Dissolved Organic Carbon Measurements Research Articles

Seasonal variabilities in sources, distribution and transport of dissolved organic carbon from a rapidly eroding coastal estuary in Mississippi River delta plain.

The fate of soil organic carbon lost from eroding coastal wetlands is of great importance. A large fraction of this carbon upon erosion is released as dissolved organic carbon (DOC) and inorganic carbon (DIC) into adjacent water bodies while the remainder is lost to atmosphere as CO2. In this study, we investigate the seasonal concentrations and transport of DOC in Barataria Basin, Louisiana, USA, which borders the northern Gulf of Mexico (nGOM). This basin is currently experiencing one of the highest rates of land-loss in the US. Seasonal measurements of DOC, DIC and colored dissolved organic matter (CDOM) were carried out during winter, spring, summer, and fall of 2020-2021. The average DOC in the Barataria Basin ranged from 648.03μM of C in winter and 867.88μM of C in spring, whereas the average DIC varied between 2582.47μM in fall to 1553.26μM in winter. Humification index and statistical analysis suggests a strong influence of terrigenous organic matter input in the northern part of the bay. The seamless creek-to-ocean SCHISM 3D model validated by data from the east coast and the coast of GOM was implemented in this study to compute the discharge rate at the Barataria Pass followed by assessing the seasonal DOC transport. The DOC exports were highest in spring, estimated at 201 tons/day and translated to approximately 0.043 Tg on an annual basis. This study highlights the importance of DOC exports from small estuaries, which should be incorporated in the global ocean carbon budget.

Seasonal and Climatic Drivers of Wet Deposition Organic Matter at the Continental Scale

AbstractDissolved organic matter (DOM) concentrations and composition within wet deposition are rarely monitored despite contributing a large input of bioavailable dissolved organic carbon (DOC) and nitrogen (DON) to the Earth's surface. Lacking from the literature are spatially comprehensive assessments of simultaneous measurements of wet deposition DOC and DON chemistry and their dependencies on metrics of climate and environmental factors. Here, we use archived precipitation samples from the US National Atmospheric Deposition Program collected in 2017 to 2018 from 17 sites across six ecoregions to investigate variability in the concentration and composition of depositional DOM. We hypothesize metrics of DOM chemistry vary with ecoregion, season, large‐scale climate drivers, and precipitation geographic source. Findings indicate differences in DOC and DON concentrations and loads among ecoregions. The highest wet deposition concentrations are from sites in the Northern Forests and lowest concentrations from sites in Marine West Coast Forests. Summer and autumn samples contained the highest DOC concentrations and DON concentrations that were consistently above detection limit, corresponding with seasonality of peak air temperatures and the phenology of the growing season in the northern hemisphere. Compositional trends suggest lighter DOM molecules in autumn and winter and heavier molecules in spring and summer. Climate drivers explain 51% of variation in DOM chemistry, revealing differing drivers on the concentrations and loads of DOC versus DON in wet deposition. This study highlights the necessity of incorporating DOC and DON measurements into national deposition monitoring networks to understand spatial and temporal feedbacks between climate change, atmospheric chemistry and landscape biogeochemistry.

Distribution and composition of redox-active species and dissolved organic carbon in Arctic lacustrine porewaters

ABSTRACT The interaction between redox-active species and dissolved organic carbon (DOC) is crucial in driving lacustrine benthic microbial processes. In lacustrine porewaters, many redox-active species exist in their reduced form, while DOC acts as a substrate and an electron acceptor. Understanding the types and abundance of redox-active species in porewaters along with their complementary DOC substrate is pivotal for gaining insights into benthic processes, particularly in regions susceptible to climate change. We report the in-situ measurement of redox-active species in sediment porewaters, alongside the ex-situ measurement of DOC extracted from cores collected from two Arctic lakes (Toolik and Fog 1). Fe2+ was abundantly detected below 4 cm of the sediment-water interface in all cores and was inversely related to dissolved O2. Additionally, two distinct Fe(III)-complexes were identified. DOC ranged in the order of 10s of mg/L and either remained stable or increased with depth. A comparison between Toolik and Fog 1 lakes revealed a higher accumulation of Fe2+ and DOC in the latter. This study marks the first of its kind to assess spatial distributions of redox-active species and DOC as a function of depth from multiple sites in Arctic lacustrine porewaters.

Role of edaphic, hydrologic, and land cover variables in determining dissolved organic carbon in Missouri (USA) reservoirs and streams

Jones JR, Graham JL, Obrecht DV, Harlan JD, Knowlton MF, Pollard C, Parris J, Thorpe AP. 2024. Role of edaphic, hydrologic, and land cover variables in determining dissolved organic carbon in Missouri (USA) reservoirs and streams. Lake Reserv Manage. 40:177–195. In Missouri, distinct geophysical gradients influence statewide patterns in water quality. Here, we quantify the spatiotemporal variability of dissolved organic carbon (DOC) in reservoirs and streams and the edaphic, hydrologic, and land cover variables that account for cross-system variation. Datasets included statewide inventories collected over decades and studies with greater temporal resolution (n = >6350 DOC measurements). Among reservoirs, the smallest DOC concentration was measured in a spring-fed system within a forested watershed, and the largest was where agricultural biosolids were applied to the land (range 1.0–15.9 mg/L, overall mean 5.8 mg/L). Reservoir values increased from the southern forested Highlands (mean 4.7 mg/L) to the northern agricultural Plains (mean 7.0 mg/L). Stream DOC was similar to reservoir values (overall mean in streams 6.3 mg/L; Highlands mean 4.0 mg/L; Plains mean 6.6 mg/L), despite differences in study design and collection period. Reservoir DOC increased in spring, indicative of allochthonous loading, with small autochthonous additions during a broad summer peak. Temporal variability in DOC was low relative to macronutrients and chlorophyll in both reservoirs and streams, indicating DOC may be a sensitive and readily detected indicator of temporal change in these systems. In regression analyses, watershed features accounted for more than 60% of overall cross-system variability in DOC in both reservoirs and streams. Driver-response relations, however, differed between regions. This analysis extends our understanding of environmental influences on surface water chemistry in Missouri and indicates DOC is nearly as predictable as macronutrients using landscape-level features.

A global database of dissolved organic matter (DOM) concentration measurements in coastal waters (CoastDOM v1)

Abstract. Measurements of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) concentrations are used to characterize the dissolved organic matter (DOM) pool and are important components of biogeochemical cycling in the coastal ocean. Here, we present the first edition of a global database (CoastDOM v1; available at https://doi.org/10.1594/PANGAEA.964012, Lønborg et al., 2023) compiling previously published and unpublished measurements of DOC, DON, and DOP in coastal waters. These data are complemented by hydrographic data such as temperature and salinity and, to the extent possible, other biogeochemical variables (e.g. chlorophyll a, inorganic nutrients) and the inorganic carbon system (e.g. dissolved inorganic carbon and total alkalinity). Overall, CoastDOM v1 includes observations of concentrations from all continents. However, most data were collected in the Northern Hemisphere, with a clear gap in DOM measurements from the Southern Hemisphere. The data included were collected from 1978 to 2022 and consist of 62 338 data points for DOC, 20 356 for DON, and 13 533 for DOP. The number of measurements decreases progressively in the sequence DOC > DON > DOP, reflecting both differences in the maturity of the analytical methods and the greater focus on carbon cycling by the aquatic science community. The global database shows that the average DOC concentration in coastal waters (average ± standard deviation (SD): 182±314 µmol C L−1; median: 103 µmol C L−1) is 13-fold higher than the average coastal DON concentration (13.6±30.4 µmol N L−1; median: 8.0 µmol N L−1), which is itself 39-fold higher than the average coastal DOP concentration (0.34±1.11 µmol P L−1; median: 0.18 µmol P L−1). This dataset will be useful for identifying global spatial and temporal patterns in DOM and will help facilitate the reuse of DOC, DON, and DOP data in studies aimed at better characterizing local biogeochemical processes; closing nutrient budgets; estimating carbon, nitrogen, and phosphorous pools; and establishing a baseline for modelling future changes in coastal waters.

Estimation of Dissolved Organic Carbon Using Sentinel-2 in the Eutrophic Lake Ebinur, China

Dissolved organic carbon (DOC) in lakes, as a regulatory agent and light-absorbing compound, is a key component of the global carbon cycling in lacustrine ecosystems. Hence, continuous monitoring of the DOC concentration in arid regions is extremely important. This study utilizes the QAA-CDOM semi-analytical model, which has good accuracy in retrieving the CDOM (colored dissolved organic matter) concentration of Lake Ebinur. We chose to invert the CDOM time-series data from May to October during the 2018–2022 period. A DOC estimation model was then established using the linear regression approach based on the CDOM inversion data and the field DOC measurements. In general, the DOC concentration in Lake Ebinur exhibited an increasing trend from 2018 to 2022, typically lower in May and higher in June. When comparing the average values of DOC in Lake Ebinur for the same months across different years, it can be observed that the month of September exhibits the greatest variability, whereas June shows the least variability. In sum, this study successfully retrieved CDOM concentrations for a saline lake within an arid region and developed a DOC estimation model, thereby providing a reference for investigating carbon cycling in typical lakes of arid areas.

Spatially distributed tracer‐aided modelling to explore DOC dynamics, hot spots and hot moments in a tropical mountain catchment

AbstractTracer‐aided rainfall‐runoff modelling is a promising tool for understanding catchment hydrology, particularly when tracers provide information about coupled hydrological‐biogeochemical processes. Such models allow for predicting the quality and quantity of water under changing climatic and anthropogenic conditions. Here, we present the Spatially‐distributed Tracer‐Aided Rainfall‐Runoff model with a coupled biogeochemical reactive tracer module (STARR‐DOC) to simulate dissolved organic carbon (DOC) dynamics and sources. The STARR‐DOC model was developed and tested for a humid high Andean ecosystem (páramo) using high‐resolution hourly DOC and hydrometeorological data to simulate hourly discharge and DOC at a fine spatial (10 × 10 m) resolution. Overall, the model was able to acceptably reproduce discharge (KGE ~ 0.45) and stream DOC (KGE ~ 0.69) dynamics. Spatially distributed DOC simulations were independently compared using point DOC measurements for different soil types across the catchment, which allowed for identifying DOC production hot spots and hot moments. Results showed higher hydrological connectivity between slopes and valleys with increasing precipitation. Wetter conditions also favoured DOC production (wet month = 82 mg L−1, dry month = 5 mg L−1) and transport to the stream network (DOC concentrations: during events ~15 mg L−1, during baseflows ~4 mg L−1). Our results also suggest that minor changes in meteorological conditions directly affect páramo soil water dynamics and biogeochemistry. Knowledge of when and where DOC production in mountain catchments is greatest is important for water managers to understand when they make decisions about water security, especially considering climate change predictions for the Andean region.

Coupled effects of dam, hydrology, and estuarine filtering on dissolved organic carbon and optical properties in the reservoir-river-estuary continuum

The land–ocean aquatic continuum is affected notably by damming, yet the effects of changing dissolved organic carbon (DOC) and optical properties in the reservoir zone of dammed rivers on the downstream waterbodies remain unclear. This was studied in the Minjiang River (SE China) in the normal, wet, and dry seasons, using the measurements of DOC, absorption spectroscopy, and fluorescence excitation-emission matrices-parallel factor analysis (EEMs-PARAFAC). The DOC and absorption spectral slope (S275-295) increased while the absorption coefficient (a280) and DOC-specific UV absorbance (SUVA254) decreased in the surface water of the reservoir. This indicated removals of chromophoric dissolved organic matter (CDOM) and potential additions of autochthonous constituents with lower aromaticity and molecular weight. The discharge of more humified DOC from the middle and deep outlets of the reservoir was evident in the adjacent downstream station in the wet season. The DOC and optical indices were generally stable in the downstream river zone in the normal and dry seasons, suggesting that these signals from the reservoir could persist downstream. In contrast, the downstream DOC and CDOM showed evident additions in the wet season, which might be related to enhanced inputs from the surrounding soils and residential areas in the rainy and hot summer. The absorption coefficient and humic-like components correlated strongly with salinity in the estuarine zone, indicating they were mainly conservative in the estuary of this study. This suggested that the refractory DOC from the reservoir and river zones could pass the estuary and contribute to the carbon storage in the coastal ocean. In contrast, the protein-like components did not correlate significantly with salinity, indicating more active modifications of labile constituents in the estuary. Overall, our results have implications for assessing the effects of damming on DOC and optical properties across the land–ocean aquatic continuum.

Water soluble polymer biodegradation evaluation using standard and experimental methods

Multiple polyethylene glycol (PEG) polymers ranging in molecular weight (MW) from 4000 to 500,000 Da, polyvinyl alcohol (PVOH) polymers with degrees of hydrolysis (DH) of 79 % and 88 % and MW 10,000 to 130,000 Da, and carboxy methyl cellulose (CMC) polymers with degrees of substitution (DS) ranging from 0.6 to 1.2 were evaluated in standard screening biodegradation tests to assess method limitations, modification potential, and reproducibility. All PEGs and PVOHs mineralized completely in OECD 301B and 302B studies reaching >80 % biodegradation with negligible dissolved organic carbon remaining at study completion. For high MW PEOs, extension of test duration was needed to reach full extent of mineralization. CMC biodegradation was directly correlated to degree of substitution with CMC 0.6 biodegrading extensively, CMC 0.79 partially biodegrading, and CMC 1.2 not biodegrading significantly in OECD 301B and 302B studies. For all materials tested in both an OECD 301B and 302B, fewer days were necessary to reach 60 % biodegradation in the OECD 302B indicating increased rates of biodegradation with higher inoculum to test chemical ratios. In a series of investigative studies using respirometry as the analytical endpoint, significant variability in the presence of competent degraders in small volume grab samples of river water was observed. Research is needed to overcome this variability and develop a standardized reproducible test method to accurately assess polymer mineralization in river water. At study completion, residual dissolved organic carbon (DOC) data confirmed respirometry data, high levels of mineralization resulted in negligible residual DOC while low levels of mineralization resulted in significant residual DOC, up to dose concentrations. DOC measurements provided confirmation of complete biodegradation when biomass incorporation and test system set up resulted in variable carbon dioxide production or oxygen demand.

Simulating dissolved organic carbon during dryness/wetness periods based on hydrological characteristics under multiple timescales

Understanding and simulating dissolved organic carbon (DOC) concentrations is a challenge with important implications for water quality projections and carbon balance estimates, particularly at the event-scale. Hydrological dryness/wetness represents water shortage/surplus in surface water. Previous studies have analyzed the dynamic patterns of DOC concentrations during either dryness or wetness (or rewetting) periods. However, the dynamic patterns of DOC concentrations and corresponding simulations during both dryness and wetness events, based on their characteristics (i.e., duration and severity), remain to be fully understood, particularly under multiple timescales. This study proposed a framework to help overcome this limitation. The standardized streamflow index (SSI) combined with run theory was used to describe the dryness/wetness characteristics at the monthly, seasonal, and annual scales. Simulation models were constructed and validated based on the multivariable linear regression model and cross-validation method. Three evaluation indicators, namely, the coefficient of determination (R2), Nash-Sutcliffe efficiency (NSE), and mean absolute percentage error (MAPE, %), were employed to determine the optimal model. This model was then applied to additional basins based on the correction coefficient method. Three nearby basins (HP3a, HP4, and HP6), with different catchment areas (9.28 to 122.80 ha) and long-term (41 years) daily streamflow and weekly (or biweekly) DOC measurements in the Harp Lake catchment, south-central Ontario, Canada, were used to demonstrate the usefulness of the framework. Data from HP3a was used to construct and validate the simulation model, and the model was applied to the HP4 and HP6 basins. Results show that the average DOC concentration during the dryness events is lower than that in the wetness events at the monthly, seasonal, and annual scales. The average, maximum, and minimum DOC concentrations during the dryness events increased with the dryness/wetness index timescales, but decreased during the wetness events. The optimal model was able to accurately simulate the DOC concentrations (R2 ≥ 0.90, NSE ≥ 0.90, and MAPE ≤ 10 %) at the monthly, seasonal, and annual scales based on the hydrological characteristics during the dryness and wetness periods, respectively, and the application results exhibit a good performance. The proposed framework can help improve the forecasting of DOC concentrations and the corresponding ecological coping strategies at the event-scale under changing environments.

High frequency UV–Vis sensors estimate error in riverine dissolved organic carbon load estimates from grab sampling

High frequency ultraviolet – visible (UV–VIS) sensors offer a way of improving dissolved organic carbon (DOC) load estimates in rivers as they can be calibrated to DOC concentration. This is an improvement on periodic grab sampling, or the use of pumped sampling systems which store samples in-field before collection. We hypothesised that the move to high frequency measurements would increase the load estimate based on grab sampling due to systemic under-sampling of high flows. To test our hypotheses, we calibrated two sensors in contrasting catchments (Exe and Bow Brook, UK) against weekly grab sampled DOC measurements and then created an hourly time series of DOC for the two sites. Taking this measurement as a ‘true’ value of DOC load, we simulated 1,000 grab sampling campaigns at weekly, fortnightly and monthly frequency to understand the likely distribution of load and error estimates. We also performed an analysis of daily grab samples collected using a pumped storage sampling system with weekly collection. Our results show that: a) grab sampling systemically underestimates DOC loads and gives positively skewed distributions of results, b) this under-estimation and positive skew decreases with increasing sampling frequency, c) commonly used estimates of error in the load value are also systemically lowered by the oversampling of low, stable flows due to their dependence on the variance in the flow-weighted mean concentration, and d) that pumped storage systems may lead to under-estimation of DOC and over estimation of specific ultra-violet absorbance (SUVA), a proxy for aromaticity, due to biodegradation during storage.

Influence of the conservation mode of seawater for dissolved organic carbon analysis

Marine dissolved organic matter (DOM) is one of the largest exchangeable organic carbon reservoir on the planet. The main proxy to track the distribution of DOM in the aquatic environments remains dissolved organic carbon (DOC). Thereby the optimal protocol for long-term DOC preservation in seawater samples must be defined. In this context, we monitored bulk DOC concentrations and its size class distribution in filtered seawater samples during yearlong experiments. With different conservation mode, we tested two types of commonly used materials (borosilicate brown glass and high-density polyethylene, HDPE) and three conditioning protocols (untreated, acidified at pH 2 and frozen at −20 °C). Offshore samples collected along the entire water column of the Pacific Ocean and stored in HDPE bottles were also analysed after 2 years of storage at pH of 2 and compared to frozen samples. Results demonstrated that bulk DOC concentrations can be accurately determined in untreated samples for one month and for years in frozen samples as well as in acidified samples, when samples are stored in acid cleaned HDPE bottles or flame sealed glass ampoules. Storage in brown glass vials with Bakelite caps seems more uncertain. The study of the size class distribution of DOC reveals the possibility to study DOM for 1 month in filtered samples with no additional treatment and for years in frozen samples when stored in acid cleaned HDPE bottles. Significant changes in DOC size fractionation were observed when samples were acidified. The high molecular weight (HMW) compounds and the humic substances from the upper 1000 m were significantly degraded at pH 2, incorporating DOC in the low molecular weight (LMW) fractions. These experiments provide preservation guidelines for future studies that aim either to study bulk DOC or the chemical properties marine DOM. It is recommended to store seawater in HDPE vials at −20 °C for DOM study, or at pH 2 for bulk DOC measurements.

Sink or source? Insights into the behavior of copper and zinc in the sediment porewater of a constructed wetland by peepers

The H-02 free water surface constructed wetland has been applied to remove heavy metals, mainly copper (Cu) and zinc (Zn), from wastewater on the Savannah River Site (Aiken, SC, USA). More and more studies focus on the metal behavior between the sediment and the overlying water, which directly reflects the stability of metals after sedimentation in constructed wetlands. This study focused on the biogeochemical pathways in metal bioavailability and remobilization in the sediment after metals were removed from the overlying water. The dialysis sampling device (peeper) was used to collect porewater samples from eight depths in the sediment for the measurement of Cu, Zn, dissolved organic carbon (DOC), and major anions (sulfate and chloride). Surface water samples were also collected for the measurement of Cu, Zn, DOC, and anions. Different temporal trends were observed for dissolved Cu between the surface and bottom waters, but not for dissolved Zn. There were no obvious changes in porewater metal concentrations with increasing depths in the sediment. Sediment served as a sink for Cu as only 3% of porewater samples showed higher labile Cu concentrations than the surface water during the entire year, and these samples were collected below the sediment-water interface. However, sediment served as a source for Zn in summer and winter as 32% of porewater samples showed higher labile Zn concentrations than the surface water, and these samples were collected at all sediment depths. We think the seasonal changes in the behaviors of Cu and Zn are primarily controlled by the sulfur dynamics and the metal removal processes in the constructed wetland, as well as the different complexing chemistry between Cu and Zn. Also, our study supports that peeper is a powerful tool for studying the biogeochemistry of metals in the sediment.

Spring coherence in dissolved organic carbon export dominates total coherence in Boreal Shield forested catchments

The wide range of forested landscapes in boreal environments store and cycle substantial amounts of carbon, although the capacity of these systems to act as either a carbon sink or source is uncertain under a changing climate. While there are clear reports of regional-scale increases in dissolved organic carbon (DOC) concentrations in streams and lakes, there remains substantial watershed-scale variability in these patterns. Coherence is a framework for examining if variables of interest within adjacent spatial units change synchronously or asynchronously through time and has been widely applied in the context of lentic hydrochemistry, and which can shed light on the relative importance of regional- vs. local-scale controls. The objective of this research was to quantify coherence in discharge, DOC concentrations, and DOC loads in forested boreal watersheds, and to what extent coherence varied by season. Coherence was assessed using data from three long-term ecological research sites spanning boreal forest environments (IISD-Experimental Lakes Area, Turkey Lakes Watershed Study, and Dorset Environmental Science Centre) that included 29 829 DOC measurements across 739 stream-years, examining correlation between stream-pairs within each site, but not between sites. Seasonal coherence in DOC export was consistent across the three sites; coherence was significantly greater in spring than all other seasons, and was strongly related to discharge coherence. Currently, the season with the greatest loads (spring) is also the most coherent season, suggesting that annual between-stream coherence may be reduced if spring becomes proportionally less important in hydrologic budgets under a changing climate. This research aids in determining which factors contribute to synchronous watershed behaviour, and which factors may contribute to the timing and extent of individual watershed-scale deviations from landscape-level patterns.

Satellite estimation of dissolved organic carbon in eutrophic Lake Taihu, China

Dissolved organic carbon (DOC) in lakes serves as a substrate for heterotrophic bacterial growth, a regulator of the global carbon cycle, and a light absorption agent. DOC in eutrophic lakes is greatly influenced by phytoplankton phenology and terrigenous input by rivers. Therefore, it is necessary and significant to dynamically monitor the concentration, storage, and riverine exchange flux of DOC. By using in-situ DOC measurements from 2004 until 2018 (N = 2019), a machine learning algorithm, namely, a multilayer back-propagation neural network (MBPNN) model, was developed in this work to improve the remote sensing estimation of DOC concentrations in eutrophic Lake Taihu. The model yielded a mean estimation error of 15.14% for the testing dataset. The monthly mean DOC concentration significantly increased from 2003 to 2018 (N = 192, p < 0.01). High DOCs were observed in lake bays with high chlorophyll a (Chl-a) levels, and phytoplankton growth explained more than 50% of the monthly DOC variations. Then, given the evenly mixed DOC in the water column on the monthly and annual scales, we further estimated the monthly mean DOC storage in the lake from 2003 until 2018 and DOC fluxes (input and output) due to rivers during 2008–2018. Although the mean net riverine DOC input (50.56 ± 32.22 × 103 t C) was approximately 5.2 times the average DOC storage (9.73 ± 1.23 × 103 t C), phytoplankton growth controlled DOC variations, which indicated that much terrigenous DOC was transformed into other carbon forms after entering Lake Taihu. This study verified the feasibility of remote sensing of DOC (surface concentration, storage, and riverine exchange flux) in eutrophic lakes.

High-frequency analysis of dissolved organic carbon storm responses in headwater streams of contrasting forest harvest history

Dissolved organic carbon (DOC) in streams is critically important for aquatic ecosystems and human water use, and is affected by changes in land use and climate. The effects of land use change (e.g. forest harvest) on DOC are not fully understood. Past studies of the effects of forestry on DOC have found mixed results, and covariate effects complicate mechanistic understanding and may underlie regional differences. DOC is typically highest during storm events, and detailed time-resolved measurements of DOC over longer periods are just becoming feasible with advances in in situ sensors. In this paper, we investigated high frequency measurements of DOC concentration in headwater streams of contrasting forest harvest history in the UBC Malcolm Knapp Research Forest near Maple Ridge, British Columbia. We found base flow DOC concentration to be lower in the clear-cut stream (Clear cut: 2.26 ± 0.43 mg/L; Forested: 4.30 ± 0.83 mg/L). This may be due to reductions in carbon inputs like leaf litter, although the difference in drainage area, slope, and the presence of a bog lake in the forested catchment may also account for some of the observed difference. We found the DOC response to storms to be larger and faster at the clear-cut site (mean increase DOC: Clear-cut: 2.42 mg/L, Forested: 1.99 mg/L; mean rate of change DOC: Clear-cut: 0.16 mg/L/h, Forested: 0.11 mg/L/h). Elevated storm responses may be due to changes in flow paths related to forest harvesting. We found low antecedent flow and greater storm intensity to significantly predict elevated DOC storm response. This is important in the context of future climate scenarios, which predict lower summer flows and more intense storms in this region. Overall, this work contributes to our understanding of the intersection between effects of land use change and climate on water quality, with a focus on drinking water treatment implications.

Comparing the performance of three methods to assess DOM dynamics within two distinct glacierized watersheds of the tropical Andes

Dissolved organic matter (DOM) is recognized as a good indicator of water quality as its concentration is influenced by land use, rainwater, windborne material and anthropogenic activities. Recent technological advances make it possible to characterize fluorescent dissolved organic matter (FDOM), the fraction of DOM that fluoresces. Among these advances, portable fluorometers and benchtop fluorescence excitation and emission spectroscopy coupled with a parallel factor analysis (EEM-PARAFAC) have shown to be reliable. Despite their rising popularity, there is still a need to evaluate the extent to which these techniques can assess DOM dynamics at the watershed scale. We compare the performance of in-situ measurements of FDOM with laboratory measurements of fluorescence spectroscopy within the context of two distinct glacierized watersheds in Peru. Glacierized watersheds represent unique testing environments with contrasting DOM conditions, flowing from pristine, vegetation-free headwaters through locations with obvious anthropogenic influences. We used an in-situ fluorometer and a portable multimeter to take 38 measurements of FDOM, pH and turbidity throughout the two catchments. Additionally, samples were analyzed in the laboratory using the EEM-PARAFAC method. Results were compared to dissolved organic carbon (DOC) measurements using standard high-temperature catalytic oxidation. Our results show that the three techniques together were able to capture the DOM dynamics for both studied watersheds. Taken individually, all three methods allowed detection of the watershed DOM main points of sources but in a more limited way. Due to the narrow bandwidth of the portable fluorometer used in the study, FDOM measurements were almost non-detectable to protein-like substances. Indeed, the more demanding EEM-PARAFAC was able to both differentiate between potential sources of DOM and provide an estimate of relative concentrations of different organic components. Finally, similar to FDOM but to a lesser extent, the DOC measurements showed some limits where protein-like substances make up most of the DOM composition.

A mechanistic microbial underpinning for the size-reactivity continuum of dissolved organic carbon degradation

The reservoir of dissolved organic carbon (DOC) in the ocean is modified by multiple input and removal processes. Incubation experiments as well as measurements of oceanic DOC have demonstrated that the high molecular weight (HMW) fraction of DOC typically has a younger radiocarbon age and is more reactive biologically than the low molecular weight (LMW) fraction of DOC. These observations have been summarized as a ‘size-reactivity continuum’ of DOC reactivity, but mechanistic explanations for these observations have been lacking. Here we describe how our recent discovery of ‘selfish’ HMW organic matter uptake among bacteria in surface ocean waters may help explain the rapid removal of HMW DOC. ‘Selfish’ substrate uptake by bacteria encompasses rapid binding and partial hydrolysis of intact polysaccharides on the outer membrane of bacteria, seamlessly followed by the transport of large oligosaccharide fragments into the periplasm with little to no loss of LMW hydrolysis products. ‘Selfish’ bacteria therefore process HMW substrates in a manner distinct from bacteria that carry out extracellular hydrolysis that yields LMW hydrolysis products in the environment. Recognition of the presence and prevalence of selfish bacteria in the ocean has profound implications for carbon flow – the source and quantity of LMW substrates made available to non-extracellular-enzyme producing bacteria – as well as for efforts to model and measure bacterial interactions during organic matter degradation. This discovery also highlights the importance of targeted substrate binding and uptake as key (often understudied) factors in geochemical investigations of microbially driven carbon cycling in the ocean. We conclude with some speculative thoughts about the factors that may determine the prevalence of selfish substrate uptake in the environment.

Importance of Mountain Glaciers as a Source of Dissolved Organic Carbon

AbstractIce sheets and glaciers have been shown to deliver large amounts of labile dissolved organic carbon (DOC) to downstream aquatic ecosystems, but recent studies may underestimate the release of DOC from mountain glaciers. To date, continuous measurements of DOC from mountain glaciers throughout entire glacier melt season are very limited. Here we present high‐density data on DOC from two Asian mountain glaciers over a full melt season in 2013 and compile a global data set of DOC from 42 mountain glaciers. Based on our study and previously published DOC data, we estimate the storage and release of DOC associated with Asian mountain glaciers to be 8.8 to 13.8 Tg C and 0.19 Tg C/a, respectively. Rough extrapolation of glacier runoff to a global scale suggests that DOC release from mountain glaciers is on the order of 0.8 Tg C/a, which is 1.4 times higher than the most current estimates. The current release of DOC from mountain glaciers is therefore far more significant than previously thought and should be considered in future evaluation of the global carbon cycle.

Removal of dissolved organic matter from raw water using zero valent iron -carbonaceous conjugated microporous polymer nanocomposites

There are a limited number of potentially scalable low-cost treatment methods for removing organic pollutants in water. In this study, a magnetic zero valent iron-carbonaceous conjugated microporous polymer nanocomposite (ZVI-CCMP) was synthesized from ZVI and waste polystyrene via the liquid phase reduction method, and used in a batch system for the removal of dissolved organic carbon (DOC) in water from a water treatment plant (WTP) in Pretoria, South Africa. The results were compared to the DOC removal efficiency of the WTP. The surface morphology of the nanocomposites as characterized using scanning electron microscopy showed heterogeneous ZVI nanastructures dispersed on the CCMP surface. Fourier transform infrared spectroscopy showed that the functional groups on the ZVI-CCMP surface were predominantly CC and C-C from quinonoid motifs, aliphatic chain conjugation, and C-S groups originating from the introduced sulphonic moieties. Batch experiment data indicated that ZVI-CCMP adsorbed significantly (p = 0.01) more DOC than ZVI, demonstrating the synergistic effect of ZVI activation. Whereas the WTP removed up to 24.3% DOC, ZVI and ZVI-CCMP removed 64 and 75%, respectively after a contact time of 30 min. Thus, compared to the processes used by the WTP, batch experiments using ZVI and ZVI-CCMP were 41 (p = 0.00) and 52% (p = 0.01) superior, respectively, demonstrating the potential of these materials to be upscaled for pilot and real life applications. While polystyrene has been used in the ZVI composites and the synthesis and evaluation of ZVI decorated carbon based materials has been widely studied, ZVI-CCMP based materials have not been reported. The objectives were: (1) to synthesise and characterise ZVI-CCMP nanocomposites; (2) to evaluate the removal of DOC in real water samples by ZVI and ZVI-CCMP; and (3) to compare the relative efficiency of ZVI-CCMP to that of the water treatment process through the measurement of DOC. Using CCMP nanocomposites to treat water is potentially a low-cost and environmentally friendly alternative for reducing the adverse public health and environmental risks associated with waste polystyrene.