dam period (0·1–0·3 mg L^?1). In contrast, median NH₄‐N concentrations in autumn and spring months were higher after dam construction (0·3–0·4 mg L^?1) than before construction (0·13–0·14 mg L^?1). Results suggest that beaver dams can increase stream inflow to riparian areas that limit water table declines and increase depths of saturated riparian soils which become more anaerobic. These changes in subsurface hydrology and chemistry have the potential to affect the transport and transformation of nitrate fluxes from adjacent cropland in agricultural landscapes. Copyright © 2009 John Wiley & Sons, Ltd.     

Seasonality of epCO2 at different scales along an integrated river continuum within the Dee basin,NE Scotland

Spatial and temporal variations of free CO₂ concentrations in surface waters are mainly controlled by dynamic processes encompassing terrestrial inputs and in‐stream biotic cycling. Free CO₂ can be expressed as ‘excess partial pressure of CO₂’ (epCO₂), indicating supersaturation or under‐saturation with respect to atmospheric CO₂. Seasonal patterns of epCO₂ at sites draining nested upland catchments between 3·40 and 1837 km² were assessed within the River Dee basin in NE Scotland. EpCO₂ values ranged from 0·14 at the lowermost site on the mainstem in autumn to 12·7 on a major tributary during spring. A seasonality index was derived to describe contrasting winter/spring maxima and summer/autumn minima as annual mean epCO₂ values could not clearly distinguish between different sites. Seasonal differences tended to increase downstream as progressive changes in physicochemical conditions enhanced the influence of autotrophic communities. However, perturbations to this continuum occurred as CO₂ inputs from high DOC, heterotrophic tributaries and land‐use changes between open moorland and forest affected downstream continuity. Major tributaries showed reduced differences between seasons compared to the mainstem. Smaller headwaters indicated a lack of seasonality as high connectivity of responsive, peaty soils enabled continual inputs of terrestrially derived free CO₂ to streams concomitant with limited autotrophic CO₂ removal, maintaining epCO₂ > 1 throughout. Seasonality of epCO₂ was mainly driven by the ability of in‐stream biota to consume CO₂ during optimal conditions in summer/autumn. This was confirmed by multiple linear regression analysis, which indicated that, apart from catchment area, baseflow and biotic activity indicators were the best predictors of epCO₂ seasonality characteristics at any particular stage of the river system. Copyright © 2009 John Wiley & Sons, Ltd.     

Dissolved organic carbon export from a cutover and restored peatland

High demand for horticultural peat has increased peatland drainage and peat extraction in Canada. The hydrology and carbon cycling of these cutover peatlands is greatly altered, necessitating active restoration efforts to permit the regeneration of Sphagnum mosses and the re‐establishment of natural peatland function. The effect of peatland extraction and restoration on the export of dissolved organic carbon (DOC) was examined for three successive seasons (May to October, 1999 to 2001) at two different sites (cutover and restored) in eastern Québec. A shift towards higher DOC concentrations was observed following peatland extraction (maximum: 182·6 mg L^?1) and concentrations remained high post‐restoration (maximum: 191·0 mg L^?1). The cutover site exported more DOC than the restored site in all three study seasons. The highest exports occurred during the wettest year (1999), with cutover and restored site export of 10·3 and 4·8 g m^?2, respectively. In 2000, 8·5 g C m^?2 was released from the cutover site, while the restored site released less than half that amount (3·4 g C m^?2). In 2001, the restored site released about the same amount of DOC as in the previous year (3·5 g C m^?2), while the cutover site load dropped to 6·2 g C m^?2. Both sites were net exporters of DOC in all years. Copyright © 2007 John Wiley & Sons, Ltd.     

Reconstructing long‐term records of dissolved CO2

The dissolved CO₂ concentration of stream waters is an important component of the terrestrial carbon cycle. This study reconstructs long‐term records of dissolved CO₂ concentration for the outlets of two large catchments (818 and 586 km²) in northern England. The study shows that:

• 1. The flux of dissolved CO₂ from the catchments (as carbon per catchment area), when adjusted for that which would be carried by the river water at equilibrium with the atmosphere, is between 0 and 0·39 t km⁻² year⁻¹ for the River Tees and between 0 and 0·65 t km⁻² year⁻¹ for the River Coquet.
• 2. The flux of dissolved CO₂ is closely correlated with dissolved organic carbon (DOC) export and is unrelated to dissolved CO₂ export from the headwaters of the study catchments.
• 3. The evasion rate of CO₂ from the rivers (as carbon per stream area) is between 0·0 and 1·49 kg m⁻² year⁻¹, and calculated in‐stream productions of CO₂ are estimated as between 0·5 and 2·5% of the stream evasion rate.
• 4. By mass balance, it is estimated that 8% of the annual flux of DOC is lost within the streams of the catchment.

The study shows that the loss of CO₂ from the streams of the Tees catchment is between 3·1 and 7·5 kt year⁻¹ (as carbon) for the River Tees, which is the same order as annual CH₄ flux from peats within the catchment and approximately 50% of the net CO₂ exchange to the peats of the catchment. Copyright © 2005 John Wiley & Sons, Ltd.     

Zur Rolle der Stickstoffixation im Stoffhaushalt eines geschichteten Sees

In the dimict lake Arend (5.1 km², 146 hm³, 49.5 m z_max), nitrogen is production-limiting with concentrations below the detection limit during the production period. Phytoplankton achieves biomasses of up to 18 mg/l fresh matter, essential contributions being made by Aphanizomenon with 2 mg/l and Anabaena with up to 10 mg/l. Nitrogen fixation was measured by the ethine reduction technique (acetylene reduction) during periods of the occurrence of heterocystforming Cyanophyceae and achieved peak values up to 6.59 μg N² · h^?1 · l^?1 or 14.87 m^?2 · h^?1 g N₂ · m^?2 · h^?1. The rates of fixation show a safe correlation with the biomass of heterocyst-containing Cyanophyceae (r = 0.88), their development beginning at values below the N : P-ratio of 2.66.     

Evidence that piezometers vent gas from peat soils and implications for pore‐water pressure and hydraulic conductivity measurements

Entrapped gas bubbles in peat can alter the buoyancy, storativity, void ratio and expansion/contraction properties of the peat. Moreover, when gas bubbles block water‐conducting pores they can significantly reduce saturated hydraulic conductivity and create zones of over‐pressuring, perhaps leading to an alteration in the magnitude and direction of groundwater flow and solute transport. Some previous researches have demonstrated that these zones of over‐pressuring are not observed by the measurements of pore‐water pressures using open‐pipe piezometers in peat; rather, they are only observed with pressure transducers sealed in the peat. In has been hypothesized that open‐pipe piezometers vent entrapped CH₄ to the atmosphere and thereby do not permit the natural development of zones of entrapped gas. Here we present findings of the study to investigate whether piezometers vent subsurface CH₄ to the atmosphere and whether the presence of piezometers alters the subsurface concentration of dissolved CH₄. We measured the flux of methane venting from the piezometers and also determined changes in pore‐water CH₄ concentration at a rich fen in southern Ontario and a poor fen in southern Quebec, in the summer of 2004. Seasonally averaged CH₄ flux from piezometers was 1450 and 37·8‐mg CH₄ m^?2 d^?1 at the southern Ontario site and Quebec site, respectively. The flux at the Ontario site was two orders of magnitude greater than the diffusive flux at the site. CH₄ pore‐water concentrations were significantly lower in open piezometers than in water taken from sealed samplers at both the Ontario and Quebec sites. The flux of CH₄ from piezometers decreased throughout the season suggesting that CH₄ venting through the piezometer exceeded the rate of methanogenesis in the peat. Consequently we conclude that piezometers may alter the gas dynamics of some peatlands. We suggest that less‐invasive techniques (e.g. buried pressure transducers, tracer experiments) are needed for the accurate measurement of pore‐water pressures and hydraulic conductivity in peatlands with a large entrapped gas component. Furthermore, we argue that caution must be made in interpreting results from previous peatland hydrology studies that use these traditional methods. Copyright © 2009 John Wiley & Sons, Ltd.     

Multi‐decadal water table manipulation alters peatland hydraulic structure and moisture retention

A peatland complex disturbed by berm construction in the 1950s was used to examine the long‐term impact of water table (WT) manipulation on peatland hydraulic properties and moisture retention at three adjacent sites with increasing depth to WT (WET, INTermediate reference and DRY). Saturated hydraulic conductivity (K_s) was found to decrease with depth by several orders of magnitude over a depth of 1–1.5 m at all sites. The depth dependence of WT response to rainfall was similar across sites: WT response increased from 1 : 1 at the surface, to 5 : 1 at 50 cm depth. While surface specific yield (S_y) values were similar across all sites, it decreased with depth at a rate of 0.014 cm^?1 in hollows and 0.007 cm^?1 in hummocks. Bulk density (ρ_b) exhibited similar depth‐dependent trends as S_y and explains a high amount of variance (r² > 0.69) in moisture retention across a range of pore water pressures (?15 to ?500 cm H₂O). Because of higher ρ_b, hollow peat had greater moisture retention, where site effects were minimal. However, the estimated residual water content for surface Sphagnum samples, while on average lower in hummocks (0.082 m³ m^?3) versus hollows (0.087 m³ m^?3), increased from WET (0.058 m³ m^?3) to INT (0.088 m³ m^?3) to DRY (0.108 m³ m^?3) which has important implications for moisture stress under conditions of persistent WT drawdown. Given the potential importance of microtopographic succession for altering peatland hydraulic structure, our findings point to the need for a better understanding of what controls the relative height and proportional coverage of hummocks in relation to long‐term disturbance‐response dynamics. Copyright © 2014 John Wiley & Sons, Ltd.     

The Accumulation of Crimidin in Some Fish Organs

The accumulation of crimidin in total samples and in some organs of five fish species was studied under experimental conditions. The species were Cyprinus carpio, Carassius carassius, Tinca tinca, Scardinus erythrophthalmus and Leucaspius delineatus. Fish were exposed to concentrations of 10 mg · l^?l and 50 mg · l^?1. During short-term experiments water and fish samples were taken at intervals of t = 0, 6, 12, 24, 48 and 72 hours, during the long-term experiment sampling was performed weekly for six weeks. Total samples were analysed for C. carassius, T. tinca, S. erythrophthalmus and L. delineatus. Samples of individual organs and tissues were taken as follows: C. carpio – gills, digestive tract, muscular tissue, kidneys, gonads; C. carassius – gills, digestive tract, muscular tissue, ovaries, testes. Crimidin was determined by gas chromatography. In samples from the short-term experiments at a concentration of 10 mg · l^?1 in the water the amount was roughly 10¹–10 mg · kg^?1, at a concentration of 50 mg · l^?1 in the water roughly 10¹ mg · kg^?1. In most cases the accumulation coefficient was lower than 1.0. The accumulation capacity of individual fish species did not differ greatly. Of the internal organs only the kidneys had a high accumulation capacity, otherwise the highest values were found in muscular tissue and the gills. After one – three weeks the amount of crimidin in most organs falls, and after transfer to clean water there is a general sharp decline. Thus crimidin is not firmly bound in the body.     

Assessing DOC export from a Sphagnum‐dominated peatland using δ13C and δ18O–H2O stable isotopes

Dissolved organic carbon (DOC) originating in peatlands can be mineralized to carbon dioxide (CO₂) and methane (CH₄), two potent greenhouse gases. Knowledge of the dynamics of DOC export via run‐off is needed for a more robust quantification of C cycling in peatland ecosystems, a prerequisite for realistic predictions of future climate change. We studied dispersion pathways of DOC in a mountain‐top peat bog in the Czech Republic (Central Europe), using a dual isotope approach. Although δ¹³C_DOC values made it possible to link exported DOC with its within‐bog source, δ¹⁸O_H2O values of precipitation and run‐off helped to understand run‐off generation. Our 2‐year DOC–H₂O isotope monitoring was complemented by a laboratory peat incubation study generating an experimental time series of δ¹³C_DOC values. DOC concentrations in run‐off during high‐flow periods were 20–30 mg L^?1. The top 2 cm of the peat profile, composed of decaying green moss, contained isotopically lighter C than deeper peat, and this isotopically light C was present in run‐off in high‐flow periods. In contrast, baseflow contained only 2–10 mg DOC L^?1, and its more variable C isotope composition intermittently fingerprinted deeper peat. DOC in run‐off occasionally contained isotopically extremely light C whose source in solid peat substrate was not identified. Pre‐event water made up on average 60% of the water run‐off flux, whereas direct precipitation contributed 40%. Run‐off response to precipitation was relatively fast. A highly leached horizon was identified in shallow catotelm. This peat layer was likely affected by a lateral influx of precipitation. Within 36 days of laboratory incubation, isotopically heavy DOC that had been initially released from the peat was replaced by isotopically lighter DOC, whose δ¹³C values converged to the solid substrate and natural run‐off. We suggest that δ¹³C systematics can be useful in identification of vertically stratified within‐bog DOC sources for peatland run‐off.     

Der Einfluß von Erdölkohlenwasserstoffen auf die Primärproduktion des Donauplanktons

Danube river water samples were saturated with mineral oil, and then the primary production (gross) was determined by means of the light-dark bottle method (oxygen) in situ at depths of 0.1… 1.5 m in comparison with untreated samples. Samples were exposed for half a solar day alternately during the first and second half-days. Investigations carried out between March and October for periods of 14 days showed a mean production of 2.56 g · m^?2d^?1 O₂, which was reduced by 36% due to 12.6 mg/l hydrocarbons. Production decreases with the water depth from 6.2 to 0.9 mg · l^?1d^?1 0₂, whereas the inhibition by hydrocarbons increases from 31 to 41%.     

Seasonal variation in carbon exchange and its ecological analysis over <Emphasis Type="Italic">Leymus chinensis</Emphasis> steppe in Inner Mongolia

Eddy covariance technique was used to measure carbon flux during two growing seasons in 2003 and 2004 over typical steppe in the Inner Mongolia Plateau, China. The results showed that there were two different CO₂ flux diurnal patterns at the grassland ecosystem. One had a dual peak in diurnal course of CO₂ fluxes with a depression of CO₂ flux after noon, and the other had a single peak. In 2003, the maximum diurnal uptake and emitting value of CO₂ were ?7.4 and 5.4 g·m^?2·d^?1 respectively and both occurred in July. While in 2004, the maximum diurnal uptake and release of CO₂ were ?12.8 and 5.8 g·m^?2·d^?1 and occurred both in August. The grassland fixed 294.66 and 467.46 g CO₂·m^?2 in 2003 and 2004, and released 333.14 and 437.17 g CO₂·m^?2 in 2003 and 2004, respectively from May to September. Water availability and photosynthetic active radiation (PAR) are two important factors of controlling CO₂ flux. Consecutive precipitation can cause reduction in the ability of ecosystem carbon exchange. Under favorable soil water conditions, daytime CO₂ flux is dependent on PAR. CO₂ flux, under soil water stress conditions, is obviously less than those under favorable soil water conditions, and there is a light saturation phenomena at PAR=1200 μmol·m^?2·s^?1. Soil respiration was temperature dependent when there was no soil water stress; otherwise, this response became accumulatively decoupled from soil temperature.     

An automated salt‐tracing gauge for flow‐velocity measurement

This article introduces the SVG (salt‐velocity gauge), a novel automated technique for measuring flow velocity by means of salt tracing. SVG allows a high measuring rate (up to one every 2 seconds), short control section length (down to 10 cm), high accuracy (+[sol ]?1·5 cm s^?1), and unbiased calculation of the mean velocity in experimental conditions with turbulent, supercritical flow. A few cubic centimetres of saturated salt solution (NaCl) are injected into the flow at regular time intervals using a programmable solenoid valve. The tracer successively passes two conductivity probes placed a short distance downstream. The transformation of the signal between the two probes is modelled as a one‐dimensional diffusion wave equation. Model calibration gives an estimation of the mean velocity and the diffusion for each salt plume. Two implementations of the SVG technique are described. The first was an outdoors simulated rainfall experiment in Senegal (conductivity probes at 40 cm apart, 8 Hz measurement rate, salt injections at 10 second intervals). Mean velocity was estimated to range between 0·1 and 0·3 m s^?1. The second was a laboratory‐based flume experiment (conductivity probes at 10 cm apart, 32 Hz, salt injections at 2 second intervals). Another SVG with probes at 34 cm apart was used for comparison. An acoustic Doppler velocimeter (ADV) was also used to give an independent assessment of velocity. Using the 10 cm salt gauge, estimated mean velocity ranged from 0·6 to 0·9 m s^?1 with a standard deviation of 1·5 cm s^?1. Comparisons between ADV, 10 cm SVG and 34 cm SVG were consistent and demonstrated that the salt‐tracing results were unbiased and independent of distance between probes. Most peaks were modelled with r² > 90 per cent. The SVG technology offers an alternative to the dye‐tracing technique, which has been severely criticized in the literature because of the wide interval of recommended values for the correction factor α to be applied to the timings. This article demonstrates that a fixed value of α is inappropriate, since the correction factor varies with velocity, diffusion and the length of the control section. Copyright © 2005 John Wiley & Sons, Ltd.     

The influence of diurnal temperatures on the hydrochemistry of a tufa‐depositing stream

At‐a‐station diurnal variations in carbonate hydrochemistry were measured during four observation periods at Davys Creek, a tufa‐depositing stream in central NSW, Australia. Major ion concentrations and continuously logged measurements of specific conductivity, pH and temperature showed that changes in the amount of CaCO₃ deposited upstream of the study reach were directly related to changes in diurnal water temperatures, which control the rate of CO₂ efflux to the atmosphere. The greatest upstream losses occurred during the mid‐afternoon water temperature peak, whereas the lowest upstream losses occurred at sunrise, when water temperatures were at their lowest. Cloudy days at all times of the year produced small diurnal water temperatures ranges (c. 2–5°C) and, consequently, relatively small changes in upstream CaCO₃ loss (23–50 mg L^?1) through the day. Clear sunny days, especially during summer months, produced large diurnal water temperature changes (up to c. 11°C), which in turn triggered diurnal changes in upstream CaCO₃ loss of up to 100 mg L^?1. By implication, the active reach of tufa deposition must advance downstream and increase in length during the evening and vice versa during the day. Given that the temperature of Davys Creek waters are a function of insolation, changes in the reach of tufa deposition under baseflow conditions are a direct function of the prevailing weather. This has implications for the palaeoclimatic interpretation of fossil tufa deposits. Copyright © 2003 John Wiley & Sons, Ltd.     

CO<Subscript>2</Subscript> flux evaluation over the evergreen coniferous and broad-leaved mixed forest in Dinghushan,China

The Dinghushan flux observation site, as one of the four forest sites of ChinaFLUX, aims to acquire long-term measurements of CO₂ flux over a typical southern subtropical evergreen coniferous and broad-leaved mixed forest ecosystem using the open path eddy covariance method. Based on two years of data from 2003 to 2004, the characteristics of temporal variation in CO₂ flux and its response to environmental factors in the forest ecosystem are analyzed. Provided two-dimensional coordinate rotation, WPL correction and quality control, poor energy-balance and underestimation of ecosystem respiration during nighttime implied that there could be a CO₂ leak during the nighttime at the site. Using daytime (PAR > 1.0 μmol^?1·m^?2·s^?1) flux data during windy conditions (u* > 0.2 m·s^?1), monthly ecosystem respiration (Reco) was derived through the Michaelis-Menten equation modeling the relationship between net ecosystem C0₂ exchange (NEE) and photosynthetically active radiation (PAR). Exponential function was employed to describe the relationship between Reco and soil temperature at 5 cm depth (Ts05), then Reco of both daytime and nighttime was calculated respectively by the function. The major results are: (i) Derived from the Michaelis-Menten equation, the apparent quantum yield (α) was 0.0027±0.0011 mgCO₂·μmol^?1 photons, and the maximum photosynthetic assimilation rate (Amax) was 1.102±0.288 mgCO₂·m^?2·s^?1. Indistinctive seasonal variation of α or Amax was consistent with weak seasonal dynamics of leaf area index (LAf) in such a lower subtropical evergreen mixed forest, (ii) Monthly accumulated Reco was estimated as 95.3±21.1 gC·m^?2mon^?1, accounting for about 68% of the gross primary product (GPP). Monthly accumulated WEE was estimated as ?43.2±29.6 gC·m^?2·mon^?1. The forest ecosystem acted as carbon sink all year round without any seasonal carbon efflux period. Annual NEE of 2003 and 2004 was estimated as ?563.0 and ?441.2 gC·m^?2·a^?1 respectively, accounting for about 32% of GPP.     

Geochemistry of meteoric diagenesis in carbonate islands of the northern Bahamas: 1. Evidence from field studies

Processes driving carbonate diagenesis in islands of the northern Bahamas are investigated using major ion, dissolved oxygen and dissolved organic carbon analyses of water samples from surface and ground waters, and measurements of soil gas P. Meteoric waters equilibrate with aragonite, but reactions are water controlled rather than mineral‐controlled and drive dissolution rather than concurrent precipitation of calcite. Surface runoff waters equilibrate with atmospheric P and rapidly recharge the vadose zone, limiting subaerial bedrock dissolution to only 6·6–15 mg l^?1 Ca. P of soil gas measured in the summer wet season ((7·4 ± 3·7) × 10^?3 atm) is elevated compared with that of the atmosphere, despite the thin skeletal organic nature of the soil and the discontinuous soil cover. Soil waters retained in surface pockets are equilibrated with respect to aragonite and have dissolved 51 ± 19 mg l^?1 Ca. This is substantially less than the 93 ± 18 mg l^?1 Ca in samples from pumping boreholes that sample meteoric waters from the freshwater lens. The high P of the freshwater lens ((16 ± 8·3) × 10^?3 atm for pumping boreholes) suggests that significant additional CO₂ may be derived by oxidation of soil‐ and surface‐derived organic carbon within the lens. The suboxic nature of the majority of the freshwater lens and the observed depletion in sulphate support this suggestion, and indicate that both aerobic and anaerobic oxidation may take place. Shallow lens samples from observation boreholes are calcite supersaturated and have a lower P than deeper lens waters, indicating that CO₂ degasses from the water table, driving precipitation of calcite cements. We suggest that the geochemical evolution of waters in the vadose zone and upper part of the freshwater lens may be determined by the presence of a body of ground air with P controlled by production in the freshwater lens and soil and by degassing to the atmosphere. Copyright © 2007 John Wiley & Sons, Ltd.     

Hydrograph separation to improve understanding of Dissolved Organic Carbon Dynamics in Headwater catchments

Dissolved organic carbon (DOC) is a key component of the global carbon cycle, but, to date, large uncertainties still exist on its source and fate in first‐order streams. In a 23 ha rangeland and steep‐slope headwater of South Africa, our aim was to quantify the contribution of overland flow (OF), soil water (SW) and ground water (GW) to DOC fluxes (DOC_F), and to interpret the results in terms of DOC sources and fate. The average 2010–2011 DOC concentration (DOC_C) at the catchment outlet was 4.7 mg C l^?1 with a standard error of ±2.5 mg C l^?1, which was significantly lower than in SW (15.2 ± 1.6 mg C l^?1) and OF (11.9 ± 0.8 mg C l^?1), but higher than in GW (2.3 ± 0.6 mg C l^?1). Based on end‐member mixing using Si and Na concentration in the water compartments, the average SW contribution to DOC_F was 66.4%, followed by OF (30.0%) and GW (3.6%). The resulting estimated DOC_F at the catchment outlet was 8.05 g C m² y^?1. This was much higher than the observed value of 2.80 g C m² y^?1, meaning that 5.25 g C m² y^?1 or 65% of the DOC is lost during its downslope and/or downstream transport to the catchment outlet. Complementary investigations revealed that the DOC_C in SW dropped from 15.2 ± 1.6 to 2.6 ± 0.3 mg C l^?1 during its downslope transport to the river system, which corresponded to a net loss of 5.10 g C m² y^?1, or 97% of the catchment DOC losses. These results on DOC sources and potential fate in headwaters are expected to improve our understanding of the impact of hydrology on the global C‐cycle. Copyright © 2013 John Wiley & Sons, Ltd.