Stream Dissolved Organic Matter Composition Research Articles

Bioavailability of dissolved organic matter varies with anthropogenic landcover in the Upper Mississippi River Basin

Anthropogenic conversion of forests and wetlands to agricultural and urban landcovers impacts dissolved organic matter (DOM) within streams draining these catchments. Research on how landcover conversion impacts DOM molecular level composition and bioavailability, however, is lacking. In the Upper Mississippi River Basin (UMRB), water from low-order streams and rivers draining one of three dominant landcovers (forest, agriculture, urban) was incubated for 28 days to determine bioavailable DOC (BDOC) concentrations and changes in DOM composition. The BDOC concentration averaged 0.49 ± 0.30 mg L−1 across all samples and was significantly higher in streams draining urban catchments (0.72 ± 0.34 mg L−1) compared to streams draining agricultural (0.28 ± 0.15 mg L−1) and forested (0.47 ± 0.17 mg L−1) catchments. Percent BDOC was significantly greater in urban (10% ± 4.4%) streams compared to forested streams (5.6% ± 3.2%), corresponding with greater relative abundances of aliphatic and N-containing aliphatic compounds in urban streams. Aliphatic compound relative abundance decreased across all landcovers during the bioincubation (average -4.1% ± 10%), whereas polyphenolics and condensed aromatics increased in relative abundance across all landcovers (average of +1.4% ± 5.9% and +1.8% ± 10%, respectively). Overall, the conversion of forested to urban landcover had a larger impact on stream DOM bioavailability in the UMRB compared to conversion to agricultural landcover. Future research examining the impacts of anthropogenic landcover conversion on stream DOM composition and bioavailability needs to be expanded to a range of spatial scales and to different ecotones, especially with continued landcover alterations.

Riparian land cover and hydrology influence stream dissolved organic matter composition in an agricultural watershed

Dissolved organic matter (DOM) represents an essential component of the carbon cycle and controls biogeochemical and ecological processes in aquatic systems. The composition and reactivity of DOM are determined by the spatial distribution of its sources and its residence time in a watershed. While the effects of agricultural land cover on DOM quality have been reported across spatial and temporal scales, the influence of riparian land cover on stream DOM composition has received little attention. Furthermore, the combined effects of riparian land cover and streamflow rates on DOM composition require investigation. To this end, a multi-year (2016–2018) DOM characterization study was conducted using bi-weekly water samples collected from seven sub-watersheds nested within the Little River Experimental Watershed (LREW) near Tifton, Georgia, USA. DOM optical properties were determined to assess compositional variations using UV–Vis and excitation-emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor (PARAFAC) analysis. PARAFAC analysis indicated that DOM in the LREW was dominated by three humic-like fluorescing components of terrestrial, microbial, and anthropogenic origin and a protein-like component. DOM composition was influenced by riparian land cover and hydrology, and shifted towards recently produced, low molecular weight DOM with low aromaticity as the percentage of agricultural land within riparian wetlands increased. During periods of high discharge and high baseflow, the DOM pool was dominated by recalcitrant and terrestrial-derived material but shifted towards protein-like and microbial-derived with increasing cropland in the riparian area. The results of this two-year study indicate that the replacement of forested riparian buffers with agricultural land can result in altered DOM composition which may affect carbon cycling and downstream water quality in agricultural watersheds.

Fire severity, time since fire, and site-level characteristics influence streamwater chemistry at baseflow conditions in catchments of the Sierra Nevada, California, USA

BackgroundFire plays an important role in controlling the cycling and composition of organic matter and nutrients in terrestrial and aquatic ecosystems. In this study, we investigated the effects of wildfire severity, time since fire, and site-level characteristics on (1) concentration of multiple solutes (dissolved organic carbon, DOC; total dissolved nitrogen, TDN; dissolved organic nitrogen, DON; calcium, Ca2+; magnesium, Mg2+; potassium, K+; sodium, Na+; chloride, Cl−; nitrate, NO3−; ammonium, NH4+; sulfate, SO42−; and phosphate, PO43−), and (2) the molecular composition of stream-dissolved organic matter (DOM) across 12 streams sampled under baseflow conditions in Yosemite National Park, California, USA. Samples were collected from low- and high-severity burned stream reaches, as well as an unburned reference stream reach.ResultsFire severity, time since fire, and variability in site-level characteristics emerged as the strongest influences on streamwater chemistry. Results from mixed-effect models indicated that DOC and DON concentrations decreased with time since fire in high-severity burned stream reaches. In low-severity burned stream reaches, DOC concentrations increased, and DON concentrations slightly decreased with time since fire. We also found that declines in aromaticity (expressed as decreased SUVA254) and mean molecular weight DOM (expressed as increased E2:E3 ratios) with time since fire were associated with high-severity fires. Mixed-effect models also indicated that site-level characteristics played a role in solute responses. Aliphatic structures dominated streamwater DOM composition across fire-impacted catchments, but neither fire severity nor time since fire was a significant predictor of the proportion of aliphatic structures in streamwater DOM. North aspect exhibited the highest concentrations of Ca2+, K+, and Mg2+, whereas the north-northwest aspect exhibited the highest concentrations of Cl− and SO42+. We also observed elevated Ca2+, K+, and Mg2+ in burned (but not reference) stream reaches with pool-riffle versus step-pool bed morphology.ConclusionsTaken together, our findings suggest that the response of stream chemistry to wildfires in the Sierra Nevada, California, can persist for years, varying with both fire severity and site-specific characteristics. These impacts may have important implications for biogeochemical cycles and productivity in aquatic ecosystems in fire-adapted landscapes.

Natural Land Cover in Agricultural Catchments Alters Flood Effects on DOM Composition and Decreases Nutrient Levels in Streams

A shift in natural hydrologic patterns, such as increases in the frequency, and changes in the magnitude of flood events are expected with climate change. A better understanding of how land use and hydrological patterns interact to affect solute levels in aquatic systems is needed so we can better navigate expected climatic changes. Here we analyzed spatiotemporal event-based data from 21 predominantly agricultural catchments with varying contributions of natural land cover. We studied the effect of hydrological events on stream dissolved phosphorus and nitrogen concentrations and dissolved organic matter (DOM) composition and bioavailability over 4 years. Our results suggest that flow regime and flood condition control stream DOM composition, nitrogen and phosphorus dynamics, modulated by seasonal processes and land use properties, like soil organic carbon content. Although higher flows generally increased solute concentrations as well as the fraction of terrestrial, humic-like DOM, this pattern was highly dependent on the catchment land use and event timing. General additive models indicated a threshold of about 30–40% natural land cover, below which DOC and nutrients showed a positive relationship with discharge, but when more than 30–40% natural features (for example, wetlands, woodlots and grasslands) were present in the catchments, this shifted to a negative relationship. This suggests that in agricultural landscapes, the presence of natural land cover is important as it can decrease solute concentrations in streams and may act as a buffer, mitigating the effect of floods on DOM and nutrient export rates.

New molecular evidence for surface and sub-surface soil erosion controls on the composition of stream DOM during storm events

Abstract. Storm events are responsible for more than 60 % of the export of dissolved organic matter (DOM) from headwater catchments due to an increase in both the discharge and concentration. The latter was attributed to changing water pathways inducing the mobilization of DOM from the surface soil horizons. Recent molecular investigations have challenged this view and hypothesized (i) a contribution of an in-stream partition of organic matter (OM) between eroded particles and the dissolved fraction and (ii) the modification of the composition of soil DOM during storm events. To investigate these assumptions, soil solutions in the macropores, surface runoff and stream outlet were sampled at high frequency during three storm events in the Kervidy–Naizin catchment, part of the French critical zone observatory AgrHyS. The molecular composition of the DOM was analysed by thermally assisted hydrolysis and methylation (THM) with tetramethylammonium hydroxide (TMAH) coupled to a gas chromatograph and a quadrupole mass spectrometer. These analyses highlighted a modification of the DOM composition in soil solution controlled by the water-table dynamic and pre-event hydrological conditions. These findings fit with the mechanism of colloidal and particulate destabilization in the soil macroporosity. The different behaviour observed for lignins, carbohydrates and fatty acids highlights a potential chemical segregation based on their hydrophobicity. The composition of surface runoff DOM is similar to the DOM composition in soil solution and could be generated by the same mechanism. The DOM composition in both soil solution and surface runoff corresponds to the stream DOM composition observed during storm events. On the basis of these results, modifications of the stream DOM composition during storm events seem to be due to surface and sub-surface soil erosion rather than in-stream production.

The relative influence of land cover, hydrology, and in‐stream processing on the composition of dissolved organic matter in boreal streams

AbstractLow‐order boreal streams are particularly sensitive interfaces where dissolved organic matter (DOM) is transported from soils to inland waters. Disentangling the relative influence of key environmental factors suspected to influence stream water DOM composition is highly relevant to predicting the reactivity and fate of terrestrial DOM entering inland waters. Here we examined changes to DOM composition using absorbance and fluorescence, from 17 boreal streams ranging from first to fourth orders, over 14 months, including the rarely studied winter season, and two snowmelt periods (n = 836). We also analyzed soil pore water samples from three forest soil lysimeters to a depth of 70 cm (n = 60). Of five identified fluorescing parallel factor analysis components, two (C4 and C5) expressed a clear mire wetland or forest signature, providing distinct molecular markers of dominant land cover. In fact, land cover alone explained 49% of the variability in DOM composition. In contrast, seasonal fluctuations in hydrology only contributed to minor shifts (8%) in the composition of stream water DOM, while in‐stream transformations to DOM composition were undetectable. These findings suggest that low‐order boreal streams act as a passive pipe, since in‐stream processing of DOM is restricted by short water residence times (6 h to 2 days). In addition, we demonstrated the sensitivity of optical approaches to distinguish between key terrestrial sources of DOM in the boreal landscape. By distinguishing the proportional leverage of key environmental controls on headwater stream DOM composition, we are better equipped to predict where and when key DOM transformations occur in the aquatic conduit.

Sources of dissolved organic matter during storm and inter-storm conditions in a lowland headwater catchment: constraints from high-frequency molecular data

Abstract. The transfer of dissolved organic matter (DOM) at soil–river interfaces controls the biogeochemistry of micropollutants and the equilibrium between continental and oceanic C reservoirs. Understanding the mechanisms controlling this transfer is fundamental to ecology and geochemistry. DOM delivery to streams during storms is assumed to come from the flushing of preexisting soil DOM reservoirs mobilized by the modification of water flow paths. We tested this hypothesis by investigating the evolution of the composition of stream DOM during inter-storm conditions and five storm events monitored with high-frequency sampling. The composition of DOM was analyzed using thermally assisted hydrolysis and methylation (THM) with tetramethylammonium hydroxide (TMAH) coupled to a gas chromatograph and mass spectrometer. In inter-storm conditions, stream DOM is derived from the flushing of soil DOM, while during storm events, the modification of the distribution of chemical biomarkers allows the identification of three additional mechanisms. The first one corresponds to the destabilization of microbial biofilms due to the increase in water velocity, resulting in the fleeting export of a microbial pool. The second mechanism corresponds to the erosion of soils and river banks, leading to a partition of organic matter between particulate and dissolved phases. The third mechanism is linked to the increase in water velocity in soils that could induce the erosion of macropore walls, leading to an in-soil partition between soil microparticles and dissolved phase. The contribution of this in-soil erosive process would be linked to the magnitude of the hydraulic gradient following the rise of the water table and could persist after the recession, which could explain why the return to inter-storm composition of DOM does not follow the same temporal scheme as the discharge. These results are the most important factors in understanding the transfer of nutrients and micropollutants at the soil–river interfaces during the hot moments that are storm events.

Qualitative differences in headwater stream dissolved organic matter and riparian water-extractable soil organic matter under four different vegetation types along an altitudinal gradient in the Wuyi Mountains of China

Terrestrial organic matter plays an important role in determining the chemical composition of headwater stream dissolved organic matter (DOM). We used excitation–emission matrix fluorescence spectroscopy (EEMs) and parallel factor analysis (PARAFAC) to detect the chemical composition of headwater stream DOM and riparian water-extractable soil organic matter (WSOM) among four different vegetation types, including evergreen broadleaf forest (EBF), coniferous forest (CF), subalpine dwarf forest (SDF) and alpine meadow (AM), along an altitudinal gradient (690–2060m above sea level) in the Wuyi Mountains of subtropical China. Fluorescence index (FI) and biological/autochthonous index (BIX) values indicated that stream DOM was from terrestrial sources. Despite this link to terrestrial sources, the composition of stream DOM differed remarkably from that of riparian WSOM. Humic-like PARAFAC component C2 was observed in headwater streams, but not in riparian WSOM. Conversely, humic-like C3 compound was identified in riparian WSOM, but not in headwater streams. The relative abundance of humic-like C1 and C2 compounds did not differ among the headwater streams under four different vegetation types across the altitudinal gradient. In contrast, the relative abundance of C1 compounds in riparian WSOM was substantially higher in EBF than in other vegetation types. The relative abundance of tryptophan-like C4 compounds decreased from 26.47–54.26% in riparian WSOM to 5.71–14.24% in headwater streams. The relative abundance of tryptophan-like C4 compounds was higher in AM and EBF streams than in SDF and CF streams, and decreased in the order of AM>CF>SDF>EBF in the riparian WSOM. Our data suggest that variation in chemical composition of WSOM among four different vegetation types along an altitudinal gradient in the Wuyi Mountains does not induce corresponding changes in the chemical composition of stream DOM. Biogeochemical processes of plant-litter input directly into streams and bio-degradation and selective adsorption of DOM along a generalized hydrologic flow path from soil to stream may cause these qualitative differences between stream DOM and riparian WSOM composition across four different vegetation types along an altitudinal gradient in the Wuyi Mountains.

Evaluation of forest disturbance legacy effects on dissolved organic matter characteristics in streams at the Hubbard Brook Experimental Forest, New Hampshire

Dissolved organic matter (DOM) source and composition are critical drivers of its reactivity, impact microbial food webs and influence ecosystem functions. It is believed that DOM composition and abundance represent an integrated signal derived from the surrounding watershed. Recent studies have shown that land-use may have a long-term effect on DOM composition. Methods for characterizing DOM, such as those that measure the optical properties and size of the molecules, are increasingly recognized as valuable tools for assessing DOM sources, cycling, and reactivity. In this study we measured DOM optical properties and molecular weight determinations to evaluate whether the legacy of forest disturbance alters the amount and composition of stream DOM. Differences in DOM quantity and composition due to vegetation type and to a greater extent, wetland influence, were more pronounced than effects due to disturbance. Our results suggest that excitation-emission matrix fluorescence with parallel factor analysis is a more sensitive metric of disturbance than the other methods evaluated. Analyses showed that streams draining watersheds that have been clearcut had lower dissolved organic carbon (DOC) concentrations and higher microbially-derived and protein-like fluorescence features compared to reference streams. DOM optical properties in a watershed amended with calcium, were not significantly different than reference watersheds, but had higher concentrations of DOC. Collectively these results improve our understanding of how the legacy of forest disturbances and natural landscape characteristics affect the quantity and chemical composition of DOM in headwater streams, having implications for stream water quality and carbon cycling.

Dissolved organic matter quality and bioavailability changes across an urbanization gradient in headwater streams.

Landscape urbanization broadly alters watersheds and stream ecosystems, yet the impact of nonpoint source urban inputs on the quantity, quality, and ultimate fate of dissolved organic matter (DOM) is poorly understood. We assessed DOM quality and microbial bioavailability in eight first-order Coastal Plain headwater streams along a gradient of urbanization (i.e., percent watershed impervious cover); none of the streams had point source discharges. DOM quality was measured using fluorescence excitation-emission matrices (EEMs) coupled with parallel factor analysis (PARAFAC). Bioavailability was assessed using biodegradable dissolved organic carbon (BDOC) incubations. Results showed that watershed impervious cover was significantly related to stream DOM composition: increasing impervious cover was associated with decreased amounts of natural humic-like DOM and enriched amounts of anthropogenic fulvic acid-like and protein-like DOM. Microbial bioavailability of DOM was greater in urbanized streams during spring and summer, and was related to decreasing proportions of humic-like DOM and increasing proportions of protein-like DOM. Increased bioavailability was associated with elevated extracellular enzyme activity of the initial microbial community supplied to samples during BDOC incubations. These findings indicate that changes in stream DOM quality due to watershed urbanization may impact stream ecosystem metabolism and ultimately the fate of organic carbon transported through fluvial systems.

Allochthonous dissolved organic matter controls bacterial carbon production in old-growth and clearfelled headwater streams

AbstractWe investigated how the source and composition of stream dissolved organic matter (DOM) influenced rates of benthic bacterial C production (BCP) in 20 forested, headwater streams in southern Tasmania. We also assessed whether the source and composition of stream DOM was influenced by clearfell forest harvesting (1–19 y after harvest). Stream DOM was dominated by humic- and fulvic-like fluorescence (86.3–95.5%) as measured by parallel factor (PARAFAC) analysis of DOM fluorescence. Several reach-scale environmental variables showed significant positive (leaf-area index, sediment total N, organic C) or negative (stream temperature) linear relationships with BCP. However, an increasing contribution of terrestrial DOM, as measured by a decreasing fluorescence index (FI), was the strongest variable driving in situ benthic BCP (R2 = 0.38, p = 0.004, n = 20). Forest harvesting did not significantly affect DOM source despite the major reach-scale disturbance that clearfell forestry represents. Nevert...

Mineralisation of dissolved organic matter by heterotrophic stream biofilm communities in a large boreal catchment

Summary Boreal headwater streams typically receive large contributions of dissolved organic matter (DOM) from wetland and forest soils and can exhibit high CO2 effluxes, but little is known about how stream biofilm respiration of DOM responds to carbon and nutrient substrates in boreal catchments. The aim of this study was to assess (i) the response of heterotrophic biofilms to increased availability of labile carbon (C), nitrogen (N) and phosphorus (P), and (ii) how this response varies across sites differing in the source of catchment organic matter. Normalised biofilm respiration rates (RBiofilm) of dissolved organic carbon (DOC) of DOM sources derived from wetland, deciduous forest and ponds were up to 10 times greater than for coniferous forest DOM. Experimental additions of bog‐ and pond‐derived DOM stimulated RBiofilm in coniferous forest sites (1.5–2.5 times), adding further weight to the evidence for differing responses to stream DOM sources in this boreal catchment. Mineralisation of added glucose by the biofilms was only increased by added N and P, coinciding with a reduction in mineralisation of the extant stream DOM. These findings suggest that increases in labile C, N and P can reduce biofilm mineralisation of stream dissolved organic nitrogen and phosphorus, perhaps due to the greater energy required to access the more complex DOM. In 15 of 19 experiments, the addition of glucose had no effect on biofilm mineralisation of stream DOC (RDOC). In the presence of added N and P, however, RDOC decreased by 30 to 38% with the addition of glucose at sites with the most active biofilms in summer. When glucose was added alone, conversely, RDOC was stimulated (40–50%) at these same sites in autumn, when biofilm respiration was lower and DOM more aromatic and less carbohydrate‐rich. Combined, these results suggest labile C sources, such as algal exudates, may (i) compete as a source of energy and/or stimulate the incorporation rather than mineralisation of the more N‐ and P‐rich stream DOM or (ii) stimulate the mineralisation of stream DOM, potentially depending upon nutrient availability and the composition of stream DOM.

Effects of Watershed History on Dissolved Organic Matter Characteristics in Headwater Streams

Dissolved organic matter (DOM) is recognized as a major component in the global carbon cycle and is an important driver in aquatic ecosystem function. Climate, land use, and forest cover changes all impact stream DOM and alter biogeochemical cycles in terrestrial environments. We determined the temporal variation in DOM quantity and quality in headwater streams at a reference watershed (REF), a watershed clear-cut 30 years ago (CC), and a watershed converted to a white pine plantation 50 years ago (WP) at the US Forest Service, Coweeta Hydrologic Laboratory, in the Nantahala Mountains of western North Carolina, USA. Average stream dissolved organic carbon (DOC) concentrations in CC or WP were 60 and 80% of those in REF, respectively. Stream DOM composition showed that the difference was mainly due to changes in humic-like components in chromophoric DOM. In addition, excitation–emission matrix fluorescence data with parallel factor analysis indicate that although the concentration of protein-like components did not differ significantly among watersheds, their relative abundance showed an enrichment in CC and WP compared to REF. The ratio of humic acid-type to fulvic acid-type components was highest and lowest at REF and WP, respectively. Our data suggest that forest ecosystem disturbance history affects the DOM quantity and quality in headwater streams over decades as a result of changes in watershed soil organic matter characteristics due to differences in organic matter inputs.

Linking soils and streams: Sources and chemistry of dissolved organic matter in a small coastal watershed

To understand the hydrologic and biogeochemical controls on the age and recalcitrance of dissolved organic matter (DOM) found in stream waters, we combined hydrometric monitoring along a topographic gradient from ridge to channel with isotopic (13C and 14C) and spectroscopic (UV and 13C nuclear magnetic resonance) analyses of soil and stream water samples in a small coastal watershed in California. With increasing discharge, dissolved organic carbon concentrations increased from 2.2 to 10.9 mg C L−1, Δ14C values increased from −125 to +120‰, δ13C values decreased from −24 to −29‰, C:N ratios increased from 6.5 to 15.4, and specific UV adsorption increased from 1.4 to 3.8 L mg C−1 m−1. These changes in DOM composition are consistent with a shift in source from old and recalcitrant soil organic matter (OM) sources found in deep soil horizons to young and relatively fresh OM sources found in the surface horizons. Results from this study suggest upland soils of the watershed become DOM production limited as indicated by a seasonal depletion and chemical shift in soil DOM, whereas highly productive soils in the hollow act as a near‐infinite DOM source. Hydrologic connectivity of this DOM‐rich riparian source region to the stream ultimately constrains DOM export, and the stream DOM composition reflects the combined influence of soil biogeochemical cycling of OM and hydrologic routing of water through the landscape.

Source‐ and substrate‐specific export of dissolved organic matter from permafrost‐dominated forested watershed in central Siberia

Terrestrial and aquatic dissolved organic matter (DOM) was characterized to trace the likely processes of DOM formation and stream export in a permafrost‐dominated watershed in central Siberia. Stream samples were collected in spring (May–June 2003) and summer (July–August 2003) at both low flow and stormflow. Dissolved organic matter was analyzed by pyrolysis/gas chromatography/mass spectrometry, and identified pyrolysis products were simultaneously analyzed for compound‐specific isotope ratios by isotope ratio mass spectrometry. Pyrograms of terrestrial and stream DOM contained a similar series of pyrolysis products, suggesting a terrestrial origin for DOM in the small stream draining our study catchment. However, despite the overall similarity of chemical composition of stream DOM at different seasons, we also observed distinct differences in isotopic fingerprint between seasons and hydrologic phases (stormflow versus low flow). This variation appears to be due to the changing origin of stream DOM from different soil layers and the catchment sources following permafrost thawing during the frost‐free period. In general, chemical and isotopic composition of stream DOM was similar to DOM produced in soils of colder north facing slopes (P < 0.01) with a shallow active layer. South facing slopes with deeper active layers produce little DOM that enters the stream, suggesting that DOM produced in the active layer is retained and stabilized in underlying, unfrozen mineral soils. Climate change that results in additional seasonal thawing of permafrost‐dominated landscapes will decrease the amount of DOM exported to riverine systems and change its chemical composition.

Landscape predictors of stream dissolved organic matter concentration and physicochemistry in a Lake Superior river watershed

We examined landscape predictors of dissolved organic matter (DOM) concentration, molecular weight (Mw), and molar absorptivity at 280 nm (ɛ280) in 60 streams from the Ontonagon River watershed in northern Michigan. During our sampling period (September 19–22, 2002), DOM concentration ranged from 4 to 35 mg C L−1 across streams. DOM Mw and ɛ280 also showed considerable variation among streams. Multiple factor regression showed that stream DOM concentrations were related to watershed area, mean watershed slope, and the percentage of watershed area in certain types of land cover (lake, total wetlands, emergent wetlands, and lowland conifer forests). Streams with higher DOM concentration also had higher DOM Mw and molar absorptivity. Moreover, DOM Mw and ɛ280 were negatively related to the % lake and positively related to the % total wetlands in the watershed. In general, landscape variables explained more among stream variation in DOM concentration than in DOM Mw or ɛ280 in this watershed. It thus appears that the many biogeochemical processes controlling DOM input, transportation, and degradation weaken relationships between stream DOM composition and terrestrial organic matter dynamics in this relatively large watershed. Our results indicate that the total proportion of wetlands alone may be inadequate to predict DOM concentration or physicochemistry in streams flowing from large watersheds of variable morphology and land cover composition.