CO2 dynamics along Danish lowland streams: water–air gradients, piston velocities and evasion rates

We measured CO₂ concentration and determined evasion rate and piston velocity across the water–air interface in flow-through chambers at eight stations along two 20 km long streams in agricultural landscapes in Zealand, Denmark. Both streams were 9–18-fold supersaturated in CO₂ with daily means of 240 and 340 μM in January–March and 130 and 180 μM in June–August. Annual CO₂ medians were 212 μM in six other streams and 460 μM in four groundwater wells, while seven lakes were weakly supersaturated (29 μM). Air concentrations immediately above stream surfaces were close to mean atmospheric conditions except during calm summer nights. Piston velocity from 0.4 to 21.6 cm h^?1 was closely related to current velocity permitting calculation of evasion rates for entire streams. CO₂ evasion rates were highest in midstream reaches (170–1,200 mmol m^?2 day^?1) where CO₂-rich soil water entered fast stream flow, while rates were tenfold lower (25–100 mmol m^?2 day^?1) in slow-flowing lower reaches. CO₂ evasion mainly derived from the input of CO₂ in soil water. The variability of CO₂ evasion along the two lowland streams covered much of the range in sub-Arctic and temperate streams reported previously. In budgets for the two stream catchments, loss of carbon from soils via the hydrological cycle was substantial (3.2–5.7 mmol m^?2 day^?1) and dominated by CO₂ consumed to form HCO₃ ^? by mineral dissolution (69–76%) and export of organic carbon (15–23%) relative to dissolved CO₂ export (7–9%).     

Leaf breakdown in streams differing in catchment land use

1. The impact of changes in land use on stream ecosystem function is poorly understood. We studied leaf breakdown, a fundamental process of stream ecosystems, in streams that represent a range of catchment land use in the Piedmont physiographic province of the south‐eastern United States. 2. We placed bags of chalk maple (Acer barbatum) leaves in similar‐sized streams in 12 catchments of differing dominant land use: four forested, three agricultural, two suburban and three urban catchments. We measured leaf mass, invertebrate abundance and fungal biomass in leaf bags over time. 3. Leaves decayed significantly faster in agricultural (0.0465 day^?1) and urban (0.0474 day^?1) streams than in suburban (0.0173 day^?1) and forested (0.0100 day^?1) streams. Additionally, breakdown rates in the agricultural and urban streams were among the fastest reported for deciduous leaves in any stream. Nutrient concentrations in agricultural streams were significantly higher than in any other land‐use type. Fungal biomass associated with leaves was significantly lower in urban streams; while shredder abundance in leaf bags was significantly higher in forested and agricultural streams than in suburban and urban streams. Storm runoff was significantly higher in urban and suburban catchments that had higher impervious surface cover than forested or agricultural catchments. 4. We propose that processes accelerating leaf breakdown in agricultural and urban streams were not the same: faster breakdown in agricultural streams was due to increased biological activity as a result of nutrient enrichment, whereas faster breakdown in urban streams was a result of physical fragmentation resulting from higher storm runoff.     

Contrasting nutrient exports from a forested and an agricultural catchment in south-eastern Australia

Dissolved organic carbon (DOC) and total and inorganic nitrogen and phosphorus concentrations were determined over 3 years in headwater streams draining two adjacent catchments. The catchments are currently under different land use; pasture/grazing vs plantation forestry. The objectives of the work were to quantify C and nutrient export from these landuses and elucidate the factors regulating export. In both catchments, stream water dissolved inorganic nutrient concentrations exhibited strong seasonal variations. Concentrations were highest during runoff events in late summer and autumn and rapidly declined as discharge increased during winter and spring. The annual variation of stream water N and P concentrations indicated that these nutrients accumulated in the catchments during dry summer periods and were flushed to the streams during autumn storm events. By contrast, stream water DOC concentrations did not exhibit seasonal variation. Higher DOC and NO₃ ⁻ concentrations were observed in the stream of the forest catchment, reflecting greater input and subsequent breakdown of leaf-litter in the forest catchment. Annual export of DOC was lower from the forested catchment due to the reduced discharge from this catchment. In contrast however, annual export of nitrate was higher from the forest catchment suggesting that there was an additional NO₃ ⁻ source or reduction of a NO₃ ⁻ sink. We hypothesize that the denitrification capacity of the forested catchment has been significantly reduced as a consequence of increased evapotranspiration and subsequent decrease in streamflow and associated reduction in the near stream saturated area.     

Primary production and potential availability of autochthonous carbon in southern Ontario streams

Primary production was measured in forested and open streams in southern Ontario to identify the main factors influencing algal growth, and to test the assumption that autochthonous inputs are negligible in forested headwater streams. In low order streams in southern Ontario, primary production is related primarily to light, temperature, and substrate, and appears to be largely independent of location on the river continuum. Net primary production on rock averaged 2.24 and 0.098 g Cm^–2 day^–1 during the summer at meadow and forested sites, respectively, showing peaks in the spring and fall at the forested sites. Net primary production on sediment was significant, averaging 20% (range of 6 to 82%) of net primary production on rock. When adjusted for assumed invertebrate assimilation efficiency, autochthonous production in riffles at forested sites could provide an estimated 30% of total carbon inputs potentially available to aquatic invertebrates.     

Dissolved organic carbon in streams from artificially drained and intensively farmed watersheds in Indiana, USA

Dissolved organic carbon (DOC) in streams draining hydrologically modified and intensively farmed watersheds has not been well examined, despite the importance of these watersheds to water quality issues and the potential of agricultural soils to sequester carbon. We investigated the dynamics of DOC for 14 months during 2006 and 2007 in 6 headwater streams in a heavily agricultural and tile-drained landscape in the midwestern US. We also monitored total dissolved nitrogen (TDN) in the streams and tile drains. The concentrations of DOC in the streams and tile drains ranged from approximately 1–6 mg L^?1, while concentrations of TDN, the composition of which averaged >94% nitrate, ranged from <1 to >10 mg L^?1. Tile drains transported both DOC and TDN to the streams, but tile inputs of dissolved N were diluted by stream water, whereas DOC concentrations were generally greater in the streams than in tile drains. Filamentous algae were dense during summer base flow periods, but did not appear to contribute to the bulk DOC pool in the streams, based on diel monitoring. Short-term laboratory assays indicated that DOC in the streams was of low bioavailability, although DOC from tile drains in summer had bioavailability of 27%. We suggest that these nutrient-rich agricultural streams are well-suited for examining how increased inputs of DOC, a potential result of carbon sequestration in agricultural soils, could influence ecosystem processes.     

The effect of land use on dissolved organic carbon and nitrogen uptake in streams

1. Agricultural and urban land use may increase dissolved inorganic nitrogen (DIN) concentrations in streams and saturate biotic nutrient demand, but less is known about their impacts on the cycling of organic nutrients. To assess these impacts we compared the uptake of DIN (as ammonium, NH₄⁺), dissolved organic carbon (DOC, as acetate), and dissolved organic nitrogen (DON, as glycine) in 18 low‐gradient headwater streams in southwest Michigan draining forested, agricultural, or urban land‐use types. Over 3 years, we quantified uptake in two streams in each of the three land‐use types during three seasons (spring, summer and autumn). 2. We found significantly higher NH₄⁺ demand (expressed as uptake velocity, V_f) in urban compared to forested streams and NH₄⁺V_f was greater in spring compared to summer and autumn. Acetate V_f was significantly higher than NH₄⁺ and glycine V_f, but neither acetate nor glycine V_f were influenced by land‐use type or season. 3. We examined the interaction between NH₄⁺ and acetate demand by comparing simultaneous short‐term releases of both solutes to releases of each solute individually. Acetate V_f did not change during the simultaneous release with NH₄⁺, but NH₄⁺V_f was significantly higher with increased acetate. Thus, labile DOC V_f was not limited by the availability of NH₄⁺, but NH₄⁺V_f was limited by the availability of labile DOC. In contrast, neither glycine nor NH₄⁺V_f changed when released simultaneously indicating either that overall N‐uptake was saturated or that glycine and NH₄⁺ uptake were controlled by different factors. 4. Our results suggest that labile DOC and DON uptake can be equivalent to, or even higher than NH₄⁺ uptake, a solute known to be highly bioreactive, but unlike NH₄⁺ uptake, may not differ among land‐use types and seasons. Moreover, downstream export of nitrogen may be exacerbated by limitation of NH₄⁺ uptake by the availability of labile DOC in headwater streams from the agricultural Midwestern United States. Further research is needed to identify the factors that influence cycling of DOC and DON in streams.     

Changes in stream benthic organic matter following watershed disturbance

Benthic organic matter was collected quarterly from streams draining a 9-yr-old clearcut, an 18-yr-old "old-field", a 25-yr-old successional forest, and two reference watersheds at Coweeta Hydrologic Laboratory in the Appalachian Mountains of North Carolina. USA. Samples were separated into large benthic organic matter (LBOM >1 mm) and fine benthic organic matter (FBOM <1 mm). An additional survey of large (>5 em diam.) and small (1–5 cm diam.) wood was conducted. Standing stocks of LBOM ranged from 124 to 235 g AFDM m⁻² (ash tree dry mass) and were significantly higher in streams draining reference watersheds and the successional forest than in either the recent clearcut or old-field. Reference sites exhibited LBOM peaks in late autumn and spring. No seasonal patterns were observed in disturbed streams. Standing stocks of FBOM averaged 113 to 387 g AFDM m⁻², and the stream draining the successional forest had significantly higher FBOM levels than the other sites. In reference streams, FBOM abundance peaked in spring. In disturbed streams, FBOM standing stocks were highest in summer or late autumn. Standing stocks of large wood ranged from 0 to 3956 g AFDM m⁻² and were significantly higher in the reference streams than in streams draining the old-field or successional forest. Small wood averaged 11 to 342 g AFDM m⁻² and was significantly lower in the stream draining the old-field than at the other sites. Comparisons of organic matter inputs with standing stocks indicated that disturbed streams at Coweeta receive less material and process it faster than reference streams. Disturbed streams also appear to be less retentive than reference streams and exhibit a gradual decline in FBOM during the winter when large, long-duration storms combined with low particle generation rates deplete accumulated FBOM.     

Diurnal variations in the carbon chemistry of two acidic peatland streams in north-east Scotland

1. Two acidic peatland upland streams in north‐east Scotland draining catchments of 1.3 and 41.4 km² were sampled each season for 2 years to investigate diurnal variations in dissolved and gaseous forms of carbon. Stream metabolism, alkalinity, discharge, pH, air and water temperatures were measured to aid data interpretation. 2. Free CO₂ showed marked diurnal variation with lowest concentrations during the period from late morning to early afternoon and highest during the hours of darkness. Although alkalinity and pH also showed some diurnal fluctuations, in comparison with other more productive alkaline systems, variation was small. Dissolved organic carbon (DOC) showed no significant diurnal pattern. However, significant changes in stream discharge influenced DOC concentrations, as well as over‐riding diurnal patterns of free CO₂, alkalinity and pH. 3. The highest diurnal ratios (maximum concentration/minimum concentration) in CO₂, gross primary productivity (GPP) and community respiration (CR) occurred in spring and summer and the lowest in autumn and winter. Variation in biotic in‐stream processes caused changes in CO₂ concentrations and temperature affected both the solubility of CO₂ and changes in up‐stream CO₂ inputs. There was no significant difference in diurnal fluctuations between the two orders of stream studied. 4. The mean GPP (as CO₂) was 0.81 g CO₂ m^?2 day^?1 and mean CR 2.67 g CO₂ m^?2 day^?1. The mean primary production/respiration (P/R) ratio was 0.26 ± 0.09 and 0.33 ± 0.15 in the first and second order streams, respectively. These values are low compared with published data because these heterotrophic headwater streams are dominated by benthic respiration and upstream allochthonous inputs with little autotrophic metabolism, particularly during the colder autumn and winter months. 5. The results have implications for the calculation of dissolved inorganic carbon (DIC) fluxes in streamwater. Samples taken during daylight hours tend to have lower concentrations of free CO₂ and HCO₃^? than samples taken during darkness. During spring, concentrations of free CO₂ were measured up to 2.4 (annual mean 1.8) times higher at night than during the day at a similar discharge. It is suggested that fluxes based on daytime measurements alone will under‐estimate the annual flux of these determinands in streamwater by as much as 40%.     

Evasion of CO2 from streams – The dominant component of the carbon export through the aquatic conduit in a boreal landscape

Evasion of gaseous carbon (C) from streams is often poorly quantified in landscape C budgets. Even though the potential importance of the capillary network of streams as C conduits across the land–water–atmosphere interfaces is sometimes mentioned, low‐order streams are often left out of budget estimates due to being poorly characterized in terms of gas exchange and even areal surface coverage. We show that evasion of C is greater than all the total dissolved C (both organic and inorganic) exported downstream in the waters of a boreal landscape. In this study evasion of carbon dioxide (CO₂) from running waters within a 67 km² boreal catchment was studied. During a 4 year period (2006–2009) 13 streams were sampled on 104 different occasions for dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC). From a locally determined model of gas exchange properties, we estimated the daily CO₂ evasion with a high‐resolution (5 × 5 m) grid‐based stream evasion model comprising the entire ~100 km stream network. Despite the low areal coverage of stream surface, the evasion of CO₂ from the stream network constituted 53% (5.0 (±1.8) g C m^?2 yr^?1) of the entire stream C flux (9.6 (±2.4) g C m^?2 yr^?1) (lateral as DIC, DOC, and vertical as CO₂). In addition, 72% of the total CO₂ loss took place already in the first‐ and second‐order streams. This study demonstrates the importance of including CO₂ evasion from low‐order boreal streams into landscape C budgets as it more than doubled the magnitude of the aquatic conduit for C from this landscape. Neglecting this term will consequently result in an overestimation of the terrestrial C sink strength in the boreal landscape.     

The effects of dissolved organic carbon,acidity and seasonality on metal geochemistry within a forested catchment on the Precambrian Shield,central Ontario,Canada

Metal pollution, in combination with other environmental stressors such as acid deposition and climate change, may disturb metal biogeochemical cycles. To investigate the influence of dissolved organic carbon, acidity and seasonality on metal geochemistry, this study has described concentrations of 19 metals as they pass through an acidified forested catchment on the Precambrian Shield in south-central Ontario, Canada. Metal, dissolved organic carbon (DOC) and sulphate (SO₄ ²⁻) concentrations fluctuate throughout the catchment compartments as the water passes through and interacts with vegetation, soils and bedrock. Relationships among metals, DOC and SO₄ ²⁻ are most pronounced in compartments where DOC and SO₄ ²⁻ exhibit high variability, namely in the throughfall, organic horizon soil water, and wetland-draining stream. Metal, DOC and SO₄ ²⁻ concentrations varied seasonally in the streams, and temporal coherence occurred among metal, DOC and SO₄ ²⁻ concentrations in the organic horizon soil water and the wetland-draining stream (PC1). In the wetland-draining stream, the highest DOC, Cr, Cu, Fe, Pb, and V concentrations occur in the summer, whereas concentrations of SO₄ ²⁻ and most other metals peak in the fall after a period of drought. Despite the rural location, provincial water quality objectives for surface water were exceeded for many metals when the peak fall values occurred.     

Effects of Straw Return in Deep Soils with Urea Addition on the Soil Organic Carbon Fractions in a Semi-Arid Temperate Cornfield

Returning straw to deep soil layers by using a deep-ditching-ridge-ploughing method is an innovative management practice that improves soil quality by increasing the soil organic carbon (SOC) content. However, the optimum quantity of straw return has not been determined. To solve this practical production problem, the following treatments with different amounts of corn straw were investigated: no straw return, CK; 400 kg ha^-1 straw, S₄₀₀; 800 kg ha^-1 straw, S₈₀₀; 1200 kg ha^-1 straw, S₁₂₀₀; and 1600 kg ha^-1 straw, S₁₆₀₀. After straw was returned to the soil for two years, the microbial biomass C (MBC), easily oxidized organic C (EOC), dissolved organic C (DOC) and light fraction organic C (LFOC) content were measured at three soil depths (0–10, 10–20, and 20–40 cm). The results showed that the combined application of 800 kg ha^-1 straw significantly increased the EOC, MBC, and LFOC contents and was a suitable agricultural practice for this region. Moreover, our results demonstrated that returning straw to deep soil layers was effective for increasing the SOC content.     

Longitudinal and seasonal variation of stream N uptake in an urbanizing watershed: effect of organic matter, stream size, transient storage and debris dams

We examined seasonal and spatial linkages between N cycling and organic matter for a suburban stream in Maryland and addressed the question: How do longitudinal NH₄ ⁺ uptake patterns vary seasonally and what is the effect of organic matter, stream size, transient storage and debris dams? We applied a longitudinal (stream channel corridor) approach in a forested stream section and conducted short-term nutrient addition experiments (adapted to account for the effect of nutrient saturation) covering 14–16 reaches, and compared two distinct seasons (late fall 2003 and late summer 2004). Longitudinal NH₄ ⁺ uptake rate patterns had a distinct seasonal reversal; fall had the highest uptake rates in the upper reaches, while summer had the highest uptake rates in the lower reaches. This seasonal reversal was attributed to organic matter and evidenced by DON patterns. Transient storage did not have an expected effect on uptake rates in fall because it was confounded by leaf litter; litter produced higher uptakes, but also may have reduced transient storage. In summer however, uptake rates had a positive correlation with transient storage. Debris dams had no distinct effect on uptake in fall because of their recent formation. In summer however, the debris dam effect was significant; although the debris dams were hydraulically inactive then, the upstream reaches had 2–5 fold higher uptake rates. The seasonal and longitudinal differences in NH₄ ⁺ uptake reflect interactions between flow conditions and the role of organic matter. Urbanization can alter both of these characteristics, hence affect stream N processing.     

An approach for the identification of exemplar sites for scaling up targeted field observations of benthic biogeochemistry in heterogeneous environments

Headwater streams influence the biogeochemical characteristics of large rivers and play important roles in regional and global carbon budgets. The combined effects of seasonality and land use change on the biogeochemistry of headwater streams, however, are not well understood. In this study we assessed the influence of catchment land use and seasonality on the composition of dissolved organic matter (DOM) and ecosystem metabolism in headwater streams of a Kenyan river. Fifty sites in 34 streams draining a gradient of catchment land use from 100% natural forest to 100% agriculture were sampled to determine temporal and spatial variation in DOM composition. Gross primary production (GPP) and ecosystem respiration (ER) were determined in 10 streams draining primarily forest or agricultural catchments. Absorbance and fluorescence spectrophotometry of DOM reflected notable shifts in composition along the land use gradient and with season. During the dry season, forest streams contained higher molecular weight and terrestrially derived DOM, whereas agricultural streams were dominated by autochthonous production and low molecular weight DOM. During the rainy season, aromaticity and high molecular weight DOM increased in agricultural streams, coinciding with seasonal erosion of soils and inputs of organic matter from farmlands. Most of the streams were heterotrophic. However, GPP and ER were generally greater in agricultural streams, driven by higher dissolved nutrient (mainly TDN) concentrations, light availability (open canopy) and temperature compared with forest streams. There were correlations between freshly and autochthonously produced DOM, GPP and ER during both the dry and wet seasons. This is one of the few studies to link land-use with organic carbon dynamics and DOM composition. Measures of ecosystem metabolism in these streams help to affirm the role of tropical streams and rivers as important components of the global carbon cycle and demonstrate that even semi-intensive, smallholder agriculture can have measurable effects on riverine ecosystem functioning.     

Forest Age Influences In-stream Ecosystem Processes in Northeastern US

A disturbance or natural event in forested streams that alter available light can have potential consequences for nutrient dynamics and primary producers in streams. In this study, we address how functional processes (primary production and nutrient uptake) in stream ecosystems respond to changes in forest canopy structure. We focus on differences in incoming irradiance, nutrient uptake (NO₃, NH₄, and PO₄) and open-channel metabolism seasonally in 13 forested streams that drain forests with different canopy structures (10 to >300 years old) in the northeastern United States. Light irradiance was related to forest age in a U-shaped pattern, with light being the greatest in both young open forests (<50 years old) and older growth forests (>245 years old), whereas the darkest conditions were found in the secondary growth middle-aged forests (80–158 years old). Streams that had adjacent open or old-growth riparian forest had similar conditions with greater standing stock biofilm biomass (chl a), and elevated ER in October compared to streams with middle-aged riparian forests. Compared to all sites, streams with old-growth riparian forest had the greatest in-stream primary production rates (GPP) and elevated background nutrient concentrations, and to a lesser degree, increased nutrient retention and uptake (V _f). Streams draining older forests tended to be more productive and retentive than middle-aged forests, likely due to increased light availability and the age and structure of surrounding forest canopies. Middle-aged forests had the least variation in response variables compared to streams in young and old-growth riparian forests, likely a result of uniform canopy conditions. As the structure of widespread middle-aged forests in NE US is altered by loss of specific tree species, climate change, and/or human activity, it will impact in-stream production and nutrient dynamics and may ultimately alter nutrient loading in downstream catchments.     

Metabolic balance of streams draining urban and agricultural watersheds in central Japan

Empirical data that describe the metabolic balance of stream ecosystems in human-dominated watersheds are scarce. We measured ecosystem metabolism in 23 open-canopied lowland streams draining urban and agricultural areas in the Fuji River Basin, central Japan. Gross primary production (GPP) and community respiration (CR) were estimated using the diurnal dissolved oxygen (DO) change technique, with the reaeration coefficient (K ₂) determined from seven empirical depth-velocity equations. Because the predicted values of K ₂ showed variation among the depth-velocity equations, the estimates of stream metabolism also varied according to the equations. However, CR was almost always greater than GPP, resulting in negative net ecosystem production (NEP) and GPP/CR ratios below unity for most of the study reaches. Highly heterotrophic streams were found in intensively farmed watersheds, suggesting that organic matter loading from agricultural lands is likely to be a source of allochthonous carbon fueling excess respiration in the study streams. In contrast, streams draining more urbanized areas were less heterotrophic. The present results suggest that lowland streams in agriculturally developed watersheds are associated strongly with terrestrial ecosystems as a source of organic carbon. The resultant strong respiration might become the dominant process in ecosystem metabolism, as reported for headwater streams, large downstream rivers, and estuaries.     

Influence of land use on stream ecosystem function in a Mediterranean catchment

1. Due to the hierarchical organization of stream networks, land use changes occurring at larger spatial scales (i.e. the catchment) can affect physical, chemical and biological characteristics at lower spatial scales, ultimately altering stream structure and function. Anthropogenic effects on streams have primarily been documented using structural metrics such as water chemistry, channel alteration and algal biomass. Functional parameters, including metrics of nutrient retention and metabolism, are now being widely used as indicators of stream condition. 2. Within this hierarchical context, we used a multivariate approach to examine how structural and functional (i.e. nutrient retention and metabolism) attributes of streams are related to catchment variables, including land use. The study was done in 13 streams located within a single Mediterranean catchment, but draining sub‐catchments with contrasting land use. 3. At the catchment scale, results showed two contrasting land use gradients: (i) from forested‐ to urban‐dominated catchments and (ii) from low to moderate agricultural‐dominated catchments. Variation in structural and functional parameters was strongly related to these land use gradients. Specifically, NH₄⁺ demand (measured as the uptake velocity, V_f) decreased along the gradient from forested‐ to urban‐dominated catchments primarily in response to increases in stream nutrient concentrations [NH₄⁺, dissolved organic nitrogen (DON) and carbon (DOC)]. Both primary production and respiration increased along the gradient of agricultural development in response to increases in algal biomass (chlorophyll a). Soluble reactive phosphorus demand was not related to any of the land use gradients. 4. Our results illustrate the connections among factors operating at different spatial scales (i.e. from catchments to streams) and their distinct influence on stream ecosystem function. Managers should take into consideration these connections when designing stream management and restoration plans. Because ecologically successful stream management and restoration is expected to restore function as well as structure to streams, the use of appropriate measures of functional processes is required. Nutrient retention and metabolism parameters are good candidates to fill this gap.     

Effects of agriculture on wood breakdown and microbial biofilm respiration in southern Appalachian streams

1. Agriculture causes high sediment, nutrient and light input to streams, which may affect rates of ecosystem processes, such as organic matter decay. In the southern Appalachians, socioeconomic trends over the past 50 years have caused widespread abandonment of farmland with subsequent reforestation. Physical and chemical properties of streams in these reforested areas may be returning to pre‐agriculture levels thereby creating the potential for recovery of ecosystem processes. 2. We examined wood breakdown and microbial activity on wood substrata in streams with different historical and current agricultural activity in their catchments. We analysed historical (1950) and recent (1998) forested land cover from large areas of the southern Appalachians and categorized streams based on percent forested land cover in these two time periods. Categories included a gradient of current agriculture from forested to heavily agricultural and reforestation from agriculture due to land abandonment. We compared microbial respiration on wood veneer substrata and breakdown of wood veneers among these land‐use categories. We also compared temperature, sediment accumulation and nitrogen and phosphorus concentrations. 3. Streams with current agriculture had higher concentrations of dissolved inorganic nitrogen than forested streams. Despite reforestation from agriculture, nitrogen concentrations were also elevated in streams with agricultural histories relative to forested streams. Temperature was also higher in agricultural streams but appeared to recover from historical agriculture through reforestation and stream shading. 4. Wood breakdown rates ranged from 0.0015 to 0.0076 day^?1 and were similar to other studies using wood veneers to determine breakdown rate. Microbial respiration increased with incubation time in streams up to approximately 150 days, after which it remained constant. Neither wood breakdown nor microbial respiration was significantly different among land‐use categories, despite the observed physical and chemical differences in streams based on land‐use. Wood breakdown rates could be predicted by microbial respiration indicating microbial control of wood breakdown in these streams. Both breakdown and microbial respiration were negatively correlated with the amount of inorganic sediment accumulated on wood veneers. 5. Higher nutrients and temperature led us to expect faster breakdown and higher microbial respiration in agricultural streams, but sediment in these streams may be limiting microbial activity and breakdown of organic material resulting in little net effect of agriculture on wood breakdown. Wood may not be desirable as a tool for functional assessment of stream integrity due to its unpredictable response to agriculture.     

Hydrologic Drivers and Seasonality of Dissolved Organic Carbon Concentration, Nitrogen Content, Bioavailability, and Export in a Forested New England Stream

We present the results of a full year of high-resolution monitoring of hydrologic event-driven export of stream dissolved organic matter (DOM) from the forested Bigelow Brook watershed in Harvard Forest, Massachusetts, USA. A combination of in situ fluorescent dissolved organic matter (FDOM) measurement, grab samples, and bioassays was utilized. FDOM was identified as a strong indicator of concentration for dissolved organic carbon (DOC, r ² = 0.96), dissolved organic nitrogen (DON, r ² = 0.81), and bioavailable DOC (BDOC, r ² = 0.81). Relationships between FDOM and concentration were utilized to improve characterization of patterns of hydrological event-driven export and the quantification of annual export. This characterization was possible because DOM composition remained relatively consistent seasonally; however, a subtle shift to increased fluorescence per unit absorbance was observed for summer and fall seasons and percent BDOC did increase slightly with increasing concentrations. The majority of export occurred during pulsed hydrological events, so the greatest impact of bioavailable exports may be on downstream aquatic ecosystems. Export from individual events was highly seasonal in nature with the highest flow weighted mean concentrations (DOC_FW) being observed in late summer and fall months, but the highest total export being observed for larger winter storms. Seasonal trends in DOC export coincide with weather driven changes in surface and subsurface flow paths, potential for depletion and rebuilding of a flushable soil organic matter pool, and the availability of terrestrial carbon sources such as leaf litter. Our approach and findings demonstrate the utility of high frequency FDOM measurement to improve estimates of intra-annual temporal trends of DOM export.     

Flow extremes and benthic organic matter shape the metabolism of a headwater Mediterranean stream

1. Single‐station diel oxygen curves were used to monitor the oxygen metabolism of an intermittent, forested third‐order stream (Fuirosos) in the Mediterranean area, over a period of 22 months. Ecosystem respiration (ER) and gross primary production (GPP) were estimated and related to organic matter inputs and photosynthetically active radiation (PAR) in order to understand the effect of the riparian forest on stream metabolism. 2. Annual ER was 1690 g O₂ m^?2 year^?1 and annual GPP was 275 g O₂ m^?2 year^?1. Fuirosos was therefore a heterotrophic stream, with P : R ratios averaging 0.16. 3. GPP rates were relatively low, ranging from 0.05 to 1.9 g O₂ m^?2 day^?1. The maximum values of GPP occurred during a few weeks in spring, and ended when the riparian canopy was fully closed. The phenology of the riparian vegetation was an important determinant of light availability, and consequently, of GPP. 4. On a daily scale, light and temperature were the most important factors governing the shape of photosynthesis–irradiance (P–I) curves. Several patterns could be generalised in the P–I relationships. Hysteresis‐type curves were characteristic of late autumn and winter. Light saturation responses (that occurred at irradiances higher than 90 μE m^?2 s^?1) were characteristic of early spring. Linear responses occurred during late spring, summer and early autumn when there was no evidence of light saturation. 5. Rates of ER were high when compared with analogous streams, ranging from 0.4 to 32 g O₂ m^?2 day^?1. ER was highest in autumn 2001, when organic matter accumulations on the streambed were extremely high. By contrast, the higher discharge in autumn 2002 prevented these accumulations and caused lower ER. The Mediterranean climate, and in its effect the hydrological regime, were mainly responsible for the temporal variation in benthic organic matter, and consequently of ER.     

The carbon dioxide evasion cycle of an intermittent first-order stream: contrasting water–air and soil–air exchange

Ephemeral streams and wetlands are characterized by complex cycles of submersion and emersion, which influence the greenhouse gas flux rates. In this study we quantify the spatiotemporal variability in CO₂ and CH₄ concentrations and fluxes of an intermittent first-order stream over three consecutive wet and dry cycles spanning 56 days, to assess how hydrologic phase transitions influence greenhouse gas evasion. Water column excess CO₂ ranged from ?11 to 1600 μM, and excess CH₄ from 1 to 15 μM. After accounting for temporal changes in the ratio of wet versus dry streambed hydraulic radius, total CO₂–C fluxes ranged from 12 to 156 mmol m^?2 day^?1, with an integrated daily mean of 61 ± 25 mmol m^?2 day^?1. Soil–air evasion rates were approximately equal to those of water–air evasion. Rainfall increased background water–air CO₂–C fluxes by up to 780% due to an increase in gas transfer velocity in the otherwise still waters. CH₄–C fluxes increased 19-fold over the duration of the initial, longer wet-cycle from 0.1 to 1.9 mmol m^?2 day^?1. Temporal shifts in water depth and site-specific ephemerality were key drivers of carbon dynamics in the upper Jamison Creek watercourse. Based on these findings, we hypothesise that the cyclic periodicity of fluxes of biogenic gases from frequently intermittent streams (wet and dry cycles ranging from days to weeks) and seasonally ephemeral watercourses (dry for months at a time) are likely to differ, and therefore these differences should be considered when integrating transient systems into regional carbon budgets and models of global change.