Seasonal,hydrological, and land management factors controlling dissolved organic carbon concentrations in the loch fleet catchments,Southwest Scotland

Dissolved organic carbon (DOC) was measured at hourly or two-hourly intervals during more than 30 events in one forested and two moorland subcatchments of the Loch Fleet catchment in southwest Scotland. The dominantly peaty soils in the catchments resulted in small discharge-related DOC variations within individual events, with a maximum range of about 2 mg 1^?1. Seasonal variations were larger with an amplitude of 8-9 mg 1^?1 and maximum concentrations in the summer months. The forested stream had the highest mean DOC, twice as large as the comparable moorland stream in the preliming phase. Applications of lime to the catchments increased stream DOC concentrations, with the largest increases in the moorland catchments.     

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.     

Modelling water,nutrients, and organic carbon in forested catchments: a HYPE application

Both monitoring and model simulation are useful for understanding and detecting changes in the environment. To understand and simulate leaching in small forested catchments, it is important to have knowledge of soil processes. Here, we describe recent development of the Hydrological Predictions for the Environment (HYPE) model for forested catchments. HYPE includes an organic carbon (OC) variable in addition to previously published nitrogen (N), phosphorus (P), and water flow models. The aspects addressed in the current study included P concentrations under low‐flow conditions and high concentrations of inorganic N. HYPE was further developed based on nine small forested catchments (0.5–200 ha) in Sweden, which were calibrated separately using local data. The model (excluding the OC variable) was tested on a larger set of forest catchments from the operational HYPE model of Sweden (S‐HYPE). We observed the following: (1) dissolved organic P could make a significant contribution to the total P concentration in a stream during low‐flow periods, (2) the inorganic N concentration simulated in a stream improved when part of the atmospheric N was retained in the soil, (3) the soil flow path formulation was critical for simulating concentration dynamics, and (4) evaluating an additional variable (OC) further elucidated the soil runoff processes in the model. Copyright © 2016 Swedish Meteorological and Hydrological Institute. Hydrological Processes published by John Wiley & Sons, Ltd.     

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.     

Nutrient concentration patterns in streams draining alpine and subalpine catchments, Fraser Experimental Forest, Colorado

Streamwater samples were collected during 1987–1988 from two adjacent gauged watersheds, the subalpine-alpine East St. Louis and the Fool Creek Alpine, in the Fraser Experimental Forest, Colorado. The study objective was to compare the relationships between streamwater discharge and ion concentration in alpine and alpine-subalpine watersheds at a site receiving low inputs of atmospheric contaminants. Streamwater discharge accounts for much of the variation in ion concentration. Trajectories of time, discharge, and ion concentration suggest that patterns of nutrient flux are controlled primarily by the magnitude of streamwater discharge, and seasonal differences in the relative contributions of snowmelt and soil water. In the subalpine catchment, increased streamwater discharge accounted for most of the decline in concentration of ions, with high concentrations in soil water relative to precipitation. This relationship was not seen in the alpine catchment, probably because of the influence of large diurnal variation in the ratio of snowmelt to soil water. In both catchments, ions with comparatively high concentrations in precipitation and the snowpack relative to soil water showed less concentration decline with increased streamwater discharge. The recurring nature of the trajectories, especially in the subalpine catchment, suggests that the time, discharge, and ion concentration patterns may represent a general characteristic in moderate-sized, undisturbed Rocky Mountain catchments which do not receive high inputs of airborne contaminants.     

Dissolved organic carbon in a eutrophic lake; dynamics,biodegradability and origin

The seasonal and spatial changes in dissolved organic carbon (DOC) in Lake Kasumigaura, a shallow, eutrophic lake, were analyzed and the lability of DOC was tested by long-term incubations. There was a nearly 1 mgCl^–1 downstream increase in refractory DOC in the lake; at the center it fluctuated little seasonally. The characteristic UV-absorbance: DOC ratios were determined for samples from the influent rivers (pedogenic: used interchangeably with allochthonous) and outdoor experimental ponds (autochthonous) during incubations. These ratios were then used to calculate the proportion of total measured lake water DOC in each of four components: pedogenic-refractory (PR), pedogenic-labile (PL), autochthonous-refractory (AR) and autochthonous-labile (AL). PR was uniform (around 1.5 mgCl^–1) or diminished very slightly over time. AR increased from nearly zero at the station closest to an influent river to 1 mgCl^–1 at the lake center. PL declined downstream from 0.3 mgCl^–1 to zero. AL was virtually constant at 0.8 mgCl^–1 except at the station closest to the influent river. The constancy of the UV-absorbance: DOC ratio during the biodegradation process was confirmed for Lake Kasumigaura; hence a two-component model (pedogenic-autochthonous) could be applied here without consideration of DOC lability. However, this assumption is not always met for other water bodies, and therefore it should be checked before applying a two-component model elsewhere.     

Landscape influences on aluminium and dissolved organic carbon in streams draining the Hubbard Brook valley,New Hampshire,USA

Concentrations of both aluminium (Al) and dissolved organic carbon (DOC) in stream waters are likely to be regulated by factors that influence water flowpaths and residence times, and by the nature of the soil horizons through which waters flow. In order to investigate landscape‐scale spatial patterns in streamwater Al and DOC, we sampled seven streams draining the Hubbard Brook valley in central New Hampshire. We observed considerable variation in stream chemistry both within and between headwater watersheds. Across the valley, concentrations of total monomeric aluminium (Al_m) ranged from below detection limits (<0·7 µmol l⁻¹) to 22·3 µmol l⁻¹. In general, concentrations of Al_m decreased as pH increased downslope. There was a strong relationship between organic monomeric aluminium (Al_o) and DOC concentrations (R² = 0·92). We observed the highest Al_m concentrations in: (i) a watershed characterized by a steep narrow drainage basin and shallow soils and (ii) a watershed characterized by exceptionally deep forest floor soils and high concentrations of DOC. Forest floor depth and drainage area together explained much of the variation in ln Al_m (R² = 0·79; N = 45) and ln DOC (R² = 0·87; N = 45). Linear regression models were moderately successful in predicting ln Al_m and ln DOC in streams that were not included in model building. However, when back‐transformed, predicted DOC concentrations were as much as 72% adrift from observed DOC concentrations and Al_m concentrations were up to 51% off. This geographic approach to modelling Al and DOC is useful for general prediction, but for more detailed predictions, process‐level biogeochemical models are required. Copyright © 2005 John Wiley & Sons, Ltd.     

Hydrological control of dissolved organic carbon dynamics in a forested headwater catchment,Kiryu Experimental Watershed,Japan

To evaluate the influence of hydrological processes on dissolved organic carbon (DOC) dynamics in a forested headwater catchment, DOC concentration was observed along the flow path from rainfall to stream water via throughfall, soil water, groundwater, and spring water for 4 years, and DOC flux through the catchment was calculated. The spatial and temporal variations in DOC concentration and flux were compared with physical hydrological observations and the mean residence time of water. In the upslope soil layer, DOC concentrations were not significantly correlated with water fluxes, suggesting that DOC concentrations were not strictly controlled by water fluxes. In the upslope perennial groundwater, DOC concentration was affected by the change in the amount of microbial degradation of DOC produced by changes in the mean residence time of water. In stream water, the temporal variation in DOC concentration was usually affected by changes in DOC concentration of the inflow component via vertical infiltration from above the perennial groundwater. During dry periods, however, the component from inflow via vertical infiltration was negligible and DOC in the upslope perennial groundwater became the major component of stream water DOC. The temporal variation in stream water DOC concentration during baseflow was affected by rainfall patterns over several preceding months. Therefore, records of rainfall over several preceding months are one of the most important factors for predicting changes in DOC concentration on a catchment scale. Copyright © 2007 John Wiley & Sons, Ltd.     

Modelling stream acidification in afforested catchments: Long-term reconstructions at two sites in central Scotland

A conceptual model of the combined effects of afforestation and acidic deposition is applied to two forested sites in central Scotland. Refinements are made to the model inputs specifically to include: increased dry deposition to the forests (in excess of the dry deposition expected for moorland sites) as the forest canopy develops; uptake of ions by the growing forests; and increased evapotranspiration (and thus decreased water yield) as the forests mature. The model is calibrated using a fuzzy optimisation technique which incorporates uncertainty in target variables (stream base cation concentrations and soil exchangeable bases) and uncertainty in selecting values for fixed and adjustable parameters which describe the physico-chemical characteristics of the catchments. Simulated present-day stream and soil chemistry closely match observed values at both sites. The calibrated models indicate that while the patterns of acidification in the two catchments are broadly similar, some differences do exist between the sites in the responses of the soils to acidic deposition and afforestation. It is concluded that the calibrated models provide a tool for: (a) comparison of the relative effects of deposition and afforestation on soil and surface water acidification; (b) assessment of the likely effects of reductions in future deposition combined with future forestry management practices.     

Seasonal controls on DOC dynamics in nested upland catchments in NE Scotland

Spatial and temporal variability of hydrological responses affecting surface water dissolved organic carbon (DOC) concentrations are important for determining upscaling patterns of DOC export within larger catchments. Annual and intra‐annual variations in DOC concentrations and fluxes were assessed over 2 years at 12 sites (3·40–1837 km²) within the River Dee basin in NE Scotland. Mean annual DOC fluxes, primarily correlated with catchment soil coverage, ranged from 3·41 to 9·48 g m^?2 yr^?1. Periods of seasonal (summer–autumn and winter–spring) DOC concentrations (production) were delineated and related to discharge. Although antecedent temperature mainly determined the timing of switchover between periods of high DOC in the summer‐autumn and low DOC in winter‐spring, inter‐annual variability of export within the same season was largely dependent on its associated water flux. DOC fluxes ranged from 1·39 to 4·80 g m^?2 season^?1 during summer–autumn and 1·43 to 4·15 g m^?2 season^?1 in winter–spring.Relationships between DOC areal fluxes and catchment scale indicated that mainstem fluxes reflect the averaging of highly heterogeneous inputs from contrasting headwater catchments, leading to convergent DOC fluxes at catchment sizes of ca 100 km². However, during summer–autumn periods, in contrast to winter–spring, longitudinal mainstem DOC fluxes continue to decrease, most likely because of increasing biological processes. This highlights the importance of considering seasonal as well as annual changes in DOC fluxes with catchment scale. This study increases our understanding of the temporal variability of DOC upscaling patterns reflecting cumulative changes across different catchment scales and aids modelling of carbon budgets at different stages of riverine systems. Copyright © 2010 John Wiley & Sons, Ltd.     

A comparison of stream bank erosion processes on forested and moorland streams in the Balquhidder Catchments,central Scotland

Stream bank erosion rates measured over a two-year period on a moorland and a forested stream in the Institute of Hydrology's Balquhidder Paired Catchments in central Scotland were compared. Bank erosion rates are generally higher on the mainstream of the moorland catchment and highest in wintger on both streams. Bank erosion is correlated with the incidence of frost: minimum temperatures measured on stream banks of the forested stream were an average of 3·7°C higher than on stream banks both outside the forest and on the moorland stream. This makes the incidence of frost on forested stream banks half as frequent. Volumes of material eroded from the mainstreams were combined with bulk density measurements and it is estimated that erosion of the mainstream banks is contributing 1·5 and 7·3 per cent of the sediment yield of the forested and moorland catchments, respectively. Analysis of the vertical distribution of erosion on the banks of both streams suggests an undercutting mechanism which is more pronounced in the moorland stream. The influence of trees on bank erosion and possible implications for the management of forest streams are discussed. © 1997 by John Wiley & Sons, Ltd.     

Searching for similarity in topographic controls on carbon,nitrogen and phosphorus export from forested headwater catchments

Topography influences hydrological processes that in turn affect biogeochemical export to surface water on forested landscapes. The differences in long‐term average annual dissolved organic carbon (DOC), organic and inorganic nitrogen [NO₃^?‐N, dissolved organic nitrogen (DON)], and phosphorus (total dissolved phosphorus, TDP) export from catchments in the Algoma Highlands of Ontario, Canada, with similar climate, geology, forest and soil were established. Topographic indicators were designed to represent topographically regulated hydrological processes that influence nutrient export, including (1) hydrological storage potential (i.e. effects of topographic flats/depressions on water storage) and (2) hydrological flushing potential (i.e. effects of topographic slopes on potential for variable source area to expand and tap into previously untapped areas). Variations in NO₃^?‐N export among catchments could be explained by indicators representing both hydrological flushing potential (91%, p < 0.001) and hydrological storage potential (65%, p < 0.001), suggesting the importance of hydrological flushing in regulating NO₃^?‐N export as well as surface saturated areas in intercepting NO₃^?‐N‐loaded runoff. In contrast, hydrological storage potential explained the majority of variations among catchments in DON (69%, p < 0.001), DOC (94%, p < 0.001) and TDP (82%, p < 0.001) export. The lower explanatory power of DON (about 15% less) compared with that of DOC and TDP suggests another mechanism influencing N export, such as controls related to alternative fates of nitrogen (e.g. as gas). This study shows that simple topographic indicators can be used to track nutrient sources, sinks and their transport and export to surface water from catchments on forest landscapes. Copyright © 2013 John Wiley & Sons, Ltd.     

Dissolved organic carbon in permafrost regions: A review

A large quantity of organic carbon(C) is stored in northern and elevational permafrost regions. A portion of this large terrestrial organic C pool will be transferred by water into soil solution(～0.4 Pg C yr~(-1))(1 Pg=10~(15) g), rivers (～0.06 Pg C yr~(-1)),wetlands, lakes, and oceans. The lateral transport of dissolved organic carbon(DOC) is the primary pathway, impacting river biogeochemistry and ecosystems. However, climate warming will substantially alter the lateral C shifts in permafrost regions.Vegetation, permafrost, precipitation, soil humidity and temperature, and microbial activities, among many other environmental factors, will shift substantially under a warming climate. It remains uncertain as to what extent the lateral C cycle is responding,and will respond, to climate change. This paper reviews recent studies on terrestrial origins of DOC, biodegradability, transfer pathways, and modelling, and on how to forecast of DOC fluxes in permafrost regions under a warming climate, as well as the potential anthropogenic impacts on DOC in permafrost regions. It is concluded that:(1) surface organic layer, permafrost soils,and vegetation leachates are the main DOC sources, with about 4.72 Pg C DOC stored in the topsoil at depths of 0–1 m in permafrost regions;(2) in-stream DOC concentrations vary spatially and temporally to a relatively small extent (1–60 mg C L~(-1)) and annual export varies from 0.1–10 g C m~(-2) yr~(-1);(3) biodegradability of DOC from the thawing permafrost can be as high as 71%, with a median at 52%;(4) DOC flux is controlled by multiple factors, mainly including vegetation, soil properties,permafrost occurrence, river discharge and other related environmental factors, and(5) many statistical and process-based models have been developed, but model predictions are inconsistent with observational results largely dependent on the individual watershed characteristics and future discharge trends. Thus, it is still difficult to predict how future lateral C flux will respond to climate change, but changes in the DOC regimes in individual catchments can be predicted with a reasonable reliability. It is advised that sampling protocols and preservation and analysis methods should be standardized, and analytical techniques at molecular scales and numerical modeling on thermokarsting processes should be prioritized.     

Dissolved organic matter in coastal waters at Jeddah,Saudi Arabia

Water samples collected during April 1982–April 1983 from Red Sea coastal waters at Jeddah, Saudi Arabia, were investigated for dissolved organic carbon, dissolved organic nitrogen, dissolved lipid, dissolved organic phosphorus, dissolved monosaccharides, total dissolved sugars and dissolved polysaccharides. All showed extremely high values at two stations. This can be attributed to the direct effect of untreated wastes on these two locations and the limited water exchange between the study area and the open sea.     

Seasonal cycles of dissolved constituents in streamwater in two forested catchments in the mid-Atlantic region of the eastern USA

Streamwater discharge and chemistry of two small catchments on Catoctin Mountain in north-central Maryland have been monitored since 1982. Repetitive seasonal cycles in stream-water chemistry have been observed each year, along with seasonal cycles in the volume of stream discharge and in groundwater levels. The hypothesis that the observed streamwater chemical cycles are related to seasonal changes in the hydrological flow paths that contribute to streamflow is examined using a combination of data on groundwater levels, shallow and deep groundwater chemistry, streamwater discharge, streamwater chemistry, soil-water chemistry, and estimates of water residence times. The concentrations of constituents derived from rock weathering, particularly bicarbonate and silica, increase in streamwater during the summer when the water table is below the regolith-bedrock interface and stream discharge consists primarily of deep groundwater from the fractured-bedrock aquifer. Conversely, the concentrations in streamwater of atmospherically derived components, particularly sulfate, increase in winter when the water table is above the regolith-bedrock interface and stream discharge consists primarily of shallow groundwater from the regolith. Tritium and chlorofluorocarbon (CFC) measurements suggest that the groundwater in these systems is young, with a residence time of less than several years. The results of this study have implications for the design of large-scale water-quality monitoring programs.     

Adjacent catchments with similar patterns of land use and climate have markedly different dissolved organic carbon concentration and runoff dynamics

Temporal patterns in specific runoff, dissolved organic carbon concentrations [DOC] and fluxes were examined during two periods: 1994–1997 (period 1) and 2007–2009 (period 2) in five adjacent tributary catchments of Lake Simcoe, the largest lake in southern Ontario, Canada. The catchments displayed similar patterns of land use change with increases in urbanization (5–16%) and forest cover (0.2–4%) and declines in agriculture (4–8%) between 1994 and 2008. Climate in the catchments was similar; temperature increased slightly, but no significant change in precipitation was observed. Despite similar pattern of climate and land use, runoff responses and tributary [DOC] were different across the catchments. Following a very dry year (i.e. 1999), runoff increased steadily until the end of record. We observed increased variability in tributary [DOC] and higher DOC exports in period 2. This led to ~10% increase in [DOC] and a 13% increase in flux between the two study periods. Between the two periods, [DOC] increased by 15% in spring and 25% in summer, whereas flux increased by 17% in spring and 48% in summer. [DOC] was consistently higher in the growing (summer + autumn) than the dormant (winter + spring, minus spring melt months) seasons, but no unique pattern or simple linear flow/concentrations relationships existed. This suggests complex spatial and temporal pattern to runoff controls on DOC and flow dynamics in adjacent catchments. We therefore caution against extrapolating from monitored to unmonitored catchments. Copyright © 2012 John Wiley & Sons, Ltd.     

Wood distribution in neotropical forested headwater streams of La Selva,Costa Rica

Surveys of wood along 30 forested headwater stream reaches in La Selva Biological Station in north‐eastern Costa Rica represent the first systematic data reported on wood loads in neotropical streams. For streams with drainage areas of 0·1–8·5 km² and gradients of 0·2–8%, wood load ranged from 3 to 34·7 m³ wood/100 m channel and 41–612 m³ wood/ha channel. These values are within the range reported for temperate streams. The variables wood diameter/flow depth, stream power, the presence of backflooding, and channel width/depth are consistently selected as significant predictors by statistical models for wood load. These variables explain half to two‐thirds of the variability in wood load. These results, along with the spatial distribution of wood with respect to the thalweg, suggest that transport processes exert a greater influence on wood loads than recruitment processes. Wood appears to be more geomorphically effective in altering bed elevations in gravel‐bed reaches than in reaches with coarser or finer substrate. Copyright © 2009 John Wiley & Sons, Ltd.     

Dissolved organic matter dynamics during the spring snowmelt at a boreal river valley mire complex in Northwest Russia

Boreal mire landscapes are rich in soil carbon and significantly contribute to the carbon input of aquatic ecosystems. They are composed of different mesoscale ecohydrological subunits, whose individual contributions to the water and carbon export of mire catchments are not well understood. The spring snowmelt period is the major hydrological event in the annual water cycle of the boreal regions and strongly influences the carbon flux between the terrestrial and aquatic systems. The aim of this study was (1) to provide a conceptual understanding of the spatial and temporal dynamics of the surface water chemistry along a swamp forest‐fen‐bog gradient during the snowmelt period, (2) to quantify the exported dissolved organic carbon (DOC) content in the runoff and (3) to identify the ecohydrological landscape unit that contributes most to DOC export during the snowmelt period in a heterogeneous mire complex in Northwest Russia. The highest DOC concentrations were detected in the swamp forest, and the lowest concentrations were observed at the treeless bog by the end of the snowmelt period (swamp forest: 37–43 mg l^?1, bog: 13–17 mg l^?1). During the spring snowmelt period, a significant amount (~1.7 g C m^?2) of DOC was transferred by the ~74 mm of runoff from the catchment into the river. Variability in the thawing periods led to differences in the relative contributions of each ecohydrological zone to the carbon export measured at a stream channel draining the studied part of the mire complex. An increased understanding of the variation in DOC concentrations and contributions from the mesoscale ecohydrological subunits to carbon export can help to predict the potential regional loss of DOC based on land cover type under climate change. Copyright © 2015 John Wiley & Sons, Ltd.