Dissolved Organic Carbon Quality Research Articles

Impacts of Hydrology and Extreme Events on Dissolved Organic Carbon Dynamics in a Heavily Urbanized Estuary and Its Major Tributaries: A View From Space

AbstractDissolved organic matter and its colored component, Colored Dissolved Organic Matter (CDOM), play a major role in global carbon budgets, and their fluxes provide an essential link between terrestrial and aquatic biogeochemical cycles. Satellite observations can uniquely capture the hydro‐biogeochemical connectivity of terrestrial and aquatic landscapes, across scales. Yet, accurate satellite retrievals of CDOM and dissolved organic carbon (DOC) dynamics remain challenging in urbanized estuaries and coasts. Here, we present an advanced unified algorithm for space‐based retrieval of coastal CDOM and DOC dynamics and its application in Long Island Sound—one of the world's most heavily urbanized estuaries that is becoming increasingly vulnerable to climate change stressors. A rich bio‐optical data set, encompassing a wide range of environmental conditions, was integrated into the algorithm training to retrieve DOC concentrations and CDOM spectral shape (i.e., spectral slope S275–295)—a proxy for DOC quality. The new algorithms were applied to full‐resolution satellite imagery from the Sentinel‐3 Ocean and Land Color Instrument (OLCI) after thoroughly evaluating the performance of six ocean color atmospheric correction approaches (ACOLITE, BAC, C2RCC, MUMM, l2gen, and Polymer). Evaluation of the algorithms yielded mean absolute percent differences of 28%, 12%, and 10% for aCDOM(300), S275–295, and DOC, respectively. Application of the algorithms to multi‐year satellite OLCI imagery captured, for the first time, the coupled impact of seasonal transitions, wind regimes, freshwater inputs, anthropogenic disturbances, and hydrological extremes (both intense precipitation and droughts) on DOC fluxes and CDOM quality at the ecosystem scale. Results have important implications for improved predictions of coastal biogeochemical fluxes in complex urban−estuary systems.

Suboxic DOM is bioavailable to surface prokaryotes in a simulated overturn of an oxygen minimum zone, Devil's Hole, Bermuda.

Oxygen minimum zones (OMZs) are expanding due to increased sea surface temperatures, subsequent increased oxygen demand through respiration, reduced oxygen solubility, and thermal stratification driven in part by anthropogenic climate change. Devil's Hole, Bermuda is a model ecosystem to study OMZ microbial biogeochemistry because the formation and subsequent overturn of the suboxic zone occur annually. During thermally driven stratification, suboxic conditions develop, with organic matter and nutrients accumulating at depth. In this study, the bioavailability of the accumulated dissolved organic carbon (DOC) and the microbial community response to reoxygenation of suboxic waters was assessed using a simulated overturn experiment. The surface inoculated prokaryotic community responded to the deep (formerly suboxic) 0.2 μm filtrate with cell densities increasing 2.5-fold over 6 days while removing 5 μmol L-1 of DOC. After 12 days, the surface community began to shift, and DOC quality became less diagenetically altered along with an increase in SAR202, a Chloroflexi that can degrade recalcitrant dissolved organic matter (DOM). Labile DOC production after 12 days coincided with an increase of Nitrosopumilales, a chemoautotrophic ammonia oxidizing archaea (AOA) that converts ammonia to nitrite based on the ammonia monooxygenase (amoA) gene copy number and nutrient data. In comparison, the inoculation of the deep anaerobic prokaryotic community into surface 0.2 μm filtrate demonstrated a die-off of 25.5% of the initial inoculum community followed by a 1.5-fold increase in cell densities over 6 days. Within 2 days, the prokaryotic community shifted from a Chlorobiales dominated assemblage to a surface-like heterotrophic community devoid of Chlorobiales. The DOM quality changed to less diagenetically altered material and coincided with an increase in the ribulose-1,5-bisphosphate carboxylase/oxygenase form I (cbbL) gene number followed by an influx of labile DOM. Upon reoxygenation, the deep DOM that accumulated under suboxic conditions is bioavailable to surface prokaryotes that utilize the accumulated DOC initially before switching to a community that can both produce labile DOM via chemoautotrophy and degrade the more recalcitrant DOM.

Hydrologic and Organic Carbon Quality Controls on Nitrous Oxide Dynamics Across a Variably Confined Karst Aquifer

AbstractAtmospheric concentrations of the potent greenhouse gas nitrous oxide (N2O) have increased approximately 20% since the pre‐industrial era. Riverine systems represent an important atmospheric source of N2O however, the influence of varying hydrogeological conditions and dissolved organic carbon (DOC) quantity and quality on N2O dynamics remains poorly understood. We investigate effects of these processes on N2O dynamics in the Santa Fe River watershed in north‐central Florida, USA where a gradient in DOC quantity and quality is caused by varying surface water‐groundwater exchange. The watershed is underlain by the karstic Floridan aquifer, which is confined in the headwaters and contains terrigenous recalcitrant DOC, while the unconfined lower reaches contain protein‐like labile DOC substrates. Where extensive surface water‐groundwater exchange occurs at the boundary between the confined and unconfined aquifer, incomplete denitrification raises N2O concentration to 6.1 μg N‐N2O L−1, approximately 20 times greater than the concentration expected from equilibration with atmospheric N2O. These elevated concentrations create average local fluxes reaching ∼950 μg N‐N2O m−2 hr−1 within the mixing zone. Summation of spatially explicit emission estimates for upstream, mixing zone, and downstream river sections yield a total average emission rate of ∼3,758 kg N‐N2O yr−1. Despite having the smallest surface area across the river transect (∼8%), emissions from the mixing zone account for 26% of the total river emission rate. These findings highlight the potential contributions from karst landscapes to N2O cycling caused by varying redox conditions during surface water‐groundwater exchange, which creates hotspots of N2O production and emissions.

Responses of dissolved organic carbon to freeze–thaw cycles associated with the changes in microbial activity and soil structure

Abstract. Arctic warming accelerates snowmelt, exposing soil surfaces with shallow or no snow cover to freeze–thaw cycles (FTCs) more frequently in early spring and late autumn. FTCs influence Arctic soil C dynamics by increasing or decreasing the amount of dissolved organic carbon (DOC); however, mechanism-based explanations of DOC changes that consider other soil biogeochemical properties are limited. To understand the effects of FTCs on Arctic soil responses, we designed microcosms with surface organic soils from Alaska and investigated several soil biogeochemical changes for seven successive temperature fluctuations of freezing at −9.0 ± 0.3 ∘C and thawing at 6.2 ± 0.3 ∘C for 12 h each. FTCs significantly changed the following soil variables: soil CO2 production (CO2), DOC and total dissolved nitrogen (TDN) contents, two DOC quality indices (SUVA254 and A365 / A254), microaggregate (53–250 µm) distribution, and small-sized mesopore (0.2–10 µm) proportion. Multivariate statistical analyses indicated that the FTCs improved soil structure at the scale of microaggregates and small-sized mesopores, facilitating DOC decomposition by soil microbes and changes in DOC quantity and quality by FTCs. This study showed that FTCs increased soil CO2 production, indicating that FTCs affected DOC characteristics without negatively impacting microbial activity. Soil microaggregation enhanced by FTCs and the subsequent increase in microbial activity and small-sized pore proportion could promote DOC decomposition, decreasing the DOC quantity. This study provides a mechanism-based interpretation of how FTCs alter DOC characteristics of the organic soil in the active layer by incorporating structural changes and microbial responses, improving our understanding of Arctic soil C dynamics.

Effects of cornstalk mulching and rainfall intensity on the quantity and quality of dissolved organic carbon in runoff: A field rainfall simulation at the plot scale

AbstractUnderscoring how crop mulching changes the dissolved organic carbon (DOC) in runoff is essential to understanding the sustainable application of cornstalk mulching. This study examined the runoff DOC quantity and quality in plots with and without cornstalk mulching under various rainfall intensities in a representative steep fallow land of southwestern China. Field rainfall simulations within a 60 min duration were conducted on the triplet plots (1.5 m × 1 m for a single plot) with bare soils under 30‐ and 90‐mm h−1 intensities and with cornstalk mulching (air‐dried whole plants with 60% coverage and weight 1.65 kg per plot) under 90 mm h−1 intensity, respectively. The concentration and loss of runoff DOC and its quality variables, including SUVA254 (aromaticity of DOC), C:C (proportion of colored humic substances in DOC), and E4:E6 (fulvic acid to humic acid in DOC), are determined. Results showed that runoff DOC concentration and loss were both significantly higher at 90 mm h−1 intensity than at 30 mm h−1 intensity (p < 0.05), simultaneously, they were 52.6% and 53.2% higher in mulched plots than in bare plots (p < 0.05), respectively. Runoff SUVA254 and C:C in bare plots were 90% and 88.9% higher in 30 mm h−1 intensity than in 90 mm h−1 intensity (p < 0.05), respectively. Whereas, E4:E6 of runoff was 12.3% higher in mulched plots than that in bare plots (p < 0.05). Our findings imply that cornstalk mulching enhances the export of runoff DOC with a decline in aromaticity and the molecular weight of humic substances.

Response of dissolved organic carbon dynamics to salinity in a Constructed Fen Peatland in the Athabasca Oil Sands region

AbstractIn northern Alberta, oil sands mining disturbs the boreal landscape, and reclamation to an ‘equivalent land capability’ is required. Industry is testing peatland construction as part of landscape reclamation. To determine if constructed peatlands can be self‐sustaining, an understanding of the cycling of solutes in pore water and their interactions with dissolved organic carbon (DOC) is needed since DOC can represent an important carbon loss from peatlands. DOC is of interest due to its biotic origin and use by the microbial community and impact on carbon budgets. Additionally, salinity as a control on DOC quantity and quality may be important in oil sands reclaimed systems due to the likelihood of elevated sodium (Na+) from saline groundwater input derived from tailings used to construct catchments, and natural sources. For this research, DOC concentration and quality, and Na+ concentration were measured in the rooting zone (10 and 30 cm depth) of Nikanotee Fen to evaluate the role of Na+ in DOC dynamics. DOC concentration and quality suggested that DOC in the fen was largely sourced from vegetation inputs, with quality also suggesting increases in vegetation inputs between years. Elevated Na+ at 30 cm below ground surface corresponded with high concentrations of labile DOC. At 10 cm below ground surface, sampling location and temperature were the best predictors of DOC concentration and quality. With expected increases in Na+, increased production of mobile and microbially active DOC may lead to higher rates of carbon export.

Soil warming decreased dissolved organic carbon quantity and quality in subtropical forests.

Soil dissolved organic carbon (DOC) is the most active part in forest soil carbon pool, the responses of which to climate warming has profound effects on forest carbon cycling. Based on a manipulative soil warming experiment in subtropical evergreen broad-leaved forests, we collected soil solutions in situ and used ultraviolet-visible, infrared and three-dimensional fluorescence spectroscopy analyses to explore the effects of soil warming (+4 ℃, 1 year) on soil DOC quantity and quality along the soil profile. The results showed that soil DOC flux remained constant along the soil profile. Soil DOC mainly included two humic-like fractions and one microbial metabolite. Warming significantly decreased soil DOC flux and the abundance of aromatic and hydrophobic components, and increased the amount of low molecular weight carbohydrates. Furthermore, soil warming increased the relative proportion of humic-like fractions in the surface soil layer (0-10 cm) and microbial metabolite in the deep soil layer (30-40 cm), indicating that warming might accelerate microbial turnover in the deep layer. Overall, soil warming not only decreased soil DOC content, but also simplified the composition of soil DOC in subtropical evergreen broad-leaved forests.

Dynamics of dissolved organic carbon in runoff discharge under different rainfall patterns in a representative agricultural catchment

Rainfall-induced runoff transported plentiful terrestrial dissolved organic carbon (DOC) to downstream waters, significantly regulating the biogeochemical cycling of carbon in downstream waters. However, the dynamics of DOC exporting from agricultural catchments in response to different rainfall patterns remain largely unknown. This study aimed to understand the effects of rainfall patterns on the runoff DOC quantity and quality in a representative agricultural catchment in Southwest China. Six rainfall events producing runoff, including three moderate rainfalls, two heavy rainfalls, and one rainstorm, were recorded to investigate the shifts in runoff DOC concentration and its quality indicators, i.e., SUVA254 (indicating the aromaticity content), C:C ratio (denoting the proportion of colored substances to uncolored substances), and E4:E6 ratio (denoting the fulvic acid to humic acid of DOC). The results showed that DOC concentration in runoff discharge was highest in the moderate and heavy rainfalls occurring at the early rainy season, while was much lower in the rainstorm. Exported DOC flux reached 1.82 kg hm-2 h−1 in the rainstorm, which was 3.95–60.67 times that in the other rainfall events. SUVA254 and C:C ratio fluctuated softly during the moderate rainfalls and rainstorm, and changed strongly during the heavy rainfall with longest antecedent dry day. The moderate and heavy rainfalls with longer antecedent dry day and the rainstorm delivered DOC with higher SUVA254 and C:C ratio than the other rainfall events did. Discharge was positively correlated with DOC concentration during rainfall across all rainfall patterns, while was negatively correlated with SUVA254 and C:C ratio. Forward stepwise regression analysis showed that DOC flux per hour was only positively linearly correlated with rainfall intensity. Our results were valuable for understanding the biogeochemical cycling of DOC in analogous catchments worldwide.

Limnology and diatom ecology of shallow lakes in a rapidly thawing discontinuous permafrost peatland

ABSTRACT Lakes in discontinuous permafrost peatlands are on the front lines of climate change, sensitive to even modest increases in air temperature. The aim of this study was to provide the first limnological characterization of shallow (∼1–2 m depth) lakes in the Scotty Creek basin (Northwest Territories, Canada), a field site of circumpolar significance due to the existence of long-term ecohydrological monitoring going back decades. We use this previous work as a foundation to advance our process-based understanding of the potential drivers of lake ecosystem change. Our results showed that dissolved organic carbon (DOC) and lake color were not correlated, a pattern that seems to be an important driver of diatom (siliceous single-celled algae) assemblages in these lakes. Diatoms in the study lakes tended to fall into 1 of 2 assemblage clusters. One cluster, composed of small benthic Fragilariaceae and small Navicula species (sensu lato), was found associated with higher lake color; the second cluster, composed of Encyonopsis and large Navicula species, was found associated with high DOC, lower color, and the presence of a benthic moss mat. From this finding, we suggest that DOC quality is a primary control on lake ecology in this region for its role in controlling light penetration to the lake bottom. Our hypothesis that the prevalence of nearshore fens and collapse scar wetlands would be important drivers of DOC was not supported in the 9 study lakes with available data to map shoreline features.

Small-scale topography explains patterns and dynamics of dissolved organic carbon exports from the riparian zone of a temperate, forested catchment

Abstract. Export of dissolved organic carbon (DOC) from riparian zones (RZs) is an important component of temperate catchment carbon budgets, but export mechanisms are still poorly understood. Here we show that DOC export is predominantly controlled by the microtopography of the RZ (lateral variability) and by riparian groundwater level dynamics (temporal variability). From February 2017 until July 2019 we studied topography, DOC quality and water fluxes and pathways in the RZ of a small forested catchment and the receiving stream in central Germany. The chemical classification of the riparian groundwater and surface water samples (n=66) by Fourier transform ion cyclotron resonance mass spectrometry revealed a cluster of plant-derived, aromatic and oxygen-rich DOC with high concentrations (DOCI) and a cluster of microbially processed, saturated and heteroatom-enriched DOC with lower concentrations (DOCII). The two DOC clusters were connected to locations with distinctly different values of the high-resolution topographic wetness index (TWIHR; at 1 m resolution) within the study area. Numerical water flow modeling using the integrated surface–subsurface model HydroGeoSphere revealed that surface runoff from high-TWIHR zones associated with the DOCI cluster (DOCI source zones) dominated overall discharge generation and therefore DOC export. Although corresponding to only 15 % of the area in the studied RZ, the DOCI source zones contributed 1.5 times the DOC export of the remaining 85 % of the area associated with DOCII source zones. Accordingly, DOC quality in stream water sampled under five event flow conditions (n=73) was closely reflecting the DOCI quality. Our results suggest that DOC export by surface runoff along dynamically evolving surface flow networks can play a dominant role for DOC exports from RZs with overall low topographic relief and should consequently be considered in catchment-scale DOC export models. We propose that proxies of spatial heterogeneity such as the TWIHR can help to delineate the most active source zones and provide a mechanistic basis for improved model conceptualization of DOC exports.

Dissolved organic carbon production and transport within a constructed fen watershed in the Athabasca Oil Sands Region, Alberta, Canada

Vast areas of wetlands in the Athabasca Oil Sands Region, Alberta, Canada have been disturbed for mining and other resource extraction activities. In recent years, these disturbed areas have become a focus for reclamation activities. To improve our understanding of the biogeochemical cycling of constructed ecosystems, it is important that we study the production of, as well as the inputs and outputs of dissolved organic carbon (DOC) in the system. The aim of our study was to investigate the hydrological DOC fluxes into a constructed fen in 2015 and 2016 (2- and 3-years post-construction respectively), relative to internal DOC production, and to assess the potential importance of each source for DOC export using DOC concentration and quality parameters. Our results show that while runoff added a small amount of DOC in both years, DOC concentration within the fen is driven largely by increased vegetation production, with DOC increasing in the years since construction. The measured quality indices (SUVA254, E2/E3, FI, HIX, and β/a) all showed that DOC present in the system is likely more recently produced and plant sourced. There was also little evidence that precipitation and runoff inputs from hillslopes impacted the quality of DOC present in the fen, while DOC export from the fen was minimal. As this constructed wetland continues to mature, it is important that monitoring of DOC continues in order to understand the trajectory of biogeochemical dynamics of the system following vegetation establishment, as well as changes under potential future climatic variations.

Aspen Soils Retain More Dissolved Organic Carbon Than Conifer Soils in a Sorption Experiment

The effect tree species have on soil organic carbon (SOC) has been hotly debated but, so far, few clear patterns have emerged. One example of a differing tree species effect on SOC are aspen forests in North America, which have been found to have more stable SOC than adjacent conifer forest stands. An important source for the formation of stable organo-mineral complexes in soil is dissolved organic carbon (DOC). DOC concentrations in mineral soil are often higher under the thick O-horizons of conifer forests than under aspen forests, but this does not correspond to more stable mineral SOC. This suggests that, instead of DOC concentration, DOC quality could be driving the observed differences in SOC. Therefore, we quantified the retention of contrasting forest detritus DOC in soils. Using a batch sorption experiment approach, we compared the retention of detritus leachates from four sources – aspen leaves (AL), aspen roots (AR), conifer (subalpine fir) needles (CN), and conifer (subalpine fir) roots (CR) – on soils sampled under aspen and conifer (subalpine fir and Douglas fir) overstories. The calculated sorption isotherms showed a higher retention of AL DOC than AR DOC, as indicated by all four sorption parameters – k and n (curve-fitting parameters), null point concentration (NPC; net sorption = net desorption), and endpoint (EP, retention at the highest initial DOC concentration). Leachates from CN and CR showed very similar retention behavior, and between the two species the retention of root leachates was more similar than the retention of foliage leachates. Soils sampled from aspen forests showed higher affinity for new DOC than conifer soils [higher sorption rate (n), lower NPC, and higher EP] regardless of the DOC source. The findings suggest that the higher DOC sorption on aspen soils might be a major driver for more stable SOC under aspen stands in North America.

Assessment of dissolved organic carbon and iron effects on water color between a forest and pasture-dominated fine-scale catchment in a Central Appalachian region, West Virginia.

Dissolved organic carbon (DOC) and iron (Fe) have been observed to be the important contributors to surface water brownification. Additionally, the DOC quality influences water color by forming Fe-DOC complexes that provide additive effects and is influenced by dominant land use type within watersheds. However, the influence of quantity and quality of DOC on Fe and water color is poorly understood in headwater streams. The aim of this study was to investigate the effects of DOC and Fe on water color in forest (FC) and pasture (GFC) fine-scale watersheds to remove the confounding effects of climate and soil parent material. Significant differences of DOC, Fe, and water absorbance at 420 nm (a420) between FC and GFC were found (p < 0.05). A dominant contribution to water color was from DOC (95.5 - 63.7%) with a decreasing trend when Fe increased from 0.011 to 0.258 mg L-1. There were no significant interactions between FC and GFC and Fe on either a420/DOC (p = 0.06) or specific ultraviolet absorbance at 254 nm (SUVA254) (p = 0.30). Increasing values of a420/DOC and SUVA254 were significantly associated with increasing Fe concentration (p < 0.01). Significant interactions were found between FC and GFC and Fe on spectral slope ratio (S ratio) (p < 0.01). The response rate of S ratio with increasing Fe per unit was 0.235 for GFC while it was - 11.043 for FC. These differences indicate that land use may change the quality of DOC, influence Fe-DOC interactions, and thus affect water color. Linking the effects of soil Fe and DOC and headwater Fe and DOC may help identify optimal management practice to mitigate surface water brownification.

Differences in the Effects of Storms on Dissolved Organic Carbon (DOC) in Boreal Lakes during an Early Summer Storm and an Autumn Storm

In boreal lakes, increased precipitation events have been linked to increased concentrations of dissolved organic carbon (DOC), however the effects of seasonal differences on DOC and how this may impact storm response remain unclear. We evaluated DOC concentration and a set of DOC quality metrics during an early summer storm and an autumn storm on a suite of six lakes in Acadia National Park in Maine, USA. to better understand differences in seasonal storm responses. Our results revealed differences in the response of DOC quality metrics to an early summer versus an autumn storm, with changes in DOC quality metrics varying by storm and lake features. During the early summer storm, we observed greater changes in various DOC quality metrics in deep lakes with longer residence times, whereas during the autumn storm, lakes with large watershed area to lake area ratios experienced the greatest changes. Land cover was highly correlated with changing DOC quality metrics in the early summer storm but did not play a significant role in the autumn storm response. Our research provides evidence of seasonal differences in the effects of storms on boreal lakes, which are ultimately mediated by a combination of lake and watershed characteristics as well as seasonal differences in climate such as solar radiation and antecedent weather conditions.

Natural levels of hydrogen peroxide (H2O2) in deep clear South temperate lakes: Field and laboratory evidence of photo- and biotic production

Hydrogen peroxide (H2O2) is a ubiquitous reactive oxygen species (ROS) in aquatic systems and is produced mainly in surface water by the interaction of ultraviolet radiation (UVR) and natural dissolved organic carbon (DOC). Andean Patagonian lakes are ultraoligotrophic, clear systems with extended photic zones (~40 m), and are exposed to challenging UVR levels due to their lati-altitudinal situation and extremely low DOC levels. This investigation describes the seasonal levels of H2O2 in relation to DOC quality in the water column of lakes Moreno East (ME) and Moreno West (MW), two deep (ca. 100 m), ultraoligotrophic, low-DOC (<0.7 mg L−1) systems of Andean Patagonia. H2O2 concentrations recorded in the lakes were below 60 nM, ranging from ~3 to ~60 nM in Lake ME and from ~5 to ~35 nM in Lake MW. In most of the samples of both lakes, the H2O2 levels were higher in the photic zone (surface to 30–40 m) than the aphotic zone (from 30–40 m to 90–100 m), particularly in summer samples. Laboratory experiments evaluated the abiotic (photochemical) and biotic (microbial) production of H2O2 in seasonal (summer, autumn) samples which varied DOM quality due to lake (ME, MW) and depth (photic and aphotic lake layers) provenance. Abiotic and biotic production of H2O2 attained higher levels in summer samples from the photic zones of both lakes. Humic DOM from deep layers (particularly from summer samples) was more susceptible to both photo- and bio-degradation than DOM from upper lake layers, which was characterized by stronger signs of degradation and progress in diagenesis.

Physicochemical and carbon quantity–quality gradients equally influence bacterial carbon metabolism across an arid riverscape

Although freshwater systems comprise a small portion of land surface, they play a pivotal role in landscape carbon (C) cycling. Carbon processing by heterotrophic bacteria is critical, contributing to ecosystem production and dissolved organic carbon (DOC) processing. Riverine bacterial C metabolism is related to a diversity of factors, including (1) physiochemical conditions and inorganic nutrient concentrations and (2) DOC quality and quantity; however, the relative importance of these factors in influencing C metabolism across arid fluvial networks remains equivocal. This study examined C metabolism by heterotrophic bacterioplankton in the Rio Grande drainage, an arid river network in Texas, USA. We examined spatial variation in physicochemical and inorganic nutrient conditions, bacterial C metabolism, and DOC lability across the drainage and assessed whether variation in bacterial C metabolism was more related to physicochemical–inorganic nutrient conditions or DOC quantity and lability. Across the drainage, hydrology and landscape position influenced physicochemical conditions, bacterial abundance, phytoplankton biomass, and bacterial C metabolism and the proportion of variation in bacterial C metabolism explained by physicochemical/inorganic nutrient and DOC quality–quantity gradients was approximately equal. Bacterial abundance and production were associated with greater NH4+ and DOC concentrations and phytoplankton biomass, whereas bacterial respiration and growth efficiency were driven by DOC color and suspended particulate concentrations. Results indicate that aspects of bacterial C metabolism are influenced by different environmental factors related to autochthonous and allochthonous inputs to riverine systems, which has implications for C transformation, sequestration, and transport to the ocean.

Monitoring dissolved organic carbon by combining Landsat-8 and Sentinel-2 satellites: Case study in Saginaw River estuary, Lake Huron

Dissolved organic carbon (DOC) in aquatic environments is an important cycled pool of organic matter on the Earth. Satellite remote sensing provides a useful tool to determine spatiotemporal distribution of water quality parameters. Previous DOC remote sensing studies in inland water suffered from either low spatial resolution or low temporal frequency. In this study, we evaluated the potential of jointly using Landsat-8 and Sentinel-2 with high spatial resolution to estimate DOC concentrations in Saginaw River plume regions of Lake Huron. Firstly, CDOM (colored dissolved organic matter) was estimated from images using the known models and then DOC can be derived in terms of the good correlations between DOC and CDOM. The results show that Landsat-8 and Sentinel-2 had acceptable accuracy and good consistency in DOC estimation so that jointly using them can improve the observation frequency. In different seasons from 2013 to 2018, DOC was typically higher in spring and autumn but lower in summer. Monthly spatiotemporal variations of DOC in 2018 were also observed. The image-derived DOC spatiotemporal variations show that DOC was covaried with Saginaw River discharge (r = 0.82) and also weakly and negatively correlated with water temperature (r = −0.6). This study demonstrated that using Landsat-8 and Sentinel-2 together can offer the potential applications for monitoring DOC and water quality dynamic in complex inland water.

Climate‐Driven Changes in Dissolved Organic Carbon and Water Clarity in Arctic Lakes of West Greenland

AbstractTo assess climate‐mediated terrestrial‐aquatic linkages in Arctic lakes and potential impacts on light attenuation and carbon cycling, we evaluated lake responses to climate drivers in two areas of west Greenland with differing climate patterns. We selected four lakes in a warmer, drier area to compare with four lakes from a cooler, wetter area proximal to the Greenland Ice Sheet. In June from 2013–2018, we measured epilimnetic water temperature, 1% depth of photosynthetically active radiation (PAR), dissolved organic carbon (DOC), specific ultraviolet absorbance (SUVA254), DOC‐normalized absorbance at 380 nm (a*380), and chlorophyll a. Interannual coherence of 1% PAR and DOC was particularly high for lakes within the warmer, drier area. This coherence suggests forcing of Arctic lake features by a large‐scale driver, likely climate. Redundancy analysis showed that monthly average precipitation, winter North Atlantic Oscillation (NAO) index (NAOW), spring average air temperature, and spring average precipitation influenced the lake variables (p = 0.003, adj. R2 = 0.58). In particular, monthly average precipitation contributed to increases in soil‐derived DOC quality metrics and chlorophyll a and decreased 1% PAR. Interannual changes in lake responses to climate drivers were more apparent in the warmer, drier area than the cooler, wetter area. The interannual lake responses within and between areas, associated with climate trends, suggest that with ongoing rapid climate change in the Arctic, there could be widespread impacts on key lake responses important for light attenuation and carbon cycling.

High-frequency measurements explain quantity and quality of dissolved organic carbon mobilization in a headwater catchment

Abstract. Increasing dissolved organic carbon (DOC) concentrations and exports from headwater catchments impact the quality of downstream waters and pose challenges to water supply. The importance of riparian zones for DOC export from catchments in humid, temperate climates has generally been acknowledged, but the hydrological controls and biogeochemical factors that govern mobilization of DOC from riparian zones remain elusive. A high-frequency dataset (15 min resolution for over 1 year) from a headwater catchment in the Harz Mountains (Germany) was analyzed for dominant patterns in DOC concentration (CDOC) and optical DOC quality parameters SUVA254 and S275−295 (spectral slope between 275 and 295 nm) on event and seasonal scales. Quality parameters and CDOC systematically changed with increasing fractions of high-frequency quick flow (Qhf) and antecedent hydroclimatic conditions, defined by the following metrics: aridity index (AI60) of the preceding 60 d and the quotient of mean temperature (T30) and mean discharge (Q30) of the preceding 30 d, which we refer to as discharge-normalized temperature (DNT30). Selected statistical multiple linear regression models for the complete time series (R2=0.72, 0.64 and 0.65 for CDOC, SUVA254 and S275−295, resp.) captured DOC dynamics based on event (Qhf and baseflow) and seasonal-scale predictors (AI60, DNT30). The relative importance of seasonal-scale predictors allowed for the separation of three hydroclimatic states (warm and dry, cold and wet, and intermediate). The specific DOC quality for each state indicates a shift in the activated source zones and highlights the importance of antecedent conditions and their impact on DOC accumulation and mobilization in the riparian zone. The warm and dry state results in high DOC concentrations during events and low concentrations between events and thus can be seen as mobilization limited, whereas the cold and wet state results in low concentration between and during events due to limited DOC accumulation in the riparian zone. The study demonstrates the considerable value of continuous high-frequency measurements of DOC quality and quantity and its (hydroclimatic) key controlling variables in quantitatively unraveling DOC mobilization in the riparian zone. These variables can be linked to DOC source activation by discharge events and the more seasonal control of DOC production in riparian soils.

Ontogenic succession of thermokarst thaw ponds is linked to dissolved organic matter quality and microbial degradation potential

AbstractWarming climate is thawing the permafrost in arctic and subarctic regions, leading to formation of thermokarst ponds. During the formation and geomorphological succession of these ponds, carbon that has been trapped in frozen soils for thousands of years is hydrologically mobilized and returned to the active carbon cycle. We sampled 12 thermokarst ponds representing three different stages of pond succession to study the potential of microbial communities to metabolize the organic carbon in the water. We investigated the quality of the dissolved organic carbon (DOC) in the water column based on the spectrophotometric and fluorometric properties of the chromophoric dissolved organic matter combined with parallel factor analysis and the potential of the microbial community for degrading these carbon compounds based on genetic markers related to carbon degradation. Our analysis showed a clear difference in the DOC quality across the different developmental stages. In the younger ponds, organic matter quality suggested that it was originating from the degrading permafrost and in the metagenomes collected from these ponds, the normalized abundance of genes related to degradation of carbon compounds was higher. There was also a shift in the degradation potential in the water column of the ponds, with higher potential for organic matter degradation in deeper, anoxic layers. In conclusion, our results show that the DOC quality and the genetic potential of the microbial community for carbon cycling change across the pond ontogeny, suggesting a capacity of the microbial communities to adapt to changing environmental conditions.