Spatial and temporal dynamics of stream water particulate and dissolved N, P and C forms along a catchment transect, NE Scotland

The dynamics of dissolved and particulate N, P and organic C were examined for field drains, through a headwater (4 km²), into a mesoscale stream (51 km²) and river (1844 km²) catchment. Distributions of N and P forms were similar in the agricultural headwater and field drains; annual P fluxes of particulate and dissolved forms were of equal magnitude, whilst N was dominated by NO₃–N. Across all scales organic P was an important, often dominant, component of the dissolved P. Temporal variation in nutrient concentrations and proportions was greatest in the headwater, where storms resulted in the generation of large concentrations of suspended particulate matter, particulate and dissolved P, particularly following dry periods. The data suggest that groundwater and minor point source inputs to the mesoscale catchment buffered the temporal variability in hydrochemistry relative to the headwater. Summer low flows were associated with large PO₄–P concentrations in the mesoscale catchment at a critical time of biological sensitivity. At the largest river catchment scale, organic forms of C, N and P dominated. Inorganic nutrient concentrations were kept small through dilution by runoff from upland areas and biological processes converted dissolved N and P to particulate forms. The different processes operating between the drain/headwater to the large river scale have implications for river basin management. Given the prevalence of organic and particulate P forms in our catchment transect, the bioavailability of these fractions needs to be better understood.     

Spatial contributions of diffuse inputs and within-channel processes to the form of stream water phosphorus over storm events

Concentrations of suspended particulate matter (SPM), NO₃-N and P fractions: PO₄-P, dissolved organic P (DOP), particulate P (PP) and bioavailable exchangeable P were examined over 5 storm events in two nested agricultural catchments in NE Scotland: a (51 km²) catchment and its headwater (4 km²). NO₃-N showed anticlockwise hysteresis for all storms in both catchments. In contrast, the headwater showed strong clockwise hysteresis of SPM, dissolved and particulate P concentrations, but which weakened through summer to spring. Less pronounced hysteresis of P forms in the larger catchment was attributed to a combination of factors: a less energetic system, nutrient leaching from the floodplain, a point source of a small sewage treatment works and the occurrence of coarser soil and sediment parent materials with less P adsorption and transport capacity. The headwater exhibited a strong ‘first flush’ effect of sediment and dissolved P, particularly following dry conditions, received a significant transfer of readily-solubilized organic P from the surrounding soils in late summer and after manure applications in winter, and was the likely cause of large sediment associated P signals observed in the 51 km² catchment. Our results suggest that steeper gradient headwaters should be targeted for riparian improvements to mitigate soil erosion from headwater fields. The efficiency of riparian erosion controls is also dependant on the size of the store of fine sediment material within the stream channel and this may be large.     

Geomorphology and vegetation drive hydrochemistry changes in two Northeast Greenland streams

Climate change is causing drastic landscape changes in the Arctic, but how these changes modify stream biogeochemistry is not clear yet. We examined how catchment properties influence stream nitrogen (N) and dissolved organic carbon concentrations (DOC) in a high-Arctic environment. We sampled two contrasting headwater streams (10–15 stations over 4.8 and 6.8 km, respectively) in Northeast Greenland (74°N). We characterized the geomorphology (i.e., bedrock, solifluction and alluvial types) and the vegetation (i.e., barren, fell field, grassland and tundra types) cover of each subcatchment area draining into each sampling station and collected water samples for hydrochemistry characterization. The two sampled streams differed in geomorphology and vegetation cover in the catchment. Aucellaelv catchment was mostly covered by a ‘bedrock’ geomorphology (71%) and ‘fellfield’ vegetation (51%), whereas Kæerelv was mostly covered by ‘alluvial’ geomorphology (65%) and ‘grassland’ and ‘tundra’ vegetation (42% and 41% respectively). Hydrochemistry also differed between the two study streams, with higher concentrations of inorganic N forms in Aucellaelv and lower DOC concentrations, compared to Kærelv. The results from the linear mixed model selection showed that vegetation and geomorphology had contrasting effects on stream hydrochemistry. Subcatchments with higher solifluction sheets and limited vegetation had higher nitrate concentrations but lower DOC concentrations. Interestingly, we also found high variability on the production and removal of nitrate across subcatchments. These results indicate landscape controls to nutrient and organic matter exports via flow paths, soil organic matter stocks and nutrient retention via terrestrial vegetation. Moreover, the results suggest that climate change induced alterations to vegetation cover and soil physical disturbance in high-Arctic catchments will affect stream hydrochemistry, with potential effects in stream productivity, trophic relations as well as change of solute export to downstream coastal areas.     

Stable hydrogen and oxygen isotope studies of rainfall and streamwaters for two contrasting holm oak areas of Catalonia, northeastern Spain

Results are presented of a study of stable hydrogen and oxygen isotopes in rainfall and streamwaters for the Montseny and Prades areas in northeastern Spain: results cover the full year of 1991. The isotopic pattern for rainfall is similar for both areas: there is a wide range in isotopic contents and the results show a strong, near-linear trend, δ²H = 7.9 × δ¹⁸O + 9.8 (N = 59; r² = 0.952), the ‘local meteoric line’. There is slight curvature to the data which may be related to the sources of water vapour forming the rainfall. Within the streams, the isotopic variability is much less than that of the rainfall although the data lie on, or very near to, the meteoric line. Data for detailed collections during storm events show more scatter than those collected regularly on a fortnightly basis. The event data show a linear feature that conforms to the local meteoric line. These results indicate that: (1) the main supply of water to the stream stormflow comes from water stored in the catchment prior to the event; (2) waters of more than one isotopic composition reside within the catchment and are transferable to the stream during storm events; (3) the main process of water transfer from the catchment back to the atmosphere comes from transpiration by the trees and (possibly) complete evaporation from the near-surface soil horizons and the tree canopy; (4) the isotopic technique cannot be used for quantitative hydrograph separation in this instance — at least two water types can be present within the catchment at any given time.     

Chemistry of streams draining grassland and forest catchments at Plynlimon,mid-Wales

Abstract

The chemistry of streamwater, bulk precipitation, throughfall and soil waters has been studied for three years in two plantation forest and two moorland catchments in mid-Wales. Na and CI are the major ions in streamwater reflecting the maritime influence on atmospheric inputs. In all streams, baseflow is characterised by high pH waters enriched in Ca, Mg, Si and HCO³. Differences in baseflow chemistry between streams reflect the varying extent of calcite and base metal sulphide mineralization within the catchments. Except for K, mean stream solute concentrations are higher in the unmineralized and mineralized forest catchments compared with their respective grassland counterparts. In the forest streams, storm flow concentrations of H⁺ are approximately 1.5 times and Al four times higher than in the moorland streams. Annual catchment losses of Na, Cl, SO⁴, NO³, Al and Si are greatest in the forest streams. In both grassland and forest systems, variations in stream chemistry be explained by mixing waters from different parts of the catchment, although NO³ concentrations may additionally be controlled by N transformations occurring between soils and streams. Differences in stream chemistry and solute budgets between forest and moorland catchments are related to greater atmospheric scavenging by the trees and changes in catchment hydrology consequent on afforestation. Mineral veins within the catchment bedrock can significantly modify the stream chemical response to afforestation.     

Changes in dissolved organic matter (DOM) amount and composition along nested headwater stream locations during baseflow and stormflow

Amount and composition of dissolved organic matter (DOM) were evaluated for multiple, nested stream locations in a forested watershed to investigate the role of hydrologic flow paths, wetlands and drainage scale. Sampling was performed over a 4‐year period (2008–2011) for five locations with drainage areas of 0.62, 3.5, 4.5, 12 and 79 ha. Hydrologic flow paths were characterized using an end‐member mixing model. DOM composition was determined using a suite of spectrofluorometric indices and a site‐specific parallel factor analysis model. Dissolved organic carbon (DOC), humic‐like DOM and fluorescence index were most sensitive to changes with drainage scale, whereas dissolved organic nitrogen, specific UV absorbance, Sr and protein‐like DOM were least sensitive. DOM concentrations and humic‐like DOM constituents were highest during both baseflow and stormflow for a 3.5‐ha catchment with a wetland near the catchment outlet. Whereas storm‐event concentrations of DOC and humic DOM constituents declined, the mass exports of DOC increased with increasing catchment scale. A pronounced dilution in storm‐event DOC concentration was observed at peak stream discharge for the 12‐ha drainage location, which was not as apparent at the 79‐ha scale, suggesting key differences in supply and transport of DOM. Our observations indicate that hydrologic flow paths, especially during storms, and the location and extent of wetlands in the catchment are key determinants of DOM concentration and composition. This study furthers our understanding of changes in DOM with drainage scale and the controls on DOM in headwater, forested catchments. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantifying groundwater discharge to a small perennial stream in southern Ontario, Canada

A study of the interaction between groundwater and surface water was undertaken within a small agricultural watershed in southern Ontario, Canada. Groundwater contributions to streamflow were measured along a section of stream during baseflow conditions and during rainfall events. Four techniques were used to estimate the contribution of groundwater to the stream along a 450 m reach (three during baseflow and one during stormflow conditions). Under baseflow conditions, streamflow measurements using the velocity–area technique indicated that the net groundwater flux to the stream during the summer months was 10 ml s⁻¹ m⁻¹. Hydrometric measurements (i.e. hydraulic gradient and hydraulic conductivity) taken using mini-piezometers installed in the sediments beneath the stream resulted in net groundwater flux estimates that were four to five times lower. Seepage meters failed to provide any measurements of water flux into or out of the stream. Therefore, based on these results, the velocity–area technique gives the best estimate of groundwater discharge. Hydrograph separations were conducted using isotopic ratios and electrical conductivity on two large rainfall events with different antecedent moisture conditions in the catchment. Both events showed that pre-event water (generally considered groundwater) dominated streamflow and tile drain flow with 64%–80% of the total discharge contributed by pre-event water. High water table conditions within the catchment resulted in greater stream discharge and a greater contribution of event water in the streamflow than that observed under low water table conditions for similar intensity storm events. The results also showed that differences in riparian zone width, vegetation and surface saturation conditions between the upper and lower catchment can influence the relative magnitude of streamflow response from the two catchment areas.     

Glacier hydrochemistry, solute provenance, and chemical denudation at a surge-type glacier in Kuannersuit Kuussuat, Disko Island, West Greenland

In 1995–1998, Han 11 km terrestrial surge of Kuannersuit Glacier, an outlet glacier of the largest ice cap on Disko Island, West Greenland, affected the catchment dramatically. In order to estimate solute fluxes and provenances, bulk meltwaters were sampled at the main subglacial outlet during the initial part of the quiescent phase. The hydrochemistry is significantly influenced by a subglacial basaltic weathering regime with absence of carbonate minerals. The results show that marine and aerosol derived solutes have minimal contribution to the total ion content, whereas sequestration of atmospheric CO₂ associated with carbonation of Ca-rich feldspar and reactive volcanic glass is more dominant than previously reported from glacierized catchments. Application of a sampling strategy dividing water samples into four groups to determine the content of dissolved HCO₃⁻ and CO₃²⁻ shows that the cationic equivalent weathering rate range is 683–860 Σmeq⁺ m⁻² a⁻¹ and solute flux ranges between 76 and 98 t km⁻² a⁻¹. The crustal denudation rate is estimated to 26 t km⁻² a⁻¹, and the transient CO₂ drawdown amounts to 8500–13700 kg C km⁻² a⁻¹.     

Combined use of spot samples and continuous integrated sampling in a study of storm runoff from a lowland catchment in the south of England

We have used two different sampling techniques to study the geochemical response of a small lowland rural catchment to episodic storm runoff. The first method involves traditional daily spot sampling and has been used to develop a standard end‐member mixing analysis (EMMA) of the relative contributions of ground water flow and surface runoff to the total stream flow. The second method utilizes a continuous sampling device, powered by an osmotic pump, to produce an integrated 24‐h sample of the stream flow. When combined with the EMMA results from the spot samples, analyses of the integrated samples reveal the presence of a third component that makes a significant contribution to the dissolved NO₃, Ca and K export from the catchment during the rising limb of the hydrographic profile of a storm event following a prolonged dry period. The storm occurred in the middle of the night, so that the response of the stream chemistry was not captured by the daily samples. We hypothesize that this third component is derived from the flushing of stored soil water that contains the geochemical signature of decaying vegetation. Copyright © 2011 John Wiley & Sons, Ltd.     

Sources of dissolved organic carbon during stormflow in a headwater agricultural catchment

Increasing dissolved organic carbon (DOC) concentrations have been reported during the last 15 years in streams from the United Kingdom, Northern Europe and North America. Identifying the sources of DOC and the controls of the delivery to the stream is important to understand the significance of these trends. This relies on the availability of observations of DOC dynamics during storm events, since much of the DOC export from soils to streams occurs during high flows. This study analyses DOC data for eight storm events during winter 2005–2006 in a small agricultural experimental catchment—the Kervidy‐Naizin experimental catchment—located in Western France. A four end‐member mixing approach was applied to the eight monitored storm events to identify DOC sources and quantify their respective contribution to DOC stream fluxes, using DOC, nitrate, sulphate and chloride as tracers. The results show that DOC concentrations in the stream at the outlet of this catchment increase markedly during storm events. The slope of the linear regression between DOC concentration and discharge was not constant for the eight events and depended on pre‐event hydrological conditions. Between 64 and 86% of the DOC that enter the stream during storms originated from the upper layers of the riparian wetland soils. The variation of the delivery of DOC seems to be controlled by hydrological processes only, the wetland soils acting as a non‐limiting store. Copyright © 2009 John Wiley & Sons, Ltd.     

The effects of antecedent moisture conditions on the relationship of hydrology to hydrochemistry in a small forested watershed

The relationship between stream discharge and the concentration of different solutes reflects the hydrological processes occurring in a catchment, but is also strongly affected by antecedent moisture conditions. In this study, the effects of antecedent conditions were examined for a small forested watershed near Montréal (Québec) by comparing data from sequences of fall events for two years of contrasting antecedent moisture conditions. The relationship between discharge and stream water chemistry was also quantified using cross‐correlation analysis. Results show that concentrations of dissolved organic carbon, nitrate and calcium increased during storm events with dry antecedent conditions but that the relationships were weaker under wet conditions. For both years, concentrations of sulfate, silicon and sodium and the pH were negatively correlated with discharge (cross‐correlations from −0·20 to −0·53). With dry antecedent conditions, there was a general decrease in the concentration of all solutes with time, whereas concentrations remained at about the same level under wet conditions. The concentration–discharge relationships for sulfate shifted frequently from higher concentrations during the rising limb to higher concentrations during the falling limb from one event to another as a result of changing antecedent conditions. Although the contrast in antecedent moisture conditions between the two years was moderate in comparison with that reported in other studies (25% dryer than average in 1995 and 12% wetter than average in 1996), the hydrochemistry of the stream was markedly different. Copyright © 1999 John Wiley & Sons, Ltd.     

Export of Acidity in Drainage Water from Acid Sulphate Soils

Disturbed acid sulphate soils are potent sources of acidity in coastal waterways. Monitoring studies of the drainage water for sites at East Trinity, Cairns and Pimpama, south-east Queensland indicate that considerable acidity is found in the drainage water from these sites. Hydrogen (H⁺), ferrous (Fe²⁺) and aluminium (Al) ions are the dominant acid cations involved. When drainage water is mixed with fresh or marine waters the effect of H⁺ on acidity generation is immediate. Aluminium can release acidity on hydrolysis, while the oxidation of Fe²⁺ to Fe³⁺ both acidifies and removes dissolved oxygen from the water. Strongly acidic waters with low levels of dissolved oxygen concentration are undesirable for most forms of aquatic life. Export of acidity from acid sulphate soil is likely to have a major effect on inshore fisheries and breeding grounds especially in periods of flood following drought or periods of low rainfall, where large volumes of acidity can be flushed/leached into sensitive aquatic/marine habitats. Impacts may include low dissolved oxygen, fish kills, epizootic ulceration syndrome and damage to oysters. During the processes of oxidation and hydrolysis, iron and aluminium flocs form, that can smother benthic communities. Heavy metals are found in the drainage water at elevated levels and may also be of concern for aquatic organisms. Chronic effects such as habitat degradation, mortality of marine worms, bivalves, invasion of acid tolerant species (both plant and animal) and avoidance of habitat have been documented elsewhere. These areas require further research.     

Influence of sea salt on stream water chemistry in an upland afforested catchment

The chemistry of bulk precipitation and stream water was monitored in an acidic afforested catchment at Llyn Brianne in upland Wales between 1985 and 1990. Throughfall, stemflow and soil water chemistry were also monitored between 1988 and 1989. Marine-derived solutes dominated the ionic composition of precipitation and stream water, which had mean Cl concentrations of 113 μequiv. 1^?1 and 245 μequiv. 1^?1, respectively. The higher concentrations in stream water reflect occult and dry deposition on the forest canopy and the effect of interception and transpiration losses. Chloride variations in stream water (112-454μequiv. 1^?1) were damped compared with bulk precipitation (28-762μequiv. 1^?1) due to the mixing of event (‘new’) water with pre-event (‘old’) water in the catchment soils. A storm episode monitored in the catchment in April 1989 was associated with high sea salt inputs and Cl concentrations in throughfall (1466μequiv. 1^?1) and storm runoff were exceptionally high (392μequiv. 1^?1). The Cl signal in stream water during the episode was consistent with an event (‘new’) water contribution to the storm response. However, a short-term hydrochemical budget estimated that although Cl outputs from the catchment during the event (1.17 kg ha^?1) were equivalent to 8% of inputs in throughfall and stemflow, the storm runoff was equivalent to 32% of effective precipitation. This indicates that pre-event (‘old’) water was the dominant source (> 75%) of storm runoff. Although sea salt inputs during the event had a marked impact on stream water chemistry, the anomalously high levels of acidity sometimes associated with sea salt events were not observed in this particular study.     

Metals discharged during different flow conditions from a mixed agricultural‐forest catchment (NW Spain)

Metal loads were determined from water samples collected under different streamflow conditions (baseflow and storm events) in a rural catchment (NW Spain) during 4 years. A study at annual, seasonal and storm‐event scales was carried out. In all analysed scales, the export order was Fe > Al > Mn > Zn > Cu. A high inter‐annual, seasonal and storm‐event scale variability of metal load was observed. The total metal loads in stream were higher during baseflow conditions than during storm events, which only represented 4% of the duration of the study period and 25% of streamflow. During storm events, both Al and Fe loads accounted 45% of the total load of the study period, whereas Mn, Cu and Zn loads represented 42%, 33% and 24%, respectively. This highlights the role of high flows on metal export. Only four big events exported around 30% of load of each metal transported in events. At all time scales, a prevalence of export of particulate metals over dissolved metals was observed, more pronounced for Al, Fe and Mn than for Cu and Zn. The export of metals in the Corbeira catchment is influenced by runoff and, to a lesser extent, by the rainfall amount. Copyright © 2014 John Wiley & Sons, Ltd.     

Dissolved organic carbon dynamics in two streams draining forested catchments at loch ard,Scotland

Dissolved organic carbon (DOC) was measured at four or eight hour intervals between mid-1989 and mid-1991 in two catchments in west central Scotland. The experimental catchment had been recently clear-felled and the control remained under forest. The amount of DOC varied during individual storm events following the stream hydro-graph. Maximum variations were found in the summer half-year and in the clear-felled catchment. There was also evidence of the exhaustion of DOC in the later events of a sequence. Differences between the catchments were related to catchment characteristics and to land-use change. The reduced magnitude of variation in DOC with discharge in the control stream was due to the influence of a wetland area through which the stream flowed. The mean DOC concentrations were similar in the two streams and annual exports were 15 g m^?2 from the control and 16g m^?2 from the felled catchment. The stream draining the clear-felled catchment had greater high flow DOC concentrations in the summer half-year, probably due to the effect of greater mean summer temperatures on DOC release and of the greater supply of organic debris in the stream channel.     

Spatial delineation of groundwater–surface water interactions through intensive in‐stream profiling

The dominance of ‘old’ pre‐event water in headwater storm runoff has been recorded in numerous upland catchment studies; however, the mechanisms by which this pre‐event water enters the stream channel are poorly understood. Understanding these processes is fundamental to determining the controls on surface water quality and associated impacts on stream ecology. Previous studies in the upland forested catchment of the Afon Hafren (River Severn) at Plynlimon, mid‐Wales, identified an active bedrock groundwater system that was discharging into the stream channel during storm response. Detailed analysis showed that these discharges were small and could not account for the majority of pre‐event storm water response identified at this site; pre‐event storm runoff had to be sourced predominantly from further upstream. An intensive stream survey was used to determine the spatial nature of groundwater–surface water (GW–SW) interactions in the Hafren Catchment. Detailed physico‐chemical in‐stream profiling identified a marked change in water quality indicating a significant discrete point of bedrock groundwater discharge upstream of the Hafren Transect study site. The in‐stream profiling showed the importance of high spatial resolution sampling as a key to understanding processes of GW–SW interaction and how quick and cost‐effective measurements of specific electrical conductance of stream waters could be used to highlight in‐stream heterogeneity. This approach is recommended for use in headwater catchments for initial characterisation of the stream channel in order to better locate instrumentation and to determine more effective targeted sampling protocols in upland catchment research. Copyright © 2012 John Wiley & Sons, Ltd.     

Groundwater flushing of solutes at wetland and hillslope positions during storm events in a small glaciated catchment in western New York,USA

While the role of groundwater in flushing of solutes has long been recognized, few studies have explicitly studied the within‐event changes in groundwater chemistry. We compared the changes in groundwater chemistry during storm events for a wetland and hillslope position in a small (1·5 ha) glaciated, forested catchment in western New York. Flushing responses for dissolved organic carbon (DOC) and nitrogen (DON), nitrate (NO₃) and sulfate (SO₄) in wetland and hillslope groundwaters were also compared against the corresponding responses in stream water. Eight storm events with varying intensity, amount, and antecedent moisture conditions were evaluated. Solute flushing patterns for wetland and hillslope groundwaters differed dramatically. While DOC concentrations in wetland groundwater followed a dilution trend, corresponding values for hillslope groundwater showed a slight increase. Concentrations for NO₃ in wetland groundwater were below detection limits, but hillslope groundwaters displayed high NO₃ concentrations with a pronounced increase during storm events. Flushing responses at all positions were also influenced by the size of the event and the time between events. We attributed the differences in flushing to the differences in hydrologic flow paths and biogeochemical conditions. Flushing of the wetland did appear to influence storm‐event stream chemistry but the same could not be said for hillslope groundwaters. This suggests that while a variety of flushing responses may be observed in a catchment, only a subset of these responses affect the discharge chemistry at the catchment outlet. Copyright © 2009 John Wiley & Sons, Ltd.     

Measurement of soluble reactive phosphorus concentration profiles and fluxes in river-bed sediments using DET gel probes

DET (diffusive equilibrium in thin films) gel probes were used for sampling river-bed sediment porewaters, to characterise in situ soluble reactive phosphorus (SRP) concentration profiles and fluxes. DET probes were deployed in three contrasting rural streams: (1) a headwater ‘pristine’ stream, with minimal P inputs from low intensity grassland and no point sources, (2) an intensively cultivated arable catchment, and (3) a stream subject to high P loadings from sewage effluent and intensive arable farming. The DET results showed highly enriched porewater SRP concentrations of between ca. 400 and 5000 μg-P l⁻¹ in the sewage-impacted stream. In contrast, the arable and pristine streams had porewater SRP concentrations <70 μg-P l⁻¹ and <20 μg-P l⁻¹, respectively. Porewater SRP concentration profiles in both the sewage-impacted and arable-impacted streams showed well-defined vertical structure, indicating internal sources and sinks of SRP within the sediment. However, there was little variability in porewater SRP concentrations in the pristine stream. The DET porewater profiles indicated net diffusion of SRP (a) from the overlying river water into the surface sediment and (b) from subsurface sediment upwards towards the sediment–water interface. A mass balance for the sewage-impacted site showed that the influx of SRP into the surface sediments from the overlying river water was small (ca. 1% of the daily river SRP load). The DET results indicated that, in the arable and sewage-impacted streams, the surface ‘cap’ of fine sediment may play an important role in inhibiting upward movement of SRP from subsurface porewaters into the overlying river water, under steady-state, low-flow conditions.     

Fluxes of inorganic carbon from two forested catchments in the Appalachian mountains

This study uses long‐term records of stream chemistry, discharge and air temperature from two neighbouring forested catchments in the southern Appalachians in order to calculate production of dissolved CO₂ and dissolved inorganic carbon (DIC). One of the pair of catchments was clear‐felled during the period of the study. The study shows that: (1) areal production rates of both dissolved CO₂ and DIC are similar between the two catchments even during and immediately after the period of clear‐felling; (2) flux of total inorganic carbon (dissolved CO₂+ DIC) rises dramatically in response to a catchment‐wide acidification event; (3) DIC and dissolved CO₂ are dominantly released on the old water portion of the discharge and concentrations peak in the early autumn when flows in the study catchments are at their lowest; (4) total fluvial carbon flux from the clear‐felled catchment is 11·6 t km⁻² year⁻¹ and for the control catchment is 11·4 t km⁻² year⁻¹. The total inorganic carbon flux represents 69% of the total fluvial carbon flux. The method presented in the study provides a useful way of estimating inorganic carbon flux from a catchment without detailed gas monitoring. The time series of dissolved CO₂ at emergence to the stream can also be a proxy for the soil flux of CO₂. Copyright © 2005 John Wiley & Sons, Ltd.     

Before and after riparian management: sediment and nutrient exports from a small agricultural catchment, Western Australia

Riparian vegetation can trap sediment and nutrients sourced from hillslopes and reduce stream bank erosion. This study presents results from a 10-year stream monitoring program (1991–2000), in a 6 km² agricultural catchment near Albany, Western Australia. After 6 years, a 1.7 km stream reach was fenced, planted with eucalyptus species and managed independently from the adjacent paddocks. Streamflow, nutrient and sediment concentration data were collected at the downstream end of the fenced riparian area, so there are data for before and after improved riparian management. Suspended sediment (SS) concentrations fell dramatically following improved riparian management; the median event mean concentration (EMC) dropped from 147 to 9.9 mg l⁻¹. Maximum SS concentrations dropped by an order of magnitude. As a result, sediment exports from the catchment decreased following improved riparian management, from over 100 to less than 10 kg ha⁻¹ yr⁻¹. Observations suggest that this was the result of reduced bank erosion and increased channel stability. Riparian management had limited impact on total phosphorus (TP) concentrations or loads, but contributed to a change in phosphorus (P) form. Before improved riparian management, around half of the P was transported attached to sediment, but after, the median filterable reactive P (FRP) to TP ratio increased to 0.75. In addition, the median FRP EMC increased by 60% and the raw median FRP concentration increased from 0.18 to 0.35 mg l⁻¹. These results suggest that there was a change in the dominant P form, from TP to FRP. Changes in total nitrogen (TN) following improved riparian management were less clear. There were reductions in TN concentrations at high flows, but little change in the loads or EMC. This study demonstrates the benefits of riparian management in reducing stream bank erosion, but suggests that in catchments with sandy, low P sorption soils, there may be limitations on the effectiveness of riparian buffers for reducing P and N exports.