Increasing riverine export of dissolved organic carbon from China

River transport of dissolved organic carbon (DOC) to the ocean is a crucial but poorly quantified regional carbon cycle component. Large uncertainties remaining on the riverine DOC export from China, as well as its trend and drivers of change, have challenged the reconciliation between atmosphere-based and land-based estimates of China's land carbon sink. Here, we harmonized a large database of riverine in-situ measurements and applied a random forest model, to quantify riverine DOC fluxes (F_DOC) and DOC concentrations (C_DOC) in rivers across China. This study proposes the first DOC modeling effort capable of reproducing well the magnitude of riverine C_DOC and F_DOC, as well as its trends, on a monthly scale and with a much wider spatial distribution over China compared to previous studies that mainly focused on annual-scale estimates and large rivers. Results show that over the period 2001–2015, the average C_DOC was 2.25 ± 0.45 mg/L and average F_DOC was 4.04 ± 1.02 Tg/year. Simultaneously, we found a significant increase in F_DOC (+0.044 Tg/year², p = .01), but little change in C_DOC (−0.001 mg/L/year, p > .10). Although the trend in C_DOC is not significant at the country scale, it is significantly increasing in the Yangtze River Basin and Huaihe River Basin (0.005 and 0.013 mg/L/year, p < .05) while significantly decreasing in the Yellow River Basin and Southwest Rivers Basin (−0.043 and −0.014 mg/L/year, p = .01). Changes in hydrology, play a stronger role than direct impacts of anthropogenic activities in determining the spatio-temporal variability of F_DOC and C_DOC across China. However, and in contrast with other basins, the significant increase in C_DOC in the Yangtze River Basin and Huaihe River Basin is attributable to direct anthropogenic activities. Given the dominance of hydrology in driving F_DOC, the increase in F_DOC is likely to continue under the projected increase in river discharge over China resulting from a future wetter climate.     

Alternative explanations for rising dissolved organic carbon export from organic soils

Since 1988, there has been, on average, a 91% increase in dissolved organic carbon (DOC) concentrations of UK lakes and streams in the Acid Waters Monitoring Network (AWMN). Similar DOC increases have been observed in surface waters across much of Europe and North America. Much of the debate about the causes of rising DOC has, as in other studies relating to the carbon cycle, focused on factors related to climate change. Data from our peat‐core experiments support an influence of climate on DOC, notably an increase in production with temperature under aerobic, and to a lesser extent anaerobic, conditions. However, we argue that climatic factors may not be the dominant drivers of DOC change. DOC solubility is suppressed by high soil water acidity and ionic strength, both of which have decreased as a result of declining sulphur deposition since the 1980s, augmented during the 1990s in the United Kingdom by a cyclical decline in sea‐salt deposition. Our observational and experimental data demonstrate a clear, inverse and quantitatively important link between DOC and sulphate concentrations in soil solution. Statistical analysis of 11 AWMN lakes suggests that rising temperature, declining sulphur deposition and changing sea‐salt loading can account for the majority of the observed DOC trend. This combination of evidence points to the changing chemical composition of atmospheric deposition, particularly the substantial reduction in anthropogenic sulphur emissions during the last 20 years, as a key cause of rising DOC. The implications of rising DOC export for the carbon cycle will be very different if linked primarily to decreasing acid deposition, rather than to changes in climate, suggesting that these systems may be recovering rather than destabilising.     

Controls on dissolved organic carbon composition and export from rice-dominated systems

Rice field outflow can contain high concentrations of dissolved organic carbon (DOC), which plays a crucial role in drinking water quality and aquatic ecosystem processes. This study examined the relationship between potential determining factors (i.e. rice area, outflow, drainwater reuse, soil properties, and time, measured as the day in the growing season) and the concentration and composition of DOC exported from 11 rice-dominated subwatersheds. Samples were collected from subwatershed inflow and outflow every 1–2 weeks from May through September 2008 and analyzed for DOC concentration, trihalomethane formation potential (THMFP), and also specific ultraviolet absorbance (SUVA₂₅₄) and the spectral slope parameter (S), which are indicators of DOC composition. Concentrations of DOC across all subwatersheds and sampling dates ranged from 1.56 to 14.43 mg L^?1 (mean = 4.32 mg L^?1). Linear mixed effects (LME) analysis indicated that DOC concentration decreased over time, and that THMFP, and DOC and THM flux, decreased over time, but increased with outflow. LME analysis of the SUVA₂₅₄ and S parameters indicated that the fraction of aromatic DOC moieties increased with time, outflow, and reuse. Additionally, apparent peaks in DOC concentrations, THMFP, and SUVA₂₅₄ coincided with the onsets of flooding and draining. Lastly, subwatersheds with outflow less than approximately 4,700 m³ ha^?1 behaved as sinks of DOC. Our findings suggest that water management factors such as outflow, reuse, and discrete irrigation events, all of which vary over the course of the growing season, were the dominant determinants of DOC concentration and composition.     

Acidity controls on dissolved organic carbon mobility in organic soils

Dissolved organic carbon (DOC) concentrations in surface waters have increased across much of Europe and North America, with implications for the terrestrial carbon balance, aquatic ecosystem functioning, water treatment costs and human health. Over the past decade, many hypotheses have been put forward to explain this phenomenon, from changing climate and land management to eutrophication and acid deposition. Resolution of this debate has been hindered by a reliance on correlative analyses of time series data, and a lack of robust experimental testing of proposed mechanisms. In a 4 year, four‐site replicated field experiment involving both acidifying and deacidifying treatments, we tested the hypothesis that DOC leaching was previously suppressed by high levels of soil acidity in peat and organo‐mineral soils, and therefore that observed DOC increases a consequence of decreasing soil acidity. We observed a consistent, positive relationship between DOC and acidity change at all sites. Responses were described by similar hyperbolic relationships between standardized changes in DOC and hydrogen ion concentrations at all sites, suggesting potentially general applicability. These relationships explained a substantial proportion of observed changes in peak DOC concentrations in nearby monitoring streams, and application to a UK‐wide upland soil pH dataset suggests that recovery from acidification alone could have led to soil solution DOC increases in the range 46–126% by habitat type since 1978. Our findings raise the possibility that changing soil acidity may have wider impacts on ecosystem carbon balances. Decreasing sulphur deposition may be accelerating terrestrial carbon loss, and returning surface waters to a natural, high‐DOC condition.     

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.     

Limitation of lowland riverine bacterioplankton by dissolved organic carbon and inorganic nutrients

Flow regulation in lowland rivers has reduced the amount of allochthonous dissolved organic carbon (DOC) entering main channels through less frequent wetting of benches, flood runners and floodplains. The hypothesis tested was that lowland riverine bacterioplankton are DOC limited when flow events are absent and simulating an increase in assimilable DOC similar to that expected during an environmental flow will lead to heterotrophic dominance. Experiments took place in the Namoi River, a highly regulated lowland river in Australia. Specifically, in situ microcosms were used to examine the responses of bacterioplankton and phytoplankton to various additions of DOC as glucose or leaf leachate, with and without additions of inorganic nutrients. The results indicated that ambient DOC availability limited the bacterioplankton for the three seasons over which we conducted the experiments. When DOC was added alone, dissolved oxygen concentrations decreased primarily because of increased bacterial respiration and bacterioplankton growth generally increased relative to controls. Additions of DOC alone led to a pattern of decreased chlorophyll a concentration relative to controls, except for willow leachate. Additions of inorganic nutrients alone increased chlorophyll a concentrations above controls, indicating limitation of phytoplankton. These findings support our hypothesis. Based on the present results, environmental flows should increase the duration of allochthonously driven heterotrophic dominance, thus shifting regulated lowland rivers to more natural (pre-regulation) conditions for greater periods.     

Long term records of riverine dissolved organic matter

This presents the longest, consistent records of dissolved organic carbon in rivers ever published. This study presents long-term records of organic matter as indicated by water colour that were constructed for three catchments in Northern England for as far back as 1962. Observations show that there have been large increases in DOC concentrations over the period of study with in one case a doubling of the concentration over a period of 29 years. However, in one of the catchments no significant change was observed over a 31-year period. All catchments show common inter-annual control on carbon release in response to droughts, but no step increases in DOC concentrations were observed in response to such perturbations with pre-drought levels being restored within a period 3–4 years. Observed increasing trends do not correlate with changes in river discharge, pH, alkalinity or rainfall, but do coincide with increasing average summer temperatures in the region. The times series of DOC concentration over the period of the record appears stationary, but the distribution of daily values suggests a change in sources of colour over the increasing trend. The evidence supports a view that increases in carbon release are in equilibrium with temperature increases accentuated by land-use factors.     

Seasonal patterns of viruses, bacteria and dissolved organic carbon in a riverine wetland

SUMMARY 1. Viral and bacterial abundances were studied in relation to environmental attributes over an annual period, for both planktonic and attached (sediment, aquatic macrophyte and submerged wood) habitats, in a riverine wetland.
2. Annual mean abundance of planktonic viruses ranged from 2.3 × 10⁵−3.8 × 10⁵ particles mL⁻¹ and varied according to sampling site. Significant seasonal patterns in viral abundance were evident and appeared to be linked to variations in bacterial abundance, dissolved organic carbon and inorganic nutrients.
3. Annual mean abundance of viruses associated with surfaces ranged from 1.3 × 10⁶ particles cm⁻² on aquatic macrophytes to 1.1 × 10⁷ particles cm⁻² on wood and also showed seasonal patterns. The difference in viral dynamics among the different sites emphasizes the importance of considering habitat diversity within wetlands when examining microbial communities.     

Controls on fluxes and export of dissolved organic carbon in grasslands with contrasting soil types

Export of dissolved organic carbon (DOC) from grassland ecosystems can be an important C flux which directly affects ecosystem C balance since DOC is leached from the soil to the groundwater. DOC fluxes and their controlling factors were investigated on two grassland sites with similar climatic conditions but different soil types (Vertisol vs. Arenosol) for a 2.5-year period. Parts of both grasslands were disturbed by deep ploughing during afforestation. Contrary to what was expected, ploughing did not increase DOC export but surface soil DOC concentrations decreased by 28% (Vertisol) and 14% (Arenosol). DOC flux from the soil profile was negatively influences by the clay content of the soil with seven times larger DOC export in the clay-poor Arenosol (55 kg C ha^?1 a^?1) than in the clay-rich Vertisol (8 kg C ha^?1 a^?1). At the Arenosol site, highest DOC concentrations were measured in late summer, whereas in the Vertisol there was a time lag of several months between surface and subsoil DOC with highest subsoil DOC concentrations during winter season. DOC export was not correlated with soil organic carbon stocks. Large differences in ¹⁴C concentrations of 22–40 pMC between soil organic carbon and DOC in the subsoil indicated that both C pools are largely decoupled. We conclude that DOC export at both sites is not controlled by the vegetation but by physicochemical parameters such as the adsorption capacity of soil minerals and the water balance of the ecosystem. Only in the acidic sandy Arenosol DOC export was a significant C flux of about 8% of net ecosystem production.     

Photooxidation of wetland and riverine dissolved organic matter: altered copper complexation and organic composition

In natural waters, the uptake of transition metals such as copper (Cu) by aquatic biota depends on the activity of the free cupric ion ({Cu²⁺}) rather than on total Cu concentration. Thus, an important ecological function of dissolved organic matter (DOM) in aquatic ecosystems is Cu–DOM complexation, which greatly decreases the {Cu²⁺}. However, Cu bioavailability is greatly modified by source and environmental history of DOM because DOM affinity for Cu varies by orders of magnitude among DOM sources; moreover, DOM is photochemically unstable. During 72-h irradiation experiments at intensities approximating sunlight with DOM from a palustrine wetland and a third-order river, we investigated photooxidative effects on DOM complexation of Cu as well as spectral and chemical changes in DOM that might explain altered Cu complexation. Irradiation decreased Cu complexation by riverine DOM, but unexpectedly increased Cu complexation by wetland DOM, resulting in 150% greater {Cu²⁺} in riverine DOM at the same dissolved organic carbon concentrations. The specific ultraviolet absorption (SUVa) and humic substances tracked photochemical changes in the conditional stability constants of Cu–DOM complexes, suggesting that the aromaticity of DOM influences its affinity for Cu. Carbonyl concentration in ¹³C nuclear magnetic resonance spectra (¹³C-NMR) covaried directly with Cu binding-site densities in DOM. However, no aspect of Cu–DOM complexation consistently covaried with fluorophores (i.e., the fluorescence index) or low molecular weight organic acids. Our results suggest that global increases in UV radiation will affect Cu–DOM complexation and subsequent Cu toxicity depending on light regime as well as DOM source. Handling editor: K. Martens     

Landscape controls on total mercury and methylmercury export from small boreal forest catchments

Biogeochemistry - Mercury (Hg) is a widespread contaminant known to pose severe risks to wildlife and human health. While Hg emissions have declined in recent decades, legacy emissions and stored...     

The significance of storms for the concentration and export of dissolved organic carbon from two Precambrian Shield catchments

Dissolved organic carbon (DOC) concentrations and DOC export arestudied during storms to examine the relationship between DOCconcentration and stream discharge and to assess the importance of stormson DOC export. Storms were monitored in seven subcatchments within twosmall watersheds (Harp 4--21 and Harp 3A) on the Precambrian Shield inCentral Ontario, Canada. Stream DOC concentrations increase during stormsby as much as 100% and 410% in Harp3A and Harp 4--21 respectively. The seasonal regression between DOC andstream discharge is significant in subcatchments without wetlands(r² > 0.7) but is not significant in thetwo subcatchments with small wetland areas (r² <0.06). On average, regressions based on weekly data yield accurate estimatesof DOC export but the variation in regressions among individual storms andthe small number of high DOC samples result in uncertainties of more than30% in DOC export. The period-weighted calculation ofDOC export from weekly data underestimates export by 14%and 22% in Harp 3A and Harp 4--21 respectively. Stormswere responsible for 57% to 68% of theDOC export in the autumn and 29% to 40%of the DOC export in the spring. A single large storm accounted for31% of the autumn DOC export in Harp 3A. The importanceof individual storms for DOC export and the variation in the relationshipbetween DOC and stream discharge among storms make it difficult to predictthe effects of climate change on DOC export and DOC concentrations.     

Forest harvest legacies control dissolved organic carbon export in small watersheds,western Oregon

This study examined physical and biological controls on dissolved organic carbon (DOC) fluxes from conifer-forest watersheds in the H.J. Andrews Experimental Forest of Oregon. We tested how DOC export was related to streamflow and legacies of wood on the forest floor three to five decades after harvest of old-growth forest in seven watersheds spanning the rain to snow elevation gradient. Three watersheds had old-growth forest and four had 30 to 50-year-old forest established after clearcutting of old-growth forest. Mean annual DOC flux in the watersheds was related to the biomass of forest floor wood, which was two or three times higher in watersheds with old-growth forest compared to young forest, and was inversely related to elevation, a measure of snowpack depth and duration. In contrast, fluxes of inorganic elements such as Si and Ca did not vary with harvest history or forest floor characteristics. Annual fluxes of DOC, Si, and Ca were linearly related to annual runoff, and annual volume-weighted concentrations of these ions declined by?<?0.6% with several-fold increases in annual runoff. Across all years, DOC concentrations peaked before the peak of the hydrograph in all watersheds, which we interpret as representing movement, likely via preferential and surficial flow, of organic materials mineralized and solubilized during the long dry summers in this ecosystem. DOC concentrations relative to stream flow exhibited clockwise hysteresis loops in each water year, also suggesting that soluble DOC produced in the dry summer is exported in the fall. DOC concentration differences between reference and harvested watersheds also peaked in late summer or early fall, suggesting that the source of the additional DOC from reference watersheds came from coarse woody debris that remains moist during the dry summers and that was significantly greater in watersheds with elevated DOC. Taken together, our results suggest that forest floor wood is a previously unappreciated control on the supply of DOC that can be exported, and runoff is a secondary control on total DOC flux to streams. The legacy of forest harvest on DOC flux can be observed for decades, as total ecosystem carbon stocks, especially coarse woody debris, may require centuries to develop after old-growth forest harvest.     

Bacterial consumption of total and dissolved organic carbon in the Great Barrier Reef

Heterotrophic bacteria typically take up directly dissolved organic matter due to the small molecular size, although both particulate and dissolved organic matter have labile (easily consumed) compounds. Tropical coastal waters are important ecosystems because of their high productivity. However, few studies have determined bacterial cycling (i.e. carbon uptake by bacteria and allocation for bacterial biomass and respiration) of dissolved organic carbon in coastal tropical waters, and none has determined bacterial cycling of total and dissolved organic carbon simultaneously. In this study we followed bacterial biomass and production, and organic carbon changes over short-term (12 days) dark incubations with (total organic carbon, TOC) and without particulate organic carbon additions (dissolved organic carbon, DOC). The study was performed at three sites along the middle stretch of the Great Barrier Reef (GBR) during the dry and wet seasons. Our results show that the bacterial growth efficiency is low (0.1–11.5%) compared to other coastal tropical systems, and there were no differences in the carbon cycling between organic matter sources, seasons or locations. Nonetheless, more carbon was consumed in the TOC compared to the DOC incubations, although the proportion allocated to biomass and respiration was similar. This suggests that having more bioavailable substrate in the particulate form did not benefit bacteria. Overall, our study indicates that when comparing the obtained respiration rates with previously measured primary production rates, the GBR is a heterotrophic system. More detailed studies are required to fully explore the mechanisms used by bacteria to cycle TOC and DOC in tropical coastal waters.

     

Bacterial growth on dissolved organic carbon from a blackwater river

Different nominal molecular weight (nMW) fractions of DOC from a southeastern blackwater river were concentrated by ultrafiltration and added to sieved river water to assess each fraction's ability to stimulate bacterial growth. Bacterial growth was measured using change in bacterial biomass from direct counts and using³H-thymidine incorporated into DNA. Bacterial growth and amount of DOC used was greatest in the low MW enrichment (< 1,000 nMW) and least in the intermediate MW enrichment (1,000–10,000 nMW). The high MW fraction (> 10,000 nMW) supported more growth than did the intermediate MW fraction, apparently because of lower MW compounds complexed with a high MW refractory core. The low MW fraction of DOC from a clearwater mountain stream, a boreal blackwater river, and leachate from water oak and willow leaves also stimulated more bacterial growth than did other fractions. However, the high MW DOC from these other sources was not as biologically available as high MW DOC from a blackwater river. Bacteria converted blackwater river DOC to bacterial biomass with an efficiency of 31%. Bacteria produced at the expense of abundant riverine DOC provide a trophic resource for protozoa and higher levels of the microbial food web of a blackwater river.     

A worldwide view of organic carbon export from catchments

Growing interest in the effects of global change on the metabolism, stoichiometry and cycling of carbon in aquatic ecosystems has motivated research on the export of organic carbon (OCE) from catchments. In this article, quantitative and functional features of the annual export rates of total, particulate and dissolved organic carbon (TOC, POC and DOC) were reviewed, and the stoichiometry of export (OC:N, OC:P and N:P) from 550 catchments worldwide was reported. TOC export ranged 2.1–92,474?kg?C?km^?2?year^?1, POC export ranged 0.4–73,979?kg?C?km^?2?year^?1 and DOC export ranged 1.2–56,946?kg?C?km^?2?year^?1. Exports of TOC and DOC were strongly linked, but POC export was unrelated to DOC. The DOC fraction comprised on average 73?±?21% of TOC export. The export rates of organic carbon were poorly related to those of total nitrogen and total phosphorus. Discrete and continuous environmental variables failed to predict TOC export, but DOC export was influenced by discharge and catchment area worldwide. Models of OCE in different catchment types were controlled by different environmental variables; hydrological variables were generally better predictors of OCE than anthropogenic and soil variables. Elemental ratios of carbon export in most catchments were above the Redfield ratio, suggesting that phosphorus may become the limiting nutrient for downstream plant growth. These ratios were marginally related to environmental data. More detailed hydrological data, consideration of in-stream processes and the use of quasi-empirical dynamical models are advocated to improve our knowledge of OCE rates and those of other nutrients.     

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.