Dissolved Organic Carbon Distribution Research Articles

Aerobic respiration in riparian exchange zones of regulated river corridors

AbstractRiver stage fluctuations drive surface water‐groundwater exchanges within river corridors. This study evaluates how repeated daily stage fluctuations, representative of hydropeaking conditions, influence aerobic respiration of river‐sourced dissolved organic carbon (DOC) in the riparian exchange zone using reactive flow and transport simulations. Over 50 hypothetical scenarios were modelled to evaluate how the duration of the daily flood signal, river DOC concentration, aquifer hydraulic conductivity and ambient groundwater flow condition affect the fate and transport of DOC and DO in the riparian aquifer. Time series subsurface snapshots highlight how the various factors influence the subsurface distribution of DOC and DO. The total mass of DOC respired per meter of river had a wide range depending on the parameters, spanning from 1.4 to 71 g over 24‐h, with high hydraulic conductivity and losing ambient groundwater flow conditions favouring the largest amount of DOC respired. The ratio of DOC mass entering the riparian zone with the mass returning to the river showed that as little as 5% to as much as 76% of the DOC that enters the bank during stage fluctuations returns to the river. This return ratio is dependent on river DOC concentration, hydraulic conductivity and ambient groundwater flow. The results illustrate that stage variations due to river regulation can be a significant control on aerobic respiration in riparian exchange zones.

Evidence of Deep DOC Enrichment via Particle Export Beneath Subarctic and Northern Subtropical Fronts in the North Pacific

Here we provide compelling evidence that deep particle export enhanced dissolved organic carbon (DOC) concentrations beneath the Pacific’s Subarctic Front (SA, ∼42°N) and Northern Subtropical Front (NST, ∼34°N). We report three main findings: First, deep export of subjectively small particles (128–512 μm) was apparent throughout the frontal zone in which the SA resides. However, export of large particles was specifically associated with the SA, rather than the entire frontal zone, and appeared to exclusively transfer DOC into the bathypelagic water column. Second, a similar DOC enrichment existed beneath the NST, though this signal was curiously not accompanied by observable particles (>128 μm). We conclude that export occurring previously in winter left this DOC behind as a residue, though the associated particles were no longer present by spring. Third, the presence of strong hydrographic fronts was not the only control on export that resulted in these unique DOC distributions. Deep export and DOC enrichment was also controlled by latitude-specific biogeochemical and hydrographic conditions, such as depth of the nutricline and seasonal mixed layer shoaling. Given these observations, the fronts within the transitional region of the North Pacific are clearly special locations for deep carbon sequestration and for providing uncommon DOC enrichment that ultimately supports the deep microbial community.

Sorption and desorption of organic matter in soils as affected by phosphate

The contribution of different adsorption processes to soil organic matter (SOM) stabilization and the consequences of the intensification of land use on the adsorption of SOM are not yet fully understood. Therefore, this study aimed to explore the adsorption of dissolved organic carbon (DOC) in soils as well as desorption of organic carbon (OC) caused by phosphate addition. We conducted desorption and sorption experiments with DOC, phosphate (Na2HPO4), and chloride (KCl) in topsoil and subsoil samples from a Ferralsol, an Andosol, and a Podzol. Furthermore, we quantified the size of DOC by size-exclusion chromatography.We found that phosphate addition caused a strong increase in the DOC concentration in all soils. The DOC concentration was elevated by up to a factor of 4.5 compared to a control (water only) within five minutes after phosphate addition and kept increasing further with time, particularly in the Ferralsol. After 10 days, the DOC concentrations in the P addition treatment were between 5.5 and 18.0 times higher than in the control treatment. In contrast, chloride addition did not lead to increased DOC concentrations compared to the control (water only). Phosphate addition led mostly to desorption of organic matter of medium molecular size (10–100 and 100–1000 kDa) in the Ferralsol and the Andosol and of large molecular size (>1000 kDa) in the Podzol. In contrast, potassium chloride addition shifted the size distribution of DOC in the soil solution towards small compounds (<10 kDa), likely because KCl addition affected the aggregation of DOC compounds, in contrast to Na2HPO4 addition. Furthermore, phosphate addition also decreased subsequent sorption of DOC in the Ferralsol and the Andosol by a factor of up to 2.9 and 2.1, respectively.Our results have far-reaching implications because they show that phosphate addition can lead to desorption, and thus to destabilization of SOM, particularly in Ferralsols, and can also prevent further sorption of organic matter in soils. Our study provides a mechanistic explanation of why phosphate addition can decrease soil organic carbon sequestration and stabilization. This mechanism should be considered in the analysis of land-use and tillage effects on SOM sequestration in the future. Furthermore, our study indicates that also medium and large molecular size organic matter is stabilized through adsorption in soils.

Surface transport of DOC acts as a trophic link among Mediterranean sub-basins

Lateral advection affects the spatial distribution of dissolved substances in the ocean but very few studies, so far, have been devoted to describe and quantify its impact on the distribution of dissolved organic carbon (DOC) which, in oligotrophic environments, accounts for the largest fraction of chemical energy. In this contribution, using an integrated approach, we explore the importance of surface advection on carbon dynamics in the Western Mediterranean Sea, where strong inter-basin differences in primary production do exist. Detailed information on the surface circulation, derived from high-resolution model simulations, is combined with the analysis of spatially resolved, accurate DOC vertical profiles, repeated over time. Our data show that surface circulation plays a crucial role in regulating DOC concentrations and distributions in the Tyrrhenian Sea and that horizontal transport of DOC into the Tyrrhenian Sea is of the same order of magnitude as in-situ DOC production. Our study addresses this process for the first time in the Mediterranean Sea, whose small size allows for fast, inter-basin transfer times which, in turn, favors the preservation of the DOC stock produced elsewhere. We highlight that this mechanism may be important also in other regions of the oceans, where surface advection may set up a sort of compensation among regions with different trophic regimes, thus smoothing trophic gradients. We posit that understanding these transport processes is a crucial and preliminary step to understand and quantify all the other processes (biological, chemical, geological) that influence DOC distribution on a variety of timescales.

Tracing riverine dissolved organic carbon and its transport to the halocline layer in the Chukchi Sea (western Arctic Ocean) using humic-like fluorescence fingerprinting

Dissolved organic carbon (DOC) and the fluorescence properties of dissolved organic matter (FDOM) were investigated using parallel factor analysis (PARAFAC) for seawater samples collected in the Chukchi Sea (65°N–78°N, 170°E–160°W) during summer 2017. River water (friver) and sea-ice meltwater (fsea ice melt) fractions were also derived using oxygen isotopes ratios (δ18O) to examine the influence of sea ice on riverine DOM. The spatial distributions of friver, riverine DOC, and the humic-like fluorescent component (C1) showed an overall south-north gradient, with higher values in the northern Chukchi Sea in summer. Pronounced accumulation of river water and riverine DOM was also observed in the anticyclonic Beaufort Gyre at the eastern stations of the northern Chukchi Sea in association with a long water residence time. Estimated riverine DOC in the surface layer accounted for 27 ± 9% (range: 17–47%) of the total DOC in the southern Chukchi Sea, and 39 ± 6% (range: 32–49%) and 31 ± 4% (range: 25–37%) for the eastern and western stations of the northern Chukchi Sea, respectively. Humic-like C1 showed negative and positive relationships with sea-ice meltwater-corrected salinity (Ssim_corrected) and friver, respectively. However, Arctic river waters with distinct humic-like C1 characteristics were likely mixed in the northern Chukchi Sea. The vertical distributions of riverine DOC, humic-like C1 fluorescence, and friver generally decreased with water depth, reflecting the strong influence of riverine DOM in the surface layer. Although riverine DOM and friver were dominant in the upper 50 m of the water column, they were also pronounced in the upper halocline (50–200 m), in which fsea ice melt dropped below zero. Our results indicated the existence of brine rejected from growing sea ice, and that sea-ice formation was a key factor for the transport of riverine DOM to the upper halocline layer in the northern Chukchi Sea.

Soil dissolved organic carbon in terrestrial ecosystems: Global budget, spatial distribution and controls

AbstractAimsSoil dissolved organic carbon (DOC) is a primary form of labile carbon in terrestrial ecosystems, and therefore plays a vital role in soil carbon cycling. This study aims to quantify the budgets of soil DOC at biome and global levels and to examine the variations in soil DOC and their environmental controls.LocationGlobal.Time period1981–2019.MethodsWe compiled a global dataset and analysed the concentration and distribution of DOC across 10 biomes.ResultsLarge variations in DOC are found among biomes across space and the soil DOC concentration declines exponentially along soil depths. Tundra has the highest soil DOC concentration in 0–30 cm soils [453.75 (95% confidence interval: 324.95–633.5) mg/kg], whereas tropical and temperate forests have relatively lower DOC concentrations, ranging from 30.20 (24.78–36.80) to 54.54 (49.77–59.77) mg/kg. DOC generally accounts for &lt; 1% of total organic carbon in soils, and DOC in 0–30 cm contributes more than half of the total DOC in the 0–100 cm soil profile. Furthermore, variations in DOC are primarily controlled by soil texture, moisture, and total organic carbon.Main conclusionsA global synthesis is combined with an empirical model to extrapolate the DOC concentration along soil profiles across the globe, and global budgets of DOC are estimated as 7.20 Pg C in the top 0–30 cm and 12.97 Pg C in the 0–100 cm soil profile, respectively, with a considerable variation among biomes. The strong soil texture control but weak total organic carbon (TOC) control on DOC variations suggest that the investigation of physical protection of soil organic carbon might need to expand to consider the labile C in soils. The global maps of DOC concentration serve as a benchmark for validating land surface models in estimating carbon storage in soils.

Contrasting dissolved organic carbon concentrations in the Bransfield Strait, Northern Antarctic Peninsula: insights into ENSO and SAM effects

The marine reservoir of dissolved organic carbon (DOC) is equivalent to the atmospheric carbon reservoir and represents the largest reduced carbon stock in the ocean. However, the role of DOC in the global carbon cycle is not fully understood. In this study, we analyzed the distribution of DOC in the water column of the Bransfield Strait, Northern Antarctic Peninsula. Data were collected during two summer cruises of the NAUTILUS project conducted by the Brazilian High Latitude Oceanography Group in February 2015 and 2016. We investigated the possible effects of the El Niño - Southern Oscillation (ENSO) and Southern Annular Mode (SAM) climate modes on DOC variability based on correlations between DOC and temperature, salinity, and dissolved oxygen. Concentrations of DOC ranged from 40.70 to 122.82 μmol kg−1 in 2015 and 33.12 to 112.20 μmol kg−1 in 2016. The analysis shows that the vertical distribution of DOC in the Bransfield Strait may be controlled by a combination of vertical stratification, vertical export of organic matter and biological activity in the upper mixed layer during summertime. The significant interannual differences observed between DOC concentration and most physicochemical parameters may be linked to ENSO and SAM climate modes. The highest DOC concentration was observed in February 2015, associated with a layer of warm surface water, due to the higher stratification of the water column. The ENSO and SAM climate modes were low and positive in that year, favoring intrusions of waters from the Antarctic Circumpolar Current. In contrast, February 2016 was characterized by record positive ENSO and SAM modes. The intensification of the Weddell Gyre induced the advection of cold, more oxygenated and recently formed shelf waters into the Bransfield Strait, which may have contributed to the lower DOC concentrations. This study contributes to a better understanding of the role of DOC in the global carbon cycle in a region of complex ocean circulation and mixing, and sensitive to global climate change.

Microbial Physiology Governs the Oceanic Distribution of Dissolved Organic Carbon in a Scenario of Equal Degradability

Dissolved organic carbon (DOC) forms one of the largest active organic carbon reservoirs on Earth and reaches average radiocarbon ages of several thousand years. Many previous large scale DOC models assume different lability classes (labile to refractory) with prescribed, globally constant decay rates. In contrast, we assume that all DOC compounds are equally degradable by a heterotrophic microbial community. Based on this central assumption, we simulate DOC concentrations using a simple biogeochemical box model. Parameterized correctly, the simple model of neutral DOC uptake produced a recalcitrant carbon pool of 33mmolC/m3, throughout the entire virtual ocean. The spatial distribution of DOC in the model was independent of the distribution of DOC sources from primary production and particle degradation. Instead, DOC concentrations were primarily driven by spatial gradients in microbial physiology, e.g. mortality rate or growth efficiency. Applying such a gradient, we find DOC concentrations of ~70mmolC/m3 at the surface and ~35mmolC/m3 in the deep ocean. Introducing model variations such as seasonally-varying supply rates or temperature-dependent DOC uptake did not significantly alter model results. DOC spatial patterns are thus not necessarily shaped by the co-cycling of separate reactivity fractions, but can also arise from gradients in physiological parameters determining DOC uptake. We conclude that neutral DOC uptake can lead to realistic large-scale patterns of DOC concentration in the ocean.

Influence of environmental variables on the distribution and community structure of mesozooplankton in the coastal waters of the eastern Arabian Sea

The spatio-temporal variation of mesozooplankton in response to environmental variables was studied for the coastal waters of southeastern Arabian Sea during the spring inter-monsoon (SIM) and fall inter-monsoon (FIM) of 2014. Overall 84 mesozooplankton taxa belonging to 17 taxonomic groups were identified. The copepod group dominated species richness (58 species) of the mesozooplankton community. Spatial variation of mesozooplankton biomass and total abundance changed significantly in relation to the distribution of sub-surface water (0.5 m) dissolved organic carbon (DOC). Carnivorous and omnivorous mesozooplankton abundance increased substantially with an increasing DOC level. Factor analysis and Mantel test revealed that the distribution of mesozooplankton community, biomass and abundance had significant positive correlation with the distribution of chlorophyll a (Chl-a) and DOC during both the seasons. Distribution of Chl-a had a positive influence on herbivorous mesozooplankton abundance, while DOC had a positive relationship with omnivorous and carnivorous mesozooplankton abundance. Our study indicates that the distribution of Chl-a and DOC influences the spatial variation of mesozooplankton community and abundance in the coastal waters of eastern Arabian Sea during the inter-monsoon seasons (SIM and FIM).

Morphometric Control on Dissolved Organic Carbon in Subarctic Streams

AbstractClimate change has the potential to alter hydrological regimes and to expand saturated areas in permafrost environments, which are important sources of organic carbon. The sources, transfer zones, and delivery mechanisms of carbon into the stream network are controlled by the morphometric properties of the catchment; however, the utility and limitations of these properties as predictors of dissolved organic carbon concentrations have rarely been systematically evaluated. This study tested the relationships between 18 morphometric indicators and observed dissolved organic carbon (DOC) concentrations in the Stordalen catchment, Sweden. Geospatial and explorative statistics were combined to assess the topographical, areal, and linear indicators influencing the distribution of DOC in the catchment. The results suggest that catchment morphometric indicators can be used as proxies to predict DOC concentrations along a longitudinal continuum in subarctic climate regions (R2 up to 0.52). Morphometry indicators that best served as predictors of DOC concentration in the model were as follows: relief, slope length and steepness factor (LS‐factor), sediment transport capacity, and catchment area. Due to the influence that catchment form exerts in DOC spatial patterns and processing, a morphometric approach can serve as a first approximation of DOC spatial patterns within a catchment. The initial step in identifying carbon sources based on the catchment topography has the potential to allow for quick and multilevel comparison within and between catchments.

High Temporal Variability of Total Organic Carbon in the Deep Northeastern Pacific

The interseasonal and interannual variability of total organic carbon (TOC) was assessed in the deep northeastern Pacific Ocean, an area characterized by high primary production and organic matter export. Samples were collected from throughout the deeper water column (>250 m) seasonally in 2017 and 2018 along the Line P transect, as well as one distribution of dissolved organic carbon (DOC) in spring 2018. High heterogeneity was observed in TOC concentrations at depths greater than 1000 meters, both within seasons (concentration differences of ~2-6 µmol kg-1) and across seasons (~2-12 µmol kg-1), coinciding with changes in fluorescence in the overlying waters. Such observations suggest that biogenic particles sinking from the upper ocean are seasonally delivering observable TOC to depth. The presence of these particles also appeared to contribute to the DOC pool, as suggested by differences in the TOC and DOC distributions in spring 2018. Seasonal TOC net accumulation and removal rates differed between the years: 0.5 and 2.1 µmol kg-1 day-1 (accumulation) and 0.6 and 2.1 µmol kg-1 day-1 (removal) for 2017 and 2018, respectively. The rate estimates indicated that introduction of organic carbon to the bathypelagic occurred at approximately the same rate as removal post-bloom, demonstrating the efficient removal of seasonally-produced organic carbon. High abundances of gelatinous zooplankton in spring 2018, supported by higher abundances of phytoplankton, enhanced the export of organic carbon to the bathypelagic zone during the seasonal bloom, resulting in localized TOC concentrations up to 148 µmol kg-1 in the bathypelagic. These results indicate high variability in bathypelagic TOC concentrations at high latitude, unlike oligotrophic systems.

Dissolved Organic Matter in the Gulf of Cádiz: Distribution and Drivers of Chromophoric and Fluorescent Properties

The Gulf of Cadiz (GoC) connects the Mediterranean Sea with the Atlantic Ocean through the Strait of Gibraltar. Particular hydrographic processes take place in the GoC, such as riverine discharges and surface circulation marked by wind-induced seasonal upwelling. Although physical processes have been widely studied, little is known about the biogeochemical processes that occur in the basin, especially those involving organic matter. Therefore, vertical and seasonal dynamics of dissolved organic carbon (DOC) and dissolved organic matter (DOM) optical properties (absorption and fluorescence) were measured in 766 samples collected between 5 and 800 m depth during 4 oceanographic cruises to obtain quantitative and qualitative information about DOM in the GoC. We performed parallel factor analysis (PARAFAC) to identify the main fluorophores present in the GoC, and an optimum multiparameter water mass analysis to differentiate the effect of water mass mixing from the biogeochemical processes in deep waters. PARAFAC analysis validated six fluorescent components; three humic-like, two protein-like, and a possible mixture of polycyclic aromatic hydrocarbon-like with protein-like material. DOC average concentration was 77.0 ± 12.7 µM, with higher values in surface and coastal waters during summer, mainly related to primary production. Linear relationships between DOC and apparent oxygen utilization indicate differences in oxygen consumption within the deep waters, which could be related to upwelling zones. Seasonal and spatial differences were also observed in fluorescent DOM distribution. Protein-like components were the most abundant fraction, with an average contribution of 64.75 % ± 7.85 %, being higher in summer and surface waters, associated with an increase in biological activity. Our results indicate that water mass mixing is the main driver of the major humic-like components, while biogeochemical processes at a local scale explain DOC and protein-like components distribution. Our findings suggest that modeling DOM dynamics in the GoC is complicated due to its complex hydrography and the presence of multiple sources and sinks of DOM.

Characterization of DOC and CDOM and their relationship in turbid waters of a high-altitude area on the western Loess Plateau, China.

Dissolved organic carbon (DOC) and chromophoric dissolved organic matter (CDOM) in rivers and reservoirs on the western Loess Plateau, which is an area of severe soil erosion, were investigated in September 2017 to analyze the CDOM characteristics and composition, DOC distribution and influence of environmental factors on these parameters. Great differences of water parameters were exhibited between different groups based on the analysis of variance (p < 0.01). The results indicated that rivers exhibited higher DOC concentrations (mean: 3.70 mg/L) than reservoir waters (mean: 2.04 mg/L). Artificial and agricultural lands exert a large influence on DOC concentrations, which verifies the hypothesis that intense anthropogenic activity results in high DOC concentrations. The CDOM absorption at 350 nm [aCDOM(350)] of tributary water samples was 2.73 m-1, which was higher than that in the Yellow River (1.71 m-1) and reservoir waters (1.33 m-1). The effects of DOC, TC and turbulence (Tur) on CDOM are positive and significant (p < 0.05) according to the multiple linear regressions. An analysis of the optical characteristics of CDOM indicated that waters on the Loess Plateau contained abundant humic acid and higher levels of allochthonous DOM with a higher molecular weight (MW) based on the spectral slopes (S) and specific UV absorbance (SUVA254) values.

Vertical dynamics of dissolved organic carbon in relation to organic input quality and microaggregate formation in a coarse– textured Ultisol

AbstractAgainst the background of current understanding of dissolved organic carbon (DOC) adsorption onto clay surfaces, it remains unclear if bulk DOC or its fractions contribute to microaggregate formation in the top layers of coarse‐textured soils. We therefore investigated the effects of long‐term inputs of biochemically contrasting organic residues on the chemical characteristics and vertical distribution of DOC in a coarse‐textured Ultisol. During 2007–2008, DOC samples were extracted from soil profiles of a long‐term residue quality field experiment initiated in 1995. In this field experiment, groundnut stover, dipterocarp and tamarind leaf litter, as well as rice straw of contrasting biochemical quality, were applied yearly at 10 Mg ha−1. Groundnut, dipterocarp and tamarind produced large amounts (7.1–11.8 g C m−2) of high‐molecular‐weight (HMW; &gt; 10 kDa) DOC, which was found in high concentrations (30–50 mg C kg−1) in the topsoil (0–15 cm). Rice straw, however, produced large amounts (3.5 g C m−2) of low‐molecular‐weight (LMW; &lt; 1 kDa) DOC during the initial stage of decomposition. Although the HMW DOC was retained in the topsoil (0–15 cm), the LMW DOC was rapidly translocated to lower soil depths (60–80 cm). This translocation was facilitated by the low adsorption potential of the rice straw‐derived LMW DOC on colloidal surfaces of the topsoil. There was a significant positive correlation of C in the HMW DOC with that in fine particles, indicating their contribution to microaggregate formation and thus C accumulation. It was concluded that biochemical quality of residues as a determinant of concentration and chemistry of DOC and its vertical dynamics along the soil profile must be considered for SOC accumulation in coarse‐textured soils. Furthermore, we found reasonable indications that HMW DOC contributes to microaggregate formation in topsoils.Highlights Residue quality determined vertical dynamics of DOC in coarse– textured Ultisols. Lignin‐ and polyphenol‐rich residues produced HMW DOC in topsoil. LMW DOC derived from cellulose‐rich residues was translocated to the subsoil. There were indications that HMW DOC supported microaggregate formation in topsoils.

Effects of intercropping mulch on the content and composition of soil dissolved organic matter in apple orchard on the loess plateau

Dissolved organic matter (DOM) is an important parameter that reflects soil fertility and quality. In this study, the effects of intercropping perennial ryegrass and straw mulch on the content, composition and spectral characteristics of soil DOM in orchards in arid and semi-arid areas were investigated by three-dimensional fluorescence excitation-emission matrix (EEM) combined with parallel factor analysis (PARAFAC). Three treatments were applied in a 6-year-old apple orchard on the Loess Plateau: (1) clean tillage (CT); (2) intercropping perennial ryegrass mulch (RE); (3) intercropping straw mulch (CS). Soil samples (0–10, 10–20, 20–40, 40–60, 60–80, 80–100 cm) were collected from different treatments. The total soil organic carbon (SOC) and dissolved organic carbon (DOC) content were ranked as CS> RE> CT, and decreased with soil depth. In addition, the vertical distribution of DOC with different soil depth was caused by organic matter input but not leaching. Three humic-like components and two protein-like components were identified by EEM-PARAFAC. Fluorescence spectra showed that humic-like compounds were the dominant fractions in soil DOM in the CS treatment. Fluorescence Index (FI) indicated that the proportion of plant residues and soil organic matter derived DOM ranked as CT> RE> CS and gradually decreased with soil depth. Our results highlight the key role of orchard intercropping mulch in improving the content and compositions of soil DOM through different forms of organic inputs and provide new theoretical support for the evaluation of soil fertility in orchards.

Dissolved organic carbon dynamics in the East China Sea and the northwest Pacific Ocean

Abstract. Oceanic dissolved organic carbon (DOC) represents one of the largest carbon reservoirs on Earth, and its distribution and biogeochemical cycles play important roles in carbon cycling and other biogeochemical processes in the ocean. We report the distribution and concentrations of DOC for water samples collected from the shelf-edge and slope regions in the East China Sea (ECS) and the Kuroshio Extension (KE) in the northwestern North Pacific during two cruises in 2014–2015. The DOC concentrations were 45–88 µM in the ECS and 35–65 µM in the KE. In addition to biological processes that are estimated to account for 7 % and 8 %–20 % in shaping the DOC distribution in the ECS and KE regions, respectively, the DOC distribution is largely controlled by hydrodynamic mixing of different water masses. By comparing the DOC results with dissolved inorganic carbon (DIC) and dissolved inorganic radiocarbon (Δ14C-DIC) measured from the same water samples, we further demonstrate that the intrusion of the Kuroshio Current could dilute the DOC concentrations at stations in the outer shelf and slope regions of the ECS. The concentrations of DOC in the KE were significantly lower in surface waters than in the ECS, and a relatively low and stable DOC level (∼40 µM) was found in deep water (below 1500 m) at all stations. Based on the previously reported DIC and Δ14C-DIC values for the stations, the observed spatial variations of DOC in the upper 700 m among the stations in the KE were mainly influenced by mixing of the two water masses carried by the Kuroshio and Oyashio, the two dominant western boundary currents in the region. The hydrodynamic processes are thus important factors in the distribution and dynamics of DOC in the KE region.

Distributive Characteristics of Riverine Nutrients in the Mun River, Northeast Thailand: Implications for Anthropogenic Inputs

The nutrient contents of Mun River water in northeast Thailand during the dry season were measured to investigate the effect of human activities on dissolved load species. Dissolved organic carbon (DOC) values varied from 2.5 to 17.1 mg/L, averaging 9.0 mg/L; dissolved inorganic nitrogen (DIN) ranged between 0.12 and 0.11 mg/L; Cl− values ranged from 1.7 to 668.6 mg/L, with an average value of 84.8 mg/L; dissolved silicon (DSi) varied from 1.7 to 9.9 mg/L; and SO42− values averaged 8.9 mg/L. DOC, Cl−, and SO42− contents decreased with the flow direction. The high concentrations of DOC, K+, Cl−, and SO42− in the upper reaches were closely related to anthropogenic inputs, specifically industrial sewage. The covariation demonstrated that these dissolved loads may have the same sources. In other regions, Cl− contents were derived from weathering products. DIN contents maintained the same level on the river, and few sampling sites with high concentrations of DIN were influenced by point source pollution. The extremely low P concentrations limited algal growth, and the DSi showed no clear relationship with N and K, indicating that DSi in the Mun River was controlled by the weathering input rather than biological effects. The exact reverse spatial distributions of DOC between the wet and dry seasons (which increased with the flow direction in the wet season) were due to different precipitation rates, and the rare rainfall in the dry season had difficulty flushing the soil and transporting soil organic matter into the rivers. The local government should control sewage discharge and optimize farming methods.

Modelling the interaction of climate, forest ecosystem, and hydrology to estimate catchment dissolved organic carbon export

AbstractThe potential for increased loads of dissolved organic carbon (DOC) in streams and rivers is a concern for regulating the water quality in water supply watersheds. With increasing hydroclimatic variability related to global warming and shifts in forest ecosystem community and structure, understanding and predicting the magnitude and variability of watershed supply and transport of DOC over multiple time scales have become important research and management goals. In this study, we use a distributed process‐based ecohydrological model (Regional Hydro‐Ecological Simulation System [RHESSys]) to explore controls and predict streamflow DOC loads in Biscuit Brook. Biscuit Brook is a forested headwater catchment of the Neversink Reservoir, part of the New York City water supply system in the Catskill Mountains. Three different model structures of RHESSys were proposed to explore and evaluate hypotheses addressing how vegetation phenology and hydrologic connectivity between deep groundwater and riparian zones influence streamflow and DOC loads. Model results showed that incorporating dynamic phenology improved model agreement with measured streamflow in spring, summer, and fall and fall DOC concentration, compared with a static phenology. Additionally, the connectivity of deep groundwater flux through riparian zones with dynamic phenology improved streamflow and DOC flux in low flow conditions. Therefore, this study suggests the importance of inter‐annual vegetation phenology and the connectivity of deep groundwater drainage through riparian zones in the hydrology and stream DOC loading in this forested watershed and the ability of process‐based ecohydrological models to simulate these dynamics. The advantage of a process‐based modelling approach is specifically seen in the sensitivity to forest ecosystem dynamics and the interactions of hydroclimate variability with ecosystem processes controlling the supply and distribution of DOC. These models will be useful to evaluate different forest management approaches toward mitigating water quality concerns.

Water residence time (age) and flow path exert synchronous effects on annual characteristics of dissolved organic carbon in terrestrial runoff

Catchment hydro-physical controls on the interannual variability of dissolved organic carbon (DOC) in terrestrial watershed runoff, important for water quality, ecosystem structure, and foodweb dynamics, are not well understood. To address this, we simulated water residence time (“age”) and flow path of terrestrial runoff and analyzed their mediating effect on relationships between annual runoff volume, DOC concentration, and DOC age. We applied this analysis to a snow-influenced watershed in California's Sierra Nevada (USA) across a range of soil types, elevations (90–4210 m), and years (1950–1999). Simulated increases in annual runoff volume were accompanied by younger ages (r2 = 0.53–0.63) of DOC in quickflow, comprised of surface runoff and lateral flow through soil. Increases in annual runoff volume were also accompanied by gentler relationships between intra-annual (weekly) values of DOC concentration and runoff volume, regression-slopes of which followed a power-law relationship to annual runoff (r2 = 0.12–0.92) for approximately 70% of the watershed. Simulations including dynamics of water age and soil temperature produced annual ages of quickflow DOC ranging from 1 to 70 days over all soil types and water years. Similarity of this range to an observed, 1–69 day range in half-lives of relatively labile DOC in previous studies suggests substantial interannual and spatial variability in the biodegradability of DOC in terrestrial runoff. Simulations excluding dynamics of water age and soil temperature predicted order-of-magnitude less interannual variability in age of quickflow DOC, demonstrating the important effect of interannual variability in soil-water interaction times. These findings suggest that the distribution of DOC bioprocessing along transitions between terrestrial and aquatic systems may be strongly influenced by year-to-year variability in age of water.

In situ production of dissolved organic carbon (DOC) by phytoplankton blooms (Cochlodinium polykrikoides) in the southern sea of Korea

We measured dissolved organic carbon (DOC) concentration in the surface seawater of Gamak Bay located in the southern Korea and groundwater nearby the bay in August 2007 to determine effect of harmful algal blooms on DOC distribution in the surface waters. The average of DOC concentration in the bloom stations (157 ± 25 μM) was 1.6-fold higher than that of the non-bloom stations (96 ± 15 μM). The DOC concentrations in groundwater and offshore seawater were significantly lower than those in the bloom stations but showed similar values in the non-bloom stations. The facts that salinity showed no significant relationship with DOC but Chl-a showed a significant correlation with DOC, indicate that the major source of DOC in the bloom stations may be in situ phytoplankton production rather than groundwater and/or river water. In addition, DOC showed a strong relationship with peridinin (biomarker of dinoflagellate) but showed no relationship with fucoxanthin (biomarker of diatom). These results suggest that the DOC distribution in Gamak Bay seems to be controlled by in situ phytoplankton production by dinoflagellate rather than diatom. Therefore, distributions of DOC in the southern sea of Korea during bloom season may be controlled by biological processes such as the intensity and duration of dinoflagellate.