River input and algae growth together determine the composition of dissolved organic matter in the large Lake Taihu.

Source, transformation, and preservation mechanisms of dissolved organic matter (DOM) remain elemental questions in contemporary marine and aquatic sciences and represent a missing link in models of global elemental cycles. Although the chemical character of DOM is central to its fate in the global carbon cycle, DOM characterizations in long‐term ecological research programs are rarely performed. We analyzed the variability in the quality of 134 DOM samples collected from 12 Long Term Ecological Research stations by quantification of organic carbon and nitrogen concentration in addition to analysis of UV‐visible absorbance and fluorescence spectra. The fluorescence spectra were further characterized by parallel factor analysis. There was a large range in both concentration and quality of the DOM, with the dissolved organic carbon (DOC) concentration ranging from less than 1 mgC/L to over 30 mgC/L. The ranges of specific UV absorbance and fluorescence parameters suggested significant variations in DOM composition within a specific study area, on both spatial and temporal scales. There was no correlation between DOC concentration and any DOM quality parameter, illustrating that comparing across biomes, large variations in DOM quality are not necessarily associated with corresponding large ranges in DOC concentrations. The data presented here emphasize that optical properties of DOM can be highly variable and controlled by different physical (e.g., hydrology), chemical (e.g., photoreactivity/redox conditions), and biological (e.g., primary productivity) processes, and as such can have important ecological consequences. This study demonstrates that relatively simple DOM absorbance and/or fluorescence measurements can be incorporated into long‐term ecological research and monitoring programs, resulting in advanced understanding of organic matter dynamics in aquatic ecosystems.

Distinct seasonal variations of dissolved organic matter across two large freshwater lakes in China: Lability profiles and predictive modeling

Water residence time is an important predictor of dissolved organic matter composition and drinking water treatability

Arctic lakes store, modify, and transport large quantities of carbon from terrestrial environments to the atmosphere; however, the spatial and temporal relationships between quantity and composition of dissolved organic matter (DOM) have not been well characterized across broad arctic regions. Moreover, most arctic lake DOM compositions have been examined during the ice‐free summer, whereas DOM cycling between the ice‐covered winter months and summer have not been addressed. To resolve these spatial and seasonal uncertainties in DOM cycling, we sampled a series of arctic lakes from the North Slope of Alaska across a latitudinal gradient in the winter and summer over 3 years. Samples were analyzed for dissolved organic carbon concentration and DOM composition was characterized using optical and fluorescence properties combined with molecular‐level analysis using Fourier transform‐ion cyclotron resonance mass spectrometry. Tundra lake DOM properties including aromaticity and molecular stoichiometries were similar to other northern high‐latitude lakes, but optical parameters related to aromaticity and molecular weight were greater in major arctic rivers and in coastal lakes in the North Slope region. DOM composition was highly seasonal, with ice exclusion concentrating microbially processed DOM in the winter water columns, potentially influencing DOM cycling the following summer. However, the greatest variations in DOM composition were related to lake depth and likely other physical features including morphology and bathymetry. As the Arctic warms, we expect changes in hydrology and ice cover to enhance under‐ice microbial DOM processing, early summer photodegradation, and ultimately carbon fluxes to the atmosphere after ice‐out.

<p>Forested watersheds are a major source of drinking water for more than two thirds of Canadians. However, drinking water security is increasingly pressured by the combination of higher demands resulting from population growth and industrial development and climate change-exacerbated landscape disturbances (e.g., wildfires, hurricanes). These may lead to deteriorated, more variable source water quality that can challenge treatment operations beyond their response capacities and have the potential to cause service disruptions. The character and concentration of dissolved organic matter (DOM), as well their shifts in response to seasonal and event-based changes in streamflow, make DOM a key driver of drinking water treatment infrastructure needs and operational challenges. As part of the <em>for</em>Water NSERC Network, which seeks to evaluate the impacts of pre-emptive forest management approaches on drinking water treatability in Canada, the objective of this study is to characterize differences in DOM concentrations and composition in headwater streams in different forested regions, including both undisturbed and disturbed catchments, and to evaluate the implications for drinking water treatability.</p><p>Our pan-Canadian study was conducted using existing long-term research sites, which span an area from 48.5° to 63° N between Canada’s east and west coasts, and represent six major forested ecozones. These ecozones exhibit significant differences in soils, vegetation, hydrological systems, and consequently surface water chemistry. At each research site, 2 to 6 headwater streams were sampled several times in 2019 and 2020 to characterize seasonal and spatial variations in water chemistry. Where relevant, both disturbed (harvested or burned) and undisturbed catchments were sampled.</p><p>The spatial and temporal variability in DOM characteristics, including the effects of disturbances, were evaluated, and the links between DOM characteristics and drinking water treatability were explored. Distinct regional differences in the concentrations of major ions, dissolved organic carbon and nutrients were observed. Variations in DOM composition, as assessed through UV-vis absorbance and excitation-emission matrix (EEM) fluorescence spectroscopy, Fourier-transform ion cyclotron mass spectrometry (FT-ICR-MS), and asymmetric flow field-flow fractionation (AF4), were also detected. To characterize drinking water treatability, relative implications to coagulant demand, membrane fouling, and distribution system stability were evaluated. The true disinfection by-product formation potential for trihalomethanes and haloacetic acids after complete oxidation resulting from chlorination was also assessed. Collectively, the results of this study underscore the importance of better understanding and anticipating natural variations in stream DOM as well as the impacts of landscape disturbance to ensure the uninterrupted supply of safe drinking water.</p>

Abstract. Iberian rivers are characterized by flow regimes with high seasonal flow variation. They also host one-fifth of Europe's reservoirs for hydropower generation, irrigation, or water supply needs, and thus many rivers in this region have heavily altered flow regimes. Such flow conditions also alter the natural dynamics of dissolved organic matter (DOM), which likely has implications for carbon cycling due to changed conditions for the transformation, transportation, production, and storage of carbon. Here we looked into the effects of flow alteration on the DOM regime, i.e. the seasonal variation in DOM concentration and composition, in 20 rivers belonging to two different natural (reference) flow regimes (i.e. Mediterranean and Atlantic) in northern Spain. To further investigate which flow regime components influence DOM properties, we linked the observed seasonal shifts in DOM composition to a range of hydrological indices. We found that Atlantic rivers with a natural flow regime tended to have lower annual mean dissolved organic carbon (DOC) concentrations than their altered equivalents; this flow alteration trend is weakly mirrored in Mediterranean rivers. We did not observe much difference in annual average DOM composition due to flow alteration in either Atlantic or Mediterranean rivers. However, the seasonal variation in DOM composition was lower in altered Atlantic rivers compared to natural ones. This flow alteration effect on the DOM regime was not as distinctive in Mediterranean rivers, which showed a higher diversity of DOM regimes across rivers. We linked the lack of seasonal variation in DOM composition in flow-altered rivers mainly to the prevention of transmission of upstream-sourced DOM from the reservoirs. It appears that in our study area, reservoirs mostly act as a temporally homogenizing buffer, averaging out naturally occurring shifts in DOM composition by transiently storing upstream-sourced carbon inputs and subjecting them to bio- and photo-degradation, thus sending relatively invariable amounts of DOM further downstream. This effect of dams on DOM regimes appears robust across both Atlantic and Mediterranean regimes despite some heterogeneity of dam types and purposes, with potentially important consequences for riverine carbon cycling.

How does anthropogenic activity influence the spatial distribution of dissolved organic matter in rivers of a typical basin located in the Loess Plateau, China

Comparison of organic matter (OM) pools in water, suspended particulate matter, and sediments in eutrophic Lake Taihu, China: Implication for dissolved OM tracking, assessment, and management

River ecosystems receive a large amount of organic matter, which will increase the production of greenhouse gases (GHGs), including carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Despite extensive research on the distribution of greenhouse gases and dissolved organic matter (DOM), little is known about the associations between greenhouse gases and DOM compositions. In this study, the distributions of GHGs (CO2, CH4 and N2O) and DOM in the overlying water of the main rivers around Taihu Lake were investigated. The results showed that the concentration of GHGs was positively correlated with dissolved organic carbon concentrations. Three-dimensional excitation-emission matrix fluorescence spectroscopy techniques were employed to identify the source of the DOM, which was related to protein-like and humic-like components. The DOM was a combination of terrigenous and endogenous origins. The GHGs (except CO2) were significantly associated with DOM composition. These results emphasize the importance of the relationship between GHGs (CO2, CH4 and N2O) and DOM compositions in river ecosystems.

Bioavailability of dissolved organic matter (DOM) derived from seaweed Gracilaria lemaneiformis meditated by microorganisms

The fate and reactivity of dissolved organic matter (DOM) in river networks is critical to understanding carbon cycling in inland water systems, and is highly regulated by physio-geographic factors and water residence time (WRT). In this study, we investigate the spatiotemporal variation of DOM concentration and composition in two SMRs in Taiwan with different landscapes and anthropogenic impacts. The WRT for these two rivers, the Keelung and Lanyang River, are around 34 and 23 hours, respectively. Dissolved organic carbon (DOC) concentration measurements and optical analyses (absorbance and fluorescence) were used to examine DOM quantity and quality along the river continuum. The comparative results showed that, along the SMR continuum, the DOC concentrations and optical indexes exhibited slight changes, with significant increases observed only at downstream sites influenced by human activities. Meanwhile, the higher biological index (BIX) and lower humification index (HIX) indicated an increase in autochthonous sources and a decrease in the degree of humic characters. In addition, we observed a positive correlation between WRT and DOC concentration variability, yet not significant for DOM compositions. When comparing the two rivers, the one with steeper topography and less human influence shows lower levels of DOC concentration and degree of humification. Overall, the SMRs seem to have lower DOC concentrations (0.26 - 1.65 mg-C L-1), lower HIX (0.28 - 0.76), and slightly higher BIX (0.8 - 1.9) on a global scale, which might be attributed to Taiwan's steep landscape and shorter water residence time, limiting soil organic carbon (SOC) production and in-stream processes rates. Through our investigation, DOC concentration and DOM composition across river networks will be better understood and potentially improve the assessment of the global carbon cycle.

Understanding how dissolved organic matter (DOM) responds to extreme climate events is critical for predicting carbon cycle feedbacks and ecosystem responses under climate change. However, the biogeochemical consequences of such events in large river‐influenced estuarine and coastal areas remain poorly understood. This study investigated seasonal variations in DOM composition off the Changjiang Estuary—a region strongly influenced by terrestrial inputs—during an extremely dry year. Field surveys were conducted in March, July, and October, using amino acid biomarkers and Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR MS) to characterize DOM composition. Seasonal analysis revealed a higher contribution of terrestrial (highly degraded) DOM in March, whereas marine (fresh, phytoplankton‐derived) DOM was more prominent in July, and intensified microbial processing of DOM in October. These trends were supported by the degradation index (2.4 ± 0.4, 3.8 ± 0.6, 3.1 ± 0.5) and the carboxyl‐rich alicyclic molecules abundance (58.3 ± 1.0%, 57.9 ± 1.8%, 61.8 ± 1.2%) in March, July, and October. Seaward transport showed a significant loss of compounds containing three or five oxygen atoms and one sulfur atom (O3S + O5S), indicating enhanced removal of terrestrial light‐sensitive DOM components. The abnormally low refractory fraction and high labile fraction in July suggested a rapid DOM turnover potentially induced by climate anomalies. These findings indicate that extreme drought conditions can significantly alter the DOM cycle and accelerate organic carbon turnover in estuarine and coastal areas. Our findings highlight the importance of incorporating episodic extreme climate events into future assessments of ocean carbon dynamics.

Spatial patterns in dissolved organic matter composition controlled by watershed characteristics in a coastal river network: The Neuse River Basin, USA

Variations of DOM quantity and compositions along WWTPs-river-lake continuum: Implications for watershed environmental management

Nutrient-driven dissolved organic matter composition and its regulation on carbon sequestration in aquatic regions: Insights from reservoirs, rivers, and offshore ecosystems.