The Role of Stream Water Carbon Dynamics and Export in the Carbon Balance of a Tropical Seasonal Rainforest,Southwest China

A two-year study (2009 ∼ 2010) was carried out to investigate the dynamics of different carbon (C) forms, and the role of stream export in the C balance of a 23.4-ha headwater catchment in a tropical seasonal rainforest at Xishuangbanna (XSBN), southwest China. The seasonal volumetric weighted mean (VWM) concentrations of total inorganic C (TIC) and dissolved inorganic C (DIC) were higher, and particulate inorganic C (PIC) and organic C (POC) were lower, in the dry season than the rainy season, while the VWM concentrations of total organic C (TOC) and dissolved organic C (DOC) were similar between seasons. With increased monthly stream discharge and stream water temperature (SWT), only TIC and DIC concentrations decreased significantly. The most important C form in stream export was DIC, accounting for 51.8% of the total C (TC) export; DOC, POC, and PIC accounted for 21.8%, 14.9%, and 11.5% of the TC export, respectively. Dynamics of C flux were closely related to stream discharge, with the greatest export during the rainy season. C export in the headwater stream was 47.1 kg C ha⁻¹ yr⁻¹, about 2.85% of the annual net ecosystem exchange. This finding indicates that stream export represented a minor contribution to the C balance in this tropical seasonal rainforest.     

Bacterial Contribution to Dissolved Organic Matter in Eutrophic Lake Kasumigaura,Japan

Incubation experiments using filtered waters from Lake Kasumigaura were conducted to examine bacterial contribution to a dissolved organic carbon (DOC) pool. Bacterial abundance, bacterial production, concentrations of DOC, total dissolved amino acids (TDAA), and total dissolved neutral sugars (TDNS) were monitored during the experiments. Bacterial production during the first few days was very high (20 to 35 μg C liter⁻¹ day⁻¹), accounting for 40 to 70% of primary production. The total bacterial production accounted for 34 to 55% of the DOC loss during the experiment, indicating high bacterial activities in Lake Kasumigaura. The DOC degradation was only 12 to 15%, whereas the degradation of TDAA and TDNS ranged from 30 to 50%, suggesting the preferential usage of TDAA and TDNS. The contribution of bacterially derived carbon to a DOC pool in Lake Kasumigaura was estimated using d-amino acids as bacterial biomarkers and accounted for 30 to 50% of the lake DOC. These values were much higher than those estimated for the open ocean (20 to 30%). The ratio of bacterially derived carbon to bulk carbon increased slightly with time, suggesting that the bacterially derived carbon is more resistant to microbial degradation than bulk carbon. This is the first study to estimate the bacterial contribution to a DOC pool in freshwater environments. These results indicate that bacteria play even more important roles in carbon cycles in freshwater environments than in open oceans and also suggests that recent increases in recalcitrant DOC in various lakes could be attributed to bacterially derived carbon. The potential differences in bacterial contributions to dissolved organic matter (DOM) between freshwater and marine environments are discussed.     

Colloidal and dissolved organic matter in lake water: Carbohydrate and amino acid composition,and ability to support bacterial growth

Bacterial utilization of dissolved organic matter (DOM) was studied in water from a humic and a clearwater oligotrophic lake. Indigenous bacteria were inoculated into either 0.2 m natural filtered lake water, or lake water enriched fivefold with colloidal DOM >100 kD but below 0.2 m. Consumption of DOM was followed from changes in concentrations of total dissolved organic carbon (DOC), dissolved combined and free carbohydrates and amino acids (DCCHO and DFCHO, and DCAA and DFAA, respectively) and by uptake of monosaccharide and amino acid radioisotopes. DCCHO and DCAA made up 8% (humic lake) to 33–44% (clear-water lake) of the natural DOC pools, while DFCHO and DFAA contributed at most 1.7% to the DOC pools. Addition of >100 kD DOM increased the DOC concentrations by 50% (clearwater lake) to 92% (humic lake), but it only resulted in a higher bacterial production (by 63%) in the humic lake. During the incubations 13 to 37% of the DOC was assimilated by the bacteria, at estimated growth efficiencies of 4–8%. Despite the measured reduction of DOC, statistically significant changes of specific organic compounds, especially of DCCHO and DCAA, generally did not occur. Probably the presence of high molecular weight DOC interfered with the applied analytical procedures. Addition of radiotracers indicated, however, that DFAA sustained 17–58% and 29–100% of the bacterial carbon and nitrogen requirements, respectively, and that glucose met 1–3% of the bacterial carbon requirements. Thus, our experiments indicate that radiotracers, rather than measurements of concentration changes, should be used in studies of bacterial utilization of DOC in freshwaters with a high content of humic or high molecular weight organic matter.     

Seasonal relationships between planktonic microorganisms and dissolved organic material in an alpine stream

The relationships between the abundance and activity of planktonic, heterotrophic microorganisms and the quantity and characteristics of dissolved organic carbon (DOC) in a Rocky Mountain stream were evaluated. Peak values of glucose uptake, 2.1 nmol L⁻¹ hr⁻¹, and glucose concentration, 333 nM, occurred during spring snowmelt when the water temperature was 4.0°C and the DOC concentration was greatest. The turnover time of thein situ glucose pool ranged seasonally from 40–1110 hours, with a mean of 272 hr. Seasonal uptake of³H-glucose, particulate ATP concentrations, and direct counts of microbial biomass were independent of temperature, but were positively correlated with DOC concentrations and negatively correlated with stream discharge. Heterotrophic activity in melted snow was generally low, but patchy. In the summer, planktonic heterotrophic activity and microbial biomass exhibited small-scale diel cycles which did not appear to be related to fluctuations in discharge or DOC, but could be related to the activity of benthic invertebrates. Leaf-packs placed under the snow progressively lost weight and leachable organic material during the winter, indicating that the annual litterfall in the watershed may be one source of the spring flush of DOC. These results indicate that the availability of labile DOC to the stream ecosystem is the primary control on seasonal variation in heterotrophic activity of planktonic microbial populations.     

Love Handles in Aquatic Ecosystems: The Role of Dissolved Organic Carbon Drawdown, Resuspended Sediments, and Terrigenous Inputs in the Carbon Balance of Lake Michigan

During the unstratified (winter) and stratified (summer) periods of 1999 and 2000, we examined carbon (C) dynamics in the upper water column of southern Lake Michigan. We found that (a) bacterial respiration (BR) and planktonic respiration (PR) were major sinks for C, (b) C flux through bacteria (CFTB) was diminished in winter because of reduced bacterial production (BP) and increased bacterial growth efficiency (BGE) at colder temperatures, and (c) PR exceeded primary production (PP) during the spring–summer transition. Drawdown of dissolved organic C (DOC), resuspended organic matter from the lake floor, and riverine organic matter likely provided organic C to compensate for this temporal deficit. DOC in the water column decreased between winter and summer (29–91 mg C m² d⁻¹) and accounted for 20%–53% of CFTB and 11%–33% of PR. Sediment resuspension events supported elevated winter heterotrophy in the years that they occurred with greatest intensities (1998 and 2000) and may be important to interannual variability in C dynamics. Further, riverine discharge, containing elevated DOC (5×) and dissolved P (10×) relative to lake water, peaked in the winter–spring season in southern Lake Michigan. Collectively, terrigenous inputs (river, stream, and groundwater discharges; storm water runoff; and atmospheric precipitation) may support approximately 10%–20% of annual in-lake heterotrophy as well as autotrophy. Terrestrial subsidies likely play a key role in the C balance of even very large lakes, representing a critical linkage between terrestrial and aquatic ecosystems. Received 11 June 2001; Accepted 14 December 2001.     

Hydrological controls on dissolved organic carbon during snowmelt in the Snake River near Montezuma,Colorado

A quantitative understanding of the factors controlling the variation of dissolved organic carbon (DOC) in headwater streams is of scientific concern for at least two reasons. First, quantifying the overall carbon budgets of lotic systems is needed for a fundamental understanding of these systems. Second, DOC interacts strongly with other dissolved substances (heavy metals in particular) and plays an important role in the transport of contaminants.In the Snake River near Montezuma, Colorado, measurements of DOC from 1980 to 1986 show rapid decreases in concentration from a peak very early in the snowmelt period. Peak DOC concentrations occur approximately one month prior to peak discharge in the stream. The decline in DOC with time is approximately exponential, suggesting that a simple flushing mechanism can explain the response. We examined hydrological mechanisms to explain the observed variability of DOC in the Snake River by simulating the hydrological response of the catchment using TOPMODEL and routing the predicted flows through a simple model that accounted for temporal changes in DOC. Conceptually the DOC model represents a terrestrial (soil) reservoir in which DOC builds up during low flow periods and is flushed out by infiltrating meltwaters. The model reproduces the main features of the observed variation in DOC in the Snake River and thus lays the foundation for quantitatively linking hydrological processes with carbon cycling through upland catchments. Model results imply that a significant fraction of the soils in the Snake River catchment contribute DOC to the stream during peak discharge. Our work represents one of the first attempts to quantitatively describe the hydrological controls on DOC dynamics in a headwater stream. These controls are studied through the model by imposing mass balance constraints on both the flux of water through the various DOC source areas and the amount of DOC that can accumulate in these areas.     

Lability of organic carbon in lakes of different trophic status

1. We used first‐order kinetic parameters of biological oxygen demand (BOD), the constant of aerobic decomposition (k) and the asymptotic value of BOD (BOD_ult), to characterise the lability of organic carbon pools in six lakes of different trophic state: L. Naroch, L. Miastro and L. Batorino (Belarus), L. Kinneret (Israel), L. Ladoga (Russia) and L. Mendota (U.S.A.). The relative contributions of labile and refractory organic carbon fractions to the pool of total organic carbon (TOC) in these lakes were quantified. We also determined the amounts of labile organic carbon within the dissolved and particulate TOC pools in the three Belarus lakes. 2. Mean annual chlorophyll concentrations (used as a proxy for lake trophic state) ranged from 2.3 to 50.6 μg L⁻¹, labile organic carbon (OC_L = 0.3BOD_ult) from 0.75 to 2.95 mg C L⁻¹ and k from 0.044 to 0.14 day⁻¹. 3. Our data showed that there were greater concentrations of OC_L but lower k values in more productive lakes. 4. In all cases, the DOC fraction dominated the TOC pool. OC_L was a minor component of the TOC pool averaging about 20%, irrespective of lake trophic state. 5. In all the lakes, most (c. 85%) of the DOC pool was refractory, corresponding with published data based on measurements of bacterial production and DOC depletion. In contrast, a larger fraction (27–55%) of the particulate organic carbon (POC) pool was labile. The relative amount of POC in the TOC pool tended to increase with increasing lake productivity. 6. Long‐term BOD incubations can be valuable in quantifying the rates of breakdown of the combined particulate and dissolved organic carbon pools and in characterising the relative proportions of the labile and recalcitrant fractions of these pools. If verified from a larger number of lakes our results could have important general implications.     

Export of DOC from forested catchments on the Precambrian Shield of Central Ontario: Clues from 13C and 14C

Export of dissolved organic carbon (DOC) from forested catchmentsis governed by competing processes of production, decomposition, sorptionand flushing. To examine the sources of DOC, carbon isotopes (¹⁴Cand ¹³C) were analyzed in DOC from surface waters, groundwatersand soils in a small forested catchment on the Canadian Shield in centralOntario. A significant fraction (greater than 50%) of DOCin major inflows to the lake is composed of carbon incorporated into organicmatter, solubilized and flushed into the stream within the last 40 years. Incontrast, ¹⁴C in groundwater DOC was old indicating extensiverecycling of forest floor derived organic carbon in the soil column beforeelution to groundwater in the lower B and C soil horizons. A small uplandbasin had a wide range in ¹⁴C from old groundwater values atbaseflow under dry basin conditions to relatively modern values during highflow or wetter antecedent conditions. Wetlands export mainly recently fixedcarbon with little seasonal range. DOC in streams entering the small lakemay be composed of two pools; an older recalcitrant pool delivered bygroundwater and a young labile pool derived from recent organic matter.The relative proportion of these two pools changes seasonally due thechanges in the water flowpaths and organic carbon dynamics. Althoughchanges in local climate (temperature and/or precipitation) may alterthe relative proportions of the old and young pools, the older pool islikely to be more refractory to sedimentation and decomposition in thelake setting. Delivery of older pool DOC from the catchment andsusceptibility of this older pool to photochemical decomposition mayconsequently be important in governing the minimum DOC concentrationlimit in lakes.     

Distribution and characterization of dissolved and particulate organic matter in Antarctic pack ice

Distribution and composition of organic matter were investigated in Antarctic pack ice in early spring and summer. Accumulation of organic compounds was observed with dissolved organic carbon (DOC) and particulate organic carbon (POC) reaching 717 and 470 μM C, respectively and transparent exopolymeric particles (TEP) up to 3,071 μg Xanthan gum equivalent l⁻¹. POC and TEP seemed to be influenced mainly by algae. Particulate saccharides accounted for 0.2–24.1% (mean, 7.8%) of POC. Dissolved total saccharides represented 0.4–29.6% (mean, 9.7%) of DOC, while dissolved free amino acids (DFAA) accounted for only 1% of DOC. Concentrations of TEP were positively correlated with those of saccharides. Monosaccharides (d-MCHO) dominated during winter–early spring, whereas dissolved polysaccharides did in spring–summer. DFAA were strongly correlated with d-MCHO, suggesting a similar pathway of production. The accumulation of monomers in winter is thought to result from limitation of bacterial activities rather than from the nature of the substrates.     

Constant release of photosynthate from marine phytoplankton.

The release rate of dissolved organic carbon (DOC) by unialgal cultures and natural phytoplankton assemblages was constant over a wide range of dissolved inorganic carbon concentrations. DOC release was not proportional to the particulate organic carbon production rate. We postulate that intracellular DOC, fated for release, exists either as a separate pool from that leading to particulate organic carbon production or that there is active metabolic control on one portion of a common pool.     

Metabolism of dissolved organic carbon by planktonic bacteria and mixotrophic algae in lake neutralisation experiments

1. Lakes formed in mining pits often contain high concentrations of dissolved ferric iron and sulphate (e.g. 2 and 16 mmol L^?1, respectively) and the pH is buffered between 2.5 and 3.5. Efforts to neutralise their water are based on the stimulation of lake internal, bacterial iron‐ and sulphate reduction. Electron donors may be supplied by organic carbon compounds or indirectly by enhancement of primary production. Here, we investigated the function of mixotrophic algae, which can potentially supplement or deplete the organic carbon pool, in the carbon metabolism and alkalinity budget of an acidic mining lake. 2. Two weeks after organic substrates had been added in a large in situ mesocosm of 30 m diameter, a bloom of Chlamydomonas occurred, reaching a biovolume of 80 mm³ L^?1. Growth experiments using filtered lake water showed that the alga reduced the overall dissolved organic carbon (DOC) concentration despite significant photosynthetic activity. However, when Chlamydomonas were grown together with natural bacterioplankton, net DOC consumption did not increase. 3. Uptake experiments using [¹⁴C]‐glucose indicated that bacteria dominated glucose uptake and remineralisation. Therefore, the DOC leached in the water column was processed mainly by planktonic bacteria. Leached DOC must be regarded as loss, not transferred by larger organisms to the sediment, where reduction processes take place. 4. From phytoplankton biomass and production 2 years after fertilisation we estimated that pelagic photosynthesis does not supply an electron donor capacity capable of reducing more than 2% of actual stock of acidity per year. We estimated that only the benthic primary production was in a range to compensate for ongoing inputs of iron and sulphate.     

Spatial and temporal patterns in the hydrogeochemistry of a poor fen in northern Wisconsin

We studied the factors causing spatial and temporal patterning of interstitial water chemistry in Crystal Bog, a 7 ha northern Wisconsin kettle-hole peatland. Over the course of the snow-free season Crystal Bog exhibited spatial and temporal patterns in chemistry, especially hydrogen-ion, dissolved organic carbon, and specific conductance. The peatland contains a 0.5 ha pond that has water more dilute than the interstitial water of the surrounding peatland. The direction of groundwater flow between the lake and the peatland was seasonally dependent. In the spring and early summer, flow was from the lake into the peatland, especially on the eastern side of the lake. This flow resulted in a plume of relatively dilute surface interstitial water in the peatland. In mid and late summer direction of groundwater flow was from the peatland into the lake and the dilute plume was reduced in areal extent. By fall the direction of water flow was again from the lake to the peatland.The spatial and temporal heterogeneity in water chemistry produced by the seasonal variation in the direction of horizontal water flow was substantial. Minimum and maximum observed concentrations of dissolved organic carbon (DOC) in the interstitial water of the peatland, for example, differed by more than a factor of three, and pH ranged between 3.8 and 5.0. Correlations of DOC with anion deficit and hydrogen ion concentration and experiments of photo-oxidation of water samples showed that organic acids were the primary cause of acidity in the peatland. Specific conductance was highly correlated with DOC, probably because of DOC's correlation with the very conductive hydrogen ion. In Crystal Bog it was possible to use the relatively simple measure of specific conductance to estimate the temporal and spatial distribution of the more difficult to determine DOC.     

Chemical character and origin of organic acids in streams and seepage lakes of central Maine

Organic acids and inorganic chemistry were examined in seventeen seepage lakes, seven streams, and one seep in central Maine. The objectives of this analysis were to determine the quantity and quality of dissolved organic carbon (DOC), and to assess the relationship between organic and inorganic surface water chemistry. Lakes and streams sampled were dilute (average conductivity of 20.3 μS cm^?1) with a wide range of DOC (125–2593 μmol C L^?1). Organic acids in DOC were evaluated by:

DOC fractionation (hydrophobic acids and neutrals, and hydrophilic acids, bases, and neutrals);

DOC isolation followed by FT-IR, base titration, and chemical analyses;

adsorption on solid phase extraction columns; and charge balance studies.

All lakes and streams were dominated by hydrophobic and hydrophilic acids (60 to 92% of DOC). Lakes and streams with low DOC had low hydrophobic to hydrophilic acid ratios (ca 1.2–1.3), regardless of pH and acid neutralizing capacities (ANC), compared to lakes and streams with moderate to high DOC concentrations (ca 1.9–2.4). Based on FT-IR spectroscopy and chemical analysis, organic acids were found to be dominated by a strong carboxylic character. Titration data of isolated DOC allowed accurate prediction of organic anions, which were strongly pH dependent (organic anions ranged from 14 to 198 μeq L^?1). Exchange acidity averaged 11.3, 13.6, and 8.7 μeq mg C^?1 for lake hydrophobic acids, lake hydrophilic acids, and stream hydrophobic acids, respectively. Overall evidence suggested that DOC and organic acid characteristics were related to their carboxylic functional group content and that the nature of these constituents was similar despite the source of origin (upland soils, wetlands, or Sphagnum deposits). Also, contact of soil leachates with B horizons seemed to be a controlling factor in DOC quantity and quality in the lakes and streams studied.     

Dynamics of dissolved and particulate organic carbon in a saline and semiarid stream of southeast Spain (Chicamo stream)

Annual variations in the concentration of dissolved (DOC) and particulate organic carbon (CPOC = Coarse; FPOC = Fine; UPOC = Ultrafine) were studied in a 100 m-reach of the Chicamo stream, an intermittent saline stream in southeast Spain. DOC represented the most important fraction of organic carbon flowing in the Chicamo stream (>98%), with concentrations of about 1.7 mgC l^–1 during most of the year, reaching 2.5 mgC l^–1 in summer. One high flow episode during a rain storm in winter was characterized by a considerably increased concentration of DOC (9.4 mgC l^–1). CPOC was the dominant POC fraction. Positive and significant correlations were found for DOC and discharge, which support the idea of allochthonous inputs due to floods. There was no significant correlation between POC and discharge. No significant correlations were found for DOC or POC with the physico-chemical parameters measured, while a negative significant correlation was found between DOC and temperature. The export of total organic carbon from the drainage basin of the Chicamo stream was low (6.2 × 10^–4 gC m^–2 yr^–1) and typical of streams in arid and semi-arid regions. The results of a Principal Component Analysis defined three different phases. The first consisted of short periods, during which floods provide pulses of allochthonous organic carbon and nutrients, the second a dry phase (summer), defined by biotic interactions, during which the stream could acts as a `sink' of organic matter, and the third and final phase which is characterised by hydrological stability.     

Variability and transport of suspended sediment,particulate and dissolved organic carbon in the tidal freshwater Hudson River

Measurements of suspended matter, particulate organic carbon and dissolved organic carbon were made over a three year period at stations spanning 150 km of the tidal freshwater Hudson River. Suspended matter concentrations varied from year-to-year and were not related to freshwater discharge. The increase in suspended matter with depth in vertical profiles suggests that, during medium to low flow conditions, resuspension of bottom sediments was as important a source of sediment as loadings from tributaries. Particulate organic carbon showed significant variability among stations, and both autochthonous primary production and detrital organic matter are contributing to POC standing stocks. Dissolved organic carbon represented over half of the total organic carbon in the water column and showed little variation among stations.Examining downstream changes in transport showed that there was significant production of both suspended matter and POC within the study reach during the ice-free season. Tributary loadings within the study reach do not appear to be the cause of these increases in downstream transport. Dissolved organic carbon behaved conservatively in that there was no evidence for net production or net consumption within the river.The spatial/temporal patterns and analyses of transport suggest that suspended matter and POC, but not DOC, were controlled to a significant extent by processes occurring within the river and were not simply related to loadings from outside.     

Global warming and the export of dissolved organic carbon from boreal peatlands

Peatlands occupy approximately 15% of boreal and sub-arctic regions, contain approximately one third of the world's soil carbon pool, and supply most of the dissolved organic carbon (DOC) entering boreal lakes and rivers and the Arctic Ocean. The high latitudes occupied by these peatlands are expected to see the greatest amount of climatic warming in the next several decades. In addition to increasing temperatures, climatic change could also affect the position of the water-table level and discharge from these peatlands. Changes in temperature, water tables, and discharge could affect delivery of DOC to downstream ecosystems where it exerts significant control over productivity, biogeochemical cycles, and attenuation of visible and UV radiation. We experimentally warmed and controlled water tables while measuring discharge in a factorial experiment in large mesocosms containing peat monoliths and intact plant communities from a bog and fen to determine the effects of climate change on DOC budgets. We show that the DOC budget is controlled largely by changes in discharge rather than by any effect of warming or position of the water-table level on DOC concentrations. Furthermore, we identify a critical discharge rate in bogs and fens for which the DOC budget switches from net export to net retention. We also demonstrate an exponential increase in trace gas CO₂–C and CH₄–C emissions coincident with increased retention of dissolved organic carbon from boreal peatlands.     

Seasonal and hydrologic drivers of dissolved organic matter and nutrients in the upper Kuparuk River, Alaskan Arctic

As the planet warms, widespread changes in Arctic hydrology and biogeochemistry have been documented and these changes are expected to accelerate in the future. Improved understanding of the behavior of water-borne constituents in Arctic rivers with varying hydrologic conditions, including seasonal variations in discharge?Cconcentration relationships, will improve our ability to anticipate future changes in biogeochemical budgets due to changing hydrology. We studied the relationship between seasonal water discharge and dissolved organic carbon and nitrogen (DOC and DON) and nutrient concentrations in the upper Kuparuk River, Arctic Alaska. Fluxes of most constituents were highest during initial snowmelt runoff in spring, indicating that this historically under-studied period contributes significantly to total annual export. In particular, the initial snowmelt period (the stream is completely frozen during the winter) accounted for upwards of 35% of total export of DOC and DON estimated for the entire study period. DOC and DON concentrations were positively correlated with discharge whereas nitrate (NO₃ ^?) and silicate were negatively correlated with discharge throughout the study. However, discharge-specific DOC and DON concentrations (i.e. concentrations compared at the same discharge level) decreased over the summer whereas discharge-specific concentrations of NO₃ ^? and silicate increased. Soluble reactive phosphorus (SRP) and ammonium (NH₄ ⁺) were negatively correlated with discharge during the spring thaw, but were less predictable with respect to discharge thereafter. These data provide valuable information on how Arctic watershed biogeochemistry will be affected by future changes in temperature, snowfall, and rainfall in the Arctic. In particular, our results add to a growing body of research showing that nutrient export per unit of stream discharge, particularly NO₃ ^?, is increasing in the Arctic.     

Immobilization of dissolved organic matter from groundwater discharging through the stream bed

SUMMARY. 1. In laboratory experiments, 9.7–25.7% of dissolved organic carbon (DOC) in groundwater (at concentrations of 18.7–24.8 mg 1-⁻¹) was immobilized after perfusion through 8-cm-deep (22-cm-diameter) cores of stony stream-bed substratum.
2. This represented immobilization rates of 7.1–23.5 mg m⁻² h⁻¹ or, extrapolated across the year, potential immobilization rates within the stream bed of 62.2–205.9g m⁻² yr⁻¹. Actual rates in the entire stream bed were probably higher because perfusion through the experimental cores did not reduce groundwater DOC concentrations to levels measured in the adjacent stream.
3. Natural concentrations of dissolved free amino acids (DFAAs) in groundwater were generally unchanged following perfusion through the cores, suggesting the maintenance of a dynamic equilibrium in their concentrations.
4. Selective enrichments of amino acids in groundwater (up to 20-fold) were entirely immobilized following perfusion, indicating their rapid retention and flux in this environment. Thus, immobilization of the bulk DOC in stream-bed cores probably did not reflect net reductions in dissolved free, low-molecular-weight material, with higher molecular weight, more 'refractory' material being immobilized instead.
5. We conclude that groundwater can contribute substantial amounts of DOC, both high and low molecular weight, to a stream ecosystem. The stream bed is the site at which much of this material could be initially immobilized and made available to the stream trophic structure.     

Groundwater controls on nutrient cycling in a Mojave desert stream

1. Groundwater fluxes of nitrogen and dissolved organic carbon (DOC) were investigated in Grape Vine Canyon Stream in the Mojave Desert focusing on the rate of inputs and the fate of groundwater-derived nutrients in the stream. Discharge rates from different ground waters were measured using an end-member mixing model coupled with injections of a conservative solute tracer into the stream channel.
2. In surface water, nitrate concentration averaged 1.13 mg N L^–1 and DOC concentration averaged 1.82 mg C L^–1.
3. Groundwater discharge into Grape Vine Canyon Stream was derived from three sources. Nitrate concentration varied among the three groundwater sources with mean concentrations of 0.56, 0.94 and 0.08 mg N L^–1. DOC, in contrast, did not vary among ground water sources, with an overall average concentration of 2.96 mg C L^–1.
4. In the surface stream, nitrate concentration was two-fold greater than the concentration predicted from groundwater input, indicating that in-stream processes generated nitrate. Stream DOC concentration was lower than predicted based upon groundwater input rate. The production of nitrate and loss of DOC suggest that DOC is lost through mineralisation of dissolved organic matter, possibly resulting in the mineralisation of dissolved organic nitrogen to ammonium and subsequent transformation to nitrate via nitrification. In further support of this hypothesised linkage, DOC loss explained 80–89% of the variance in nitrate production in Grape Vine Canyon Stream.     

Biogeochemical and hydrological controls on carbon export from a forested catchment in central Japan

Here we review research on the links between hydrological processes and the biogeochemical environment controlling the dynamics of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) in temperate forested catchments. In addition, we present the results of original experiments. The spatial and temporal changes in DIC and DOC concentrations were investigated in tandem with observations of elementary belowground hydrological processes for a forested headwater catchment in central Japan. The soil CO₂ gas concentration, which is the source of DIC, increased with depth. The hydrological characteristics of groundwater also affected the spatial variation of partial pressure of dissolved CO₂ (pCO₂) in groundwater. The temporal variations in the soil CO₂ gas concentration and the pCO₂ values of groundwater suggested that the dynamics of DIC were strongly affected by biological activity. However, the geographical differences in DIC leaching were affected not only by the link between climatological conditions and biological activity, but also by other factors such as geomorphologic conditions. The DOC concentrations decreased with selective removal of hydrophobic acid during vertical infiltration. The major DOC-removal mechanisms were retention of metal-organic complexes to soil solids in the upper mineral soil layer and decomposition of DOC in the lower mineral soil layer. The responses of the DIC and DOC concentrations to changes in discharge during storm events were explained by the spatial variation in the DIC and DOC concentrations. Seasonal variation, which represents a long-term change, in stream water DOC concentrations was affected not only by the temporal variation in DOC concentrations in the topsoil, which may be affected by biological activity, but also by water movement, which transports DOC from the topsoil to stream water. These results indicate that both a biogeochemical approach and a method for evaluating the hydrological effects on carbon dynamics are critical for clarifying the carbon accumulation-and-release processes in forested ecosystems.