Stream Dissolved Organic Carbon Research Articles

Effects of changes in precipitation patterns and stream flow on stream DOC and [formula omitted] concentrations in a headwater catchment in a subtropical region of southern China

Study regionThe Guantian headwater watershed is located in the subtropical region of South China. Study focusThis study examined how changes in precipitation and consequent shifts in streamflow affect dissolved organic carbon (DOC) and nitrate (NO3−) concentrations in a headwater stream. New hydrological insights for the regionThe results indicate that stream DOC and NO3− concentrations decrease from spring to winter (0.16 mg/L/month for DOC and 0.18 mg/L/month for NO3−) and have a generally positive relationship with streamflow (p < 0.05). Changes in precipitation-concentration, precipitation-amount and consequent shifts in streamflow remarkably affect stream DOC and NO3− concentrations. When precipitation events are more heavily concentrated (when precipitation concentration index (PCI) changed from 12.81 to 18.33) from May to July, the stream DOC and NO3− concentrations tend to increase from May to July but decrease before and after this period. In addition, an increase in precipitation-amount tends to increase stream DOC and NO3− concentrations before the occurrence of peak flow and in the dry period from September to December, but decreases them in the wet period from June to September. When the precipitation-amount decreases, the trend of the change in concentration is exactly the opposite. These findings suggest that stream DOC and NO3− concentrations in the headwater stream are highly chemodynamic: they are transport-limited before the occurrence of high flow and in the dry period from September to December, while they are supply-limited in the June-September period.

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.

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.

Coupling of Water‐Carbon Interactions During Snowmelt in an Arctic Finland Catchment

AbstractSnowmelt spring floods regulate carbon transport from land to streams. However, these coupled processes are rarely documented through high‐resolution measurements focused on water‐carbon interactions. We collated a state‐of‐the‐art high‐frequency data set throughout a snowmelt and early post snowmelt period, alongside regular samples of stream water, precipitation, and snowmelt isotopes (δ18O). Our study was conducted during the 2019 snowmelt and initial post snowmelt season in a subarctic, peatland influenced headwater catchment in Pallas, Northern Finland. We measured high‐frequency dissolved organic carbon (DOC), and in‐stream carbon dioxide (pCO2). We identified a change in hydrological processes as the snowmelt season progressed and the post snowmelt season began. We found (a) Overland flow dominated stream DOC dynamics in early snowmelt, while increased catchment connectivity opened new distal pathways in the later snowmelt period; (b) CO2 processes were initially driven by rapid bursts of CO2 from the meltwaters in snowmelt, followed by dilution and source limitation emerging post snowmelt as deep soil pathways replaced the snowpack as the main source of CO2; (c) stream carbon concentration shifted from being relatively balanced between CO2 and DOC during the early snowmelt period to being increasingly DOC dominated as snowmelt progressed due to changes in DOC and CO2 source supply. The study highlights the importance of using high‐frequency measurements combined with high‐frequency data analyses to identify changes in the processes driving water‐carbon interactions. The degree to which water‐carbon interactions respond to the continuation of Arctic water cycle amplification is central to delineating the evolving complexity of the future Arctic.

Disentangling the responses of dissolved organic carbon and nitrogen concentrations to overlapping drivers in a northeastern United States forested watershed

The concurrent reduction in acid deposition and increase in precipitation impact stream solute dynamics in complex ways that make predictions of future water quality difficult. To understand how changes in acid deposition and precipitation have influenced dissolved organic carbon (DOC) and nitrogen (N) loading to streams, we investigated trends from 1991 to 2018 in stream concentrations (DOC, ~3,800 measurements), dissolved organic nitrogen (DON, ~1,160 measurements), and dissolved inorganic N (DIN, ~2,130 measurements) in a forested watershed in Vermont, USA. Our analysis included concentration-discharge (C-Q) relationships and Seasonal Mann-Kendall tests on long-term, flow-adjusted concentrations. To understand whether hydrologic flushing and changes in acid deposition influenced long-term patterns by liberating DOC and dissolved N from watershed soils, we measured their concentrations in the leachate of 108 topsoil cores of 5 cm diameter that we flushed with solutions simulating high and low acid deposition during four different seasons. Our results indicate that DOC and DON often co-varied in both the long-term stream dataset and the soil core experiment. Additionally, leachate from winter soil cores produced especially high concentrations of all three solutes. This seasonal signal was consistent with C-Q relation showing that organic materials (e.g., DOC and DON), which accumulate during winter, are flushed into streams during spring snowmelt. Acid deposition had opposite effects on DOC and DON compared to DIN in the soil core experiment. Low acid deposition solutions, which mimic present day precipitation, produced the highest DOC and DON leachate concentrations. Conversely, high acid deposition solutions generally produced the highest DIN leachate concentrations. These results are consistent with the increasing trend in stream DOC concentrations and generally decreasing trend in stream DIN we observed in the long-term data. These results suggest that the impact of acid deposition on the liberation of soil carbon (C) and N differed for DOC and DON vs. DIN, and these impacts were reflected in long-term stream chemistry patterns. As watersheds continue to recover from acid deposition, stream C:N ratios will likely continue to increase, with important consequences for stream metabolism and biogeochemical processes.

Woody debris dominates the exports of carbon and nitrogen from headwater streams in an alpine forest

AbstractThe exports of carbon and nitrogen with water flowing in the headwater streams could be important components of material mitigation in forest ecosystems. Plant debris is a major source of dissolved organic matter for headwater streams, but few studies have investigated the differences between the impacts of woody debris and non‐woody debris inputs on headwater streams in terms of carbon and nitrogen exports. Here, we assessed the effects of plant debris (i.e., woody debris, non‐woody debris and mixed debris) on the concentrations and exports of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) in the headwater streams of an alpine forest on the eastern Tibetan Plateau. Woody and non‐woody debris only weakly affected the DOC and TDN concentrations in the headwater streams. Compared with those in the reference stream excluding plant debris, woody debris increased the exports of DOC and TDN by 19% and 13%, whereas non‐woody debris decreased the exports of DOC and TDN by 22% and 25%, respectively. However, when fall approached, the role of non‐woody debris reversed to enhance the stream‐water exports of DOC and TDN. The effect of non‐woody debris during the fall season differed from that during the overall growing season, indicating that restricting non‐woody debris inputs might improve the ability of the stream to retain forest carbon and nitrogen. Notwithstanding the relatively limited experimental period, this work revealed the critical importance of plant debris for carbon and nutrients within water conservation regions like alpine forests.

Machine‐Learning Reveals Equifinality in Drivers of Stream DOC Concentration at Continental Scales

AbstractResearch at long‐term catchment monitoring sites has generated a great volume, variety, and velocity of data for analysis of stream water chemistry dynamics. To harness the potential of these big data and extract patterns that are indicative of underlying functional relationships, machine learning tools have advantages over traditional statistical methods, and are increasingly being applied for dimension reduction, feature extraction, and trend identification. Still, as examples of complex systems, catchments are characterized by multivariate factor interactions and equifinality that are not easily identified by most machine‐learning methods. Using dissolved organic carbon (DOC) dynamics as an illustration, we applied a new evolutionary algorithm (EA) to extract geologic, topographic, meteorologic, hydrologic, and land use attributes that were correlated to mean stream DOC concentration in forested catchments distributed across the continental United States. The EA reduced dimensionality of our attribute dataset to identify the combination of factors, and their specific value ranges, that interacted to drive membership in High or Low mean DOC clusters. High mean DOC concentrations were associated with two distinct geographic locations of variable climatic and vegetative conditions, indicating equifinality. Our findings underscore the importance of critical zone structure in mediating hydrological and biogeochemical processes to govern DOC dynamics at the catchment scale. This multi‐scale, pattern‐to‐process approach is being applied to refine hypotheses for process‐based modeling of DOC dynamics in forested headwater streams at catchment to site scales.

Groundwater flow paths drive longitudinal patterns of stream dissolved organic carbon (DOC) concentrations in boreal landscapes

Abstract. Preferential groundwater flow paths can influence dissolved organic carbon (DOC) concentration and export in the fluvial network because they facilitate the inflow of terrestrial DOC from large upslope contributing areas to discrete sections of the stream, referred to as discrete riparian inflow points (DRIPs). However, the mechanisms by which DRIPs influence longitudinal patterns of stream DOC concentrations are still poorly understood. In this study, we ask how DRIPs affect longitudinal patterns of stream DOC concentrations under different hydrologic conditions, as they can simultaneously act as major sources of terrestrial DOC and important locations for in-stream processes. To answer this question, we tested four model structures that account for different representations of hydrology (distributed inflows of DRIPs vs. diffuse groundwater inflow) and in-stream processes (no DOC uptake vs. in-stream DOC uptake downstream of DRIPs) to simulate stream DOC concentrations along a 1.5 km headwater reach for 14 sampling campaigns with flow conditions ranging from droughts to floods. Despite the magnitude and longitudinal patterns of stream DOC concentration varying across campaigns, at least one model structure was able to capture longitudinal trends during each campaign. Specifically, our results showed that during snowmelt periods or high-flow conditions (&gt;50 L s−1), accounting for distributed inputs of DRIPs improved simulations of stream DOC concentrations along the reach, because groundwater inputs from DRIPs diluted the DOC in transport. Moreover, accounting for in-stream DOC uptake immediately downstream of DRIPs improved simulations during five sampling campaigns that were performed during spring and summer, indicating that these locations served as a resource of DOC for aquatic biota. These results show that the role of DRIPs in modulating DOC concentration, cycling, and export varies over time and depends strongly on catchment hydrology. Therefore, accounting for DRIPs can improve stream biogeochemistry frameworks and help inform management of riparian areas under current and future climatic conditions.

Analysis of spatiotemporal variation in dissolved organic carbon concentrations for streams with cropland-dominated watersheds

It remains a challenge to understand how dissolved organic carbon (DOC) is cycled from farmlands to rivers due to the complex interaction between farming practices, the baseflow hydrology of predominantly flat lowlands, and seasonal environmental influences such as snowpack. To address this, field DOC concentrations were measured monthly throughout the year at sub-basin scales across the Chippewa River Watershed, which falls within the Corn Belt of the Midwestern United States. These DOC dynamics in stream water from croplands were benchmarked against the data sampled from hilly forested areas in the Connecticut River Watershed. The Soil Water Assessment Tool (SWAT) simulation was applied to provide potential predictive variables associated with daily baseflow. Our study outlines a framework using the combination of primary field data, hydrological modeling, and knowledge-based reclassification of Land Use/Land Cover (LULC) data to analyze the viability of modeling the spatial and temporal variations of cropland stream DOC concentrations. Calibration of the SWAT model resulted in the overall daily Nash–Sutcliffe model efficiency coefficient (NSE) of 0.67 and the corresponding R2 = 0.89. Our main results show: 1) baseflow DOC concentrations from croplands were substantially higher throughout the year relative to other landcover areas, especially for spring runoff/snowmelt scenarios, 2) an empirical analysis explained ~82 % of the spatial gradient of annual mean observed DOC concentrations, and 3) with the addition of hydrological simulated variables, a linear model explained ~81 % of monthly and 54 % of daily variations of observed DOC concentrations for cropland sub-basins. Our study identified key factors regulating the spatiotemporal DOC concentrations in cropland streamflow; the contribution here promotes to strengthen future analytical models that link watershed characteristics to carbon cycling processes in a large freshwater ecosystem.

Riparian Microtopography Affects Event‐Driven Stream DOC Concentrations and DOM Quality in a Forested Headwater Catchment

AbstractThe transport of dissolved organic carbon (DOC) from the soils to inland waters plays an important role in the global carbon cycle. Widespread increases in DOC concentrations have been observed in surface waters over the last few decades, affecting carbon balances, ecosystem functioning and drinking water treatment. However, the primary hydrological controls on DOC mobilization are still uncertain. The aim of this study was to investigate the role of microtopography in the riparian zone for DOC export and DOM quality. DOC concentration and DOM quality in the shallow groundwater of a riparian zone and in streamflow in a forested headwater catchment was investigated using fluorescence and absorbance characteristics. We found higher DOC concentrations with a higher aromaticity in the microtopographical depressions, which were influenced by highly dynamic shallow groundwater levels, than in the flat forest soil. As a result of the frequent wet‐dry cycles in the upper soil layers, aromatic DOC accumulated in the shallow groundwater within and below the microtopographical depressions. Rising groundwater levels during precipitation events led to the connection of the microtopographical depressions to the stream, resulting in a change toward more aromatic DOC in the stream. Increasing stream DOC concentrations were accompanied by increasing concentrations of iron and aluminum, suggesting the coupled release of these metals with DOC from the riparian zone. Our results highlight the importance of the interplay between microtopography and groundwater level dynamics in the riparian zone for DOC export from headwater catchments.

The emerging role of drought as a regulator of dissolved organic carbon in boreal landscapes

One likely consequence of global climate change is an increased frequency and intensity of droughts at high latitudes. Here we use a 17-year record from 13 nested boreal streams to examine direct and lagged effects of summer drought on the quantity and quality of dissolved organic carbon (DOC) inputs from catchment soils. Protracted periods of drought reduced DOC concentrations in all catchments but also led to large stream DOC pulses upon rewetting. Concurrent changes in DOC optical properties and chemical character suggest that seasonal drying and rewetting trigger soil processes that alter the forms of carbon supplied to streams. Contrary to expectations, clearest drought effects were observed in larger watersheds, whereas responses were most muted in smaller, peatland-dominated catchments. Collectively, our results indicate that summer drought causes a fundamental shift in the seasonal distribution of DOC concentrations and character, which together operate as primary controls over the ecological and biogeochemical functioning of northern aquatic ecosystems.

Groundwater travel times predict DOC in streams and riparian soils across a heterogeneous boreal landscape

Dissolved organic carbon (DOC) in surface waters is an important component of the boreal landscape carbon budget and a critical variable in water quality. A dominant terrestrial DOC source in the boreal landscape is the riparian zone. These near stream areas play a key role in regulating DOC transport between land and aquatic ecosystems. The groundwater dynamics at this interface have been considered a major controlling variable for DOC export to streams. This study focuses on the regulating role of groundwater levels and mean travel times (MTT) on riparian DOC concentrations and, subsequently, stream DOC. This is done by comparing them as explanatory variables to capture the spatial and intra-annual variability of the stream and riparian groundwater DOC. We used a physically based 3D hydrological model, Mike SHE, to simulate DOC concentrations of the riparian zones for 14 sub-catchments within the Krycklan catchment (Sweden). The model concept assumes that DOC concentrations will be higher in groundwater moving through shallow flow paths. In the model, this can be linked to the position of the groundwater table at a point of observation or the travel time, which will generally be shorter for water that has travelled through shallow and more conductive soil layers. We compared the results with both observed stream and groundwater concentrations. The analysis revealed that the correlation between modelled and observed annual averages of stream DOC increased from r = 0.08 to r = 0.87 by using MTT instead of groundwater level. MTT also better captured the observed spatial variability in riparian DOC concentrations and more successfully represented seasonal variability of stream DOC. We, therefore, suggest that MTT is a better predictor than groundwater level for riparian DOC concentration because it can capture a greater variety of catchment heterogeneities, such as variation in soil properties, catchment size, and input from deep groundwater sources.

A systematic increase in the slope of the concentration discharge relation for dissolved organic carbon in a forested catchment in Vermont, USA

The production, mobilization and fluvial transport of dissolved organic carbon (DOC) in temperate forests are important components of the carbon cycle that are influenced by ongoing changes in climate. Numerous studies have reported temporal trends in stream water DOC concentrations and have attributed changes in concentrations to climatic and hydrologic variables. Fewer studies have reported trends in concentration-discharge (C-Q) relations for DOC. The goal of this study was to detect and quantify changes in DOC concentration and slope of the C-Q relation from 1991 to 2018 in an intensively sampled forested research watershed in northern Vermont. Stream water DOC concentration and slope of the C-Q relation increased over time as did precipitation, stream discharge, and air temperature. The increases in DOC concentration and slope of the C-Q were substantially greater in the summer and fall (autumn) than in winter and spring. The largest increases in the magnitude of C-Q slopes occurred in the December, October and September. The increases in slope of the C-Q relation in summer and fall were larger for baseflow than for storm flow. The increases in DOC concentration and slope of the C-Q relation over time may be related to increasing temperature, longer growing seasons, and associated increases in production and microbial decomposition of soil organic matter that supplies DOC for mobilization to streams. The results suggest that in a changing climate, C-Q relations may not necessarily be stationary and therefore analyses that attempt to estimate future DOC concentrations and loads should consider potentially changing C-Q relations over time.

Shifting stoichiometry: Long-term trends in stream-dissolved organic matter reveal altered C:N ratios due to history of atmospheric acid deposition.

Dissolved organic carbon (DOC) and nitrogen (DON) are important energy and nutrient sources for aquatic ecosystems. In many northern temperate, freshwater systems DOC has increased in the past 50 years. Less is known about how changes in DOC may vary across latitudes, and whether changes in DON track those of DOC. Here, we present long‐term DOC and DON data from 74 streams distributed across seven sites in biomes ranging from the tropics to northern boreal forests with varying histories of atmospheric acid deposition. For each stream, we examined the temporal trends of DOC and DON concentrations and DOC:DON molar ratios. While some sites displayed consistent positive or negative trends in stream DOC and DON concentrations, changes in direction or magnitude were inconsistent at regional or local scales. DON trends did not always track those of DOC, though DOC:DON ratios increased over time for ~30% of streams. Our results indicate that the dissolved organic matter (DOM) pool is experiencing fundamental changes due to the recovery from atmospheric acid deposition. Changes in DOC:DON stoichiometry point to a shifting energy‐nutrient balance in many aquatic ecosystems. Sustained changes in the character of DOM can have major implications for stream metabolism, biogeochemical processes, food webs, and drinking water quality (including disinfection by‐products). Understanding regional and global variation in DOC and DON concentrations is important for developing realistic models and watershed management protocols to effectively target mitigation efforts aimed at bringing DOM flux and nutrient enrichment under control.

Low hydrological connectivity after summer drought inhibits DOC export in a forested headwater catchment

Abstract. Understanding the controls on event-driven dissolved organic carbon (DOC) export is crucial as DOC is an important link between the terrestrial and the aquatic carbon cycles. We hypothesized that topography is a key driver of DOC export in headwater catchments because it influences hydrological connectivity, which can inhibit or facilitate DOC mobilization. To test this hypothesis, we studied the mechanisms controlling DOC mobilization and export in the Große Ohe catchment, a forested headwater in a mid-elevation mountainous region in southeastern Germany. Discharge and stream DOC concentrations were measured at an interval of 15 min using in situ UV-Vis (ultraviolet–visible) spectrometry from June 2018 until October 2020 at two topographically contrasting subcatchments of the same stream. At the upper location (888 m above sea level, a.s.l.), the stream drains steep hillslopes, whereas, at the lower location (771 m a.s.l.), it drains a larger area, including a flat and wide riparian zone. We focus on four events with contrasting antecedent wetness conditions and event size. During the events, in-stream DOC concentrations increased up to 19 mg L−1 in comparison to 2–3 mg L−1 during baseflow. The concentration–discharge relationships exhibited pronounced but almost exclusively counterclockwise hysteresis loops which were generally wider in the lower catchment than in the upper catchment due to a delayed DOC mobilization in the flat riparian zone. The riparian zone released considerable amounts of DOC, which led to a DOC load up to 7.4 kg h−1. The DOC load increased with the total catchment wetness. We found a disproportionally high contribution to the total DOC export of the upper catchment during events following a long dry period. We attribute this to the low hydrological connectivity in the lower catchment during drought, which inhibited DOC mobilization, especially at the beginning of the events. Our data show that not only event size but also antecedent wetness conditions strongly influence the hydrological connectivity during events, leading to a varying contribution to DOC export of subcatchments, depending on topography. As the frequency of prolonged drought periods is predicted to increase, the relative contribution of different subcatchments to DOC export may change in the future when hydrological connectivity will be reduced more often.

Drivers of Dissolved Organic Carbon Mobilization From Forested Headwater Catchments: A Multi Scaled Approach

Understanding and predicting catchment responses to a regional disturbance is difficult because catchments are spatially heterogeneous systems that exhibit unique moderating characteristics. Changes in precipitation composition in the Northeastern U.S. is one prominent example, where reduction in wet and dry deposition is hypothesized to have caused increased dissolved organic carbon (DOC) export from many northern hemisphere forested catchments; however, findings from different locations contradict each other. Using shifts in acid deposition as a test case, we illustrate an iterative “process and pattern” approach to investigate the role of catchment characteristics in modulating the steam DOC response. We use a novel dataset that integrates regional and catchment-scale atmospheric deposition data, catchment characteristics and co-located stream Q and stream chemistry data. We use these data to investigate opportunities and limitations of a pattern-to-process approach where we explore regional patterns of reduced acid deposition, catchment characteristics and stream DOC response and specific soil processes at select locations. For pattern investigation, we quantify long-term trends of flow-adjusted DOC concentrations in stream water, along with wet deposition trends in sulfate, for USGS headwater catchments using Seasonal Kendall tests and then compare trend results to catchment attributes. Our investigation of climatic, topographic, and hydrologic catchment attributes vs. directionality of DOC trends suggests soil depth and catchment connectivity as possible modulating factors for DOC concentrations. This informed our process-to-pattern investigation, in which we experimentally simulated increased and decreased acid deposition on soil cores from catchments of contrasting long-term DOC response [Sleepers River Research Watershed (SRRW) for long-term increases in DOC and the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) for long-term decreases in DOC]. SRRW soils generally released more DOC than SSHCZO soils and losses into recovery solutions were higher. Scanning electron microscope imaging indicates a significant DOC contribution from destabilizing soil aggregates mostly from hydrologically disconnected landscape positions. Results from this work illustrate the value of an iterative process and pattern approach to understand catchment-scale response to regional disturbance and suggest opportunities for further investigations.

Event Scale Relationships of DOC and TDN Fluxes in Throughfall and Stemflow Diverge From Stream Exports in a Forested Catchment

AbstractAquatic fluxes of carbon and nutrients link terrestrial and aquatic ecosystems. Within forests, storm events drive both the delivery of carbon and nitrogen to the forest floor and the export of these solutes from the land via streams. To increase understanding of the relationships between hydrologic event character and the relative fluxes of carbon and nitrogen in throughfall, stemflow and streams, we measured dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) concentrations in each flow path for 23 events in a forested watershed in Vermont, USA. DOC and TDN concentrations increased with streamflow, indicating their export was limited by water transport of catchment stores. DOC and TDN concentrations in throughfall and stemflow decreased exponentially with increasing precipitation, suggesting that precipitation removed a portion of available sources from tree surfaces during the events. DOC and TDN fluxes were estimated for 76 events across a 2‐year period. For most events, throughfall and stemflow fluxes greatly exceeded stream fluxes, but the imbalance narrowed for larger storms (&gt;30 mm). The largest 10 stream events exported 40% of all stream event DOC whereas those same 10 events contributed 14% of all throughfall export. Approximately 2–5 times more DOC and TDN was exported from trees during rain events than left the catchment via streams annually. The diverging influence of event size on tree versus stream fluxes has important implications for forested ecosystems as hydrological events increase in intensity and frequency due to climate change.

Is a simple model based on two mixing reservoirs able to reproduce the intra-annual dynamics of DOC and NO3 stream concentrations in an agricultural headwater catchment?

Agriculture disturbs the biogeochemical cycles of major elements, which alters the elemental stoichiometry of surface stream waters, with potential impacts on their ecosystems. However, models of catchment hydrology and water quality remain relatively disconnected, even though the observation that dissolved organic carbon (DOC) and nitrate (NO3−) have opposite spatial and temporal patterns seems relevant for improving our representation of hydrological transport pathways within catchments. We tested the ability of a parsimonious model to simultaneously reproduce intra-annual dynamics of stream flow, DOC and NO3− concentrations using 15 years of daily data from a small headwater agricultural catchment (AgrHyS observatory). The model consists of an unsaturated reservoir, a slow reservoir representing the groundwater and a fast reservoir representing the riparian zone and preferential flow paths. The sources of DOC and NO3− are assumed to behave as infinite pools with a fixed concentration in each reservoir that contributes to the stream. Stream concentrations thus result from simple mixing of slow and fast reservoir contributions. The model simultaneously reproduced annual and storm-event dynamics of discharge, DOC and NO3− concentrations in the stream, with calibration KGE scores of 0.77, 0.64 and 0.58 respectively, and validation KGE scores of 0.72, 0.58 and 0.43 respectively. These results suggest that the dynamics of these concentrations can be explained by hydrological transport processes and thus by temporally variable contributions from slow (NO3− rich and DOC poor) and fast reservoirs (DOC rich and NO3− poor), with a poor representation of the biogeochemical transformations. Unexpectedly, using the concentration time series to calibrate the model increased uncertainty in the parameters that control hydrological fluxes of the model. The legacy storage of NO3− resulting from agricultural history in the studied catchment supports the assumption that the main DOC and NO3− sources behave as infinite pools at the scale of several years. Nevertheless, reproducing the long-term trends in solute concentration would require additional information about DOC and NO3− trends within the reservoirs.

Hillslopes in Headwaters of Qinghai‐Tibetan Plateau as Hotspots for Subsurface Dissolved Organic Carbon Processing During Permafrost Thaw

AbstractClimate warming has accelerated thawing of northern permafrost, resulting in changes to the supply of dissolved organic carbon (DOC) to inland waters with uncertain fate. Extensive surface water and groundwater interactions occur in alpine permafrost watersheds and likely influence DOC processing differently than systems with limited interactions. Here, we quantify and characterize DOC in waters collected from eight types of water sampled across a small (25 km2) alpine (elevation 2,960–4,820 m a.s.l) watershed in the Qinghai‐Tibetan Plateau (QTP) containing variably degraded permafrost. Three types of water (thermokarst ponds, red mud gully, and seepage‐I) contained high DOC concentrations (5.2–22.6 mg L−1, n = 38), with carbon contributions predominantly from frozen soil meltwater. Spatial patterns of DOC in stream (0.3–4.8 mg L−1, n = 41), and subsurface waters (0.4–3.8 mg L−1, n = 34), all contained frozen soil meltwater carbon as constrained by δ18O and electrical conductivity, reflecting surface‐groundwater exchanges in the upper‐, middle‐, and lower stretches of the watershed. Further, patterns of increasing DOC loss in subsurface waters with decreased proportions of protein‐like organic matter and specific UV absorbance at 254 nm, suggest subsurface microbial processing. Using previously established biodegradation DOC kinetics (0.06 day−1) from the QTP, the groundwater transit time is estimated to be between 6 and 20 days based on DOC loss changes of 32% and 74% for July and September, respectively. Mass balance of DOC inputs and export fluxes demonstrate nearly half of all DOC was lost in this small watershed, indicating hillslopes are hotspots for DOC processing, with subsurface environments playing a key role.

Stream Water Chemistry in Mixed-Conifer Headwater Basins: Role of Water Sources, Seasonality, Watershed Characteristics, and Disturbances

Understanding the transport of dissolved organic carbon (DOC) and nitrogen (N) as water flows through headwater basins is important for predicting downstream water quality. With increased recognition of climatic impact on nutrient transport, more studies are needed in headwater basins experiencing a Mediterranean-type climate, such as those of the Sierra Nevada, California. We analyzed water samples collected over 5 years from eight low-order and mixed-conifer watersheds to elucidate the temporal variation of water chemistry and evaluate their responses to prolonged drought and low-intensity forest thinning. We observed higher stream DOC concentrations in October compared to other months within water years prior to drought and thinning, suggesting the importance of antecedent moisture conditions on seasonal C export. In unthinned watersheds, stream DOC concentrations were lower (62%) and DOC aromaticity was higher (68 and 92%, depending on the index used) during drought compared to non-drought years. In thinned watersheds during drought years, stream water had higher DOC concentrations (66–94% in three consecutive years following thinning) and dissolved inorganic N (24%, in the third year following thinning) compared to unthinned watersheds during drought. Additionally, lower stream DOC concentrations were found in watersheds with higher elevations and lower drainage densities in the year with near-average precipitation; however, these correlations were not significant in years with greater or extremely low precipitation. Taken together, our results suggest that stream concentrations of DOC and dissolved N in Mediterranean headwater basins are extremely variable over time due to the high temporal climatic variabilities and periodic management practices.