Response of dissolved organic matter chemistry to flood control of a large river reservoir during an extreme storm event

Disentangling the assembly patterns and drivers of microbial communities during thermal stratification and mixed periods in a deep-water reservoir

Effect of periodic thermal stratification in deep-water reservoirs on aquatic ecosystems has been a research hotspot. Nevertheless, there is limited information on the response patterns of microbial communities to environmental changes under such specialized conditions. To fill this gap, samples were collected from a typical deep-water reservoir during the thermal stratification period (SP) and mixed period (MP). Three crucial questions were answered: 1) How microbial communities develop with stratified to mixed succession, 2) how the relative importance of stochastic and deterministic processes to microbial community assembly, shifted in two periods, and 3) how environmental variables drive microbial co-occurrence networks and functional group alteration. We used Illumina Miseq high-throughput sequencing to investigate the dynamics of the microbial community over two periods, constructed molecular ecological networks (MENs), and unraveled assembly processes based on null and neutral models. The results indicated that a total of 33.9 % and 27.7 % of bacterial taxa, and 23.1 % and 19.4 % of fungal taxa were enriched in the stratified and mixed periods, respectively. Nitrate, water temperature, and total phosphorus drove the variation of microbial community structure. During the thermal stratification period, stochastic processes (dispersal limitation) and deterministic processes (variable selection) dominated the assembly of bacterial and fungal communities, followed by a shift to stochastic processes dominated by dispersal limitation in two communities. The MENs results revealed that thermal stratification-induced environmental stresses increased the complexity of microbial networks but decreased its robustness, resulting in more vulnerable ecological networks. Therefore, this work provides critical ecological insights for the longevity and sustainability of water quality management in an artificially regulated engineered system.

The loss of dissolved organic matter from biological soil crust at various successional stages under rainfall of different intensities: Insights into the changes of molecular components at different rainfall stages

Biological soil crusts (BSCs) are typical covers in arid and semiarid regions. The dissolved organic matter (DOM) of BSCs can be transported to various aquatic ecosystems by rainfall-runoff processes. However, the spatiotemporal variation in quality and quantity of DOM in runoff remains unclear. Herein, four kinds of runoff plots covered by four successional stages of BSCs were set up on slopes, including bare runoff plot (BR), cyanobacteria crust covered runoff plot (CR), mixed crust covered runoff plot (MIR), and moss crust covered runoff plot (MOR). The quantity and quality of DOM in runoff during rainfall was investigated based on the stimulated rainfall experiments combined with optical spectroscopy and ultra-high resolution mass spectrometry analyses. The results showed that the DOM concentrations (i.e., 0.30 to 45.25 mg L-1) in runoff followed the pattern of MOR>MIR>CR>BR, and they were exponentially decreased with rainfall duration. The DOM loss rate of BR (8.26 to 11.64 %) was significantly greater than those of CR, MIR, and MOR (0.84 to 3.22 %). Highly unsaturated compounds (HUCs), unsaturated aliphatic compounds (UACs), saturated compounds (SCs), and peptide-like compounds (PLCs) were the dominated compounds of the water extractable DOM from the original soils. Thereinto, PLCs and UACs were more easily leached into runoff during rainfall. The relatively intensity of HUCs in runoff generally decreased with rainfall duration, while the relatively intensities of UACs, PLCs, and SCs slightly increased with rainfall duration. These findings suggested that the DOM loss rate was effectively decreased with the successional of BSCs during rainfall; meanwhile, some labile compounds (e.g., PLCs and UACs) were transported into various aquatic ecosystems by rainfall-runoff processes.

Characterization of dissolved organic matter in rivers impacted by acid mine drainage: Components and complexation with metals

Dissolved organic matter (DOM), a ubiquitous and active ingredient, is extensively involved in the transformation and migration of environmental pollutants in aquatic ecosystems. However, its chemical composition in acid mine drainage (AMD)-impacted rivers remains poorly characterized, hindering our understanding of its role in the biogeochemistry of key elements in contaminated fluvial environments. Here, we investigated the concentration of dissolved organic carbon (DOC) and spectroscopic and molecular characteristics of DOM in a headwater river contaminated with polymetallic mine-derived AMD in southern China. Terrestrial humic-like (C1) and typically groundwater-supplied aromatic protein/tyrosine-like (C2) substances which were partially from AMD, were identified as the predominant fluorescent components in the river water. Notably, tryptophan-like (C3) substances originating from tailings pond spills were only occasionally detected in the river. Although DOM biogeochemical transformations and degradation occurred in the lateral soil-water riparian interface and longitudinal in-stream transport processes, the molecular compositions identified by FT-ICR MS showed a core set of molecular formulae in the lignin/saturated compound/tannin region of the van Krevelen diagram of the water samples across the rivers. The complexation of DOM with typical metals in AMD was investigated using fluorescence quenching experiments. The results showed that the highest binding ability of Fe(III) to C2 followed by C1, with both detected in the experimental water samples. Mg(II) and Ca(II) strengthened the binding of DOM-Fe(III) when the ferric/DOM ratio was low, while Cu(II) weakened the binding of DOM-Fe(III) due to competition. Ca(II) inhibited the binding of Fe(III) to C1 but promoted the binding of the complex to C2 when both Cu(II) and Mg(II) were present. Since DOM-Fe(III) complexation was associated with the cotransport of AMD-derived metals/metalloids in diverse aqueous environments with multiple co-existing ions (typically Ca(II) input for remediation), our study on the composition of DOM and its complexation with metals can contribute to managing and remediating AMD-impacted rivers.

Enhanced Role of Streamflow Processes in the Evolutionary Trends of Dissolved Organic Carbon

Investigating dissolved organic carbon (DOC) dynamics and drivers in rivers enhances the understanding of carbon-environment linkages and support sustainability. Previous studies did not fully consider the dynamic nature of key drivers that influence the long-term changing trends in DOC concentration over time (the controlling factors and their roles in DOC trend can undergo alterations over time). We analyzed 42 years (1979-2018) of hydrometeorology, sulfate SO4, and DOC data from a 5.42 km2 watershed in central-southern Ontario, Canada. Our findings reveal a significant (p ≤ 0.01) overall increase in DOC concentrations, mainly due to the coevolution of SO4 and streamflow trends, especially the extreme flows. Over the 42-year period, the changing trend of streamflow (especially the extreme high or low flows) have significantly (p < 0.05) intensified their influence on DOC trends, increasing by an average of 30%. Conversely, the impact of SO4 has weakened, experiencing an average decrease of 32.6%. The upward trend in the annual average DOC concentration is attributed to the increasing number of maximum flow days within a year, while the decreasing trend in the number of minimum flow days has a contrasting effect. In other words, changes in maximum and minimum flow days have a counteracting effect on the DOC concentration trends. These results underscore the importance of considering the effects of altered streamflow processes on carbon cycle changes under evolving environmental conditions.

Dissolved organic matter composition and characteristics during extreme flood events in the Yangtze River Estuary

Understanding the molecular composition and fate of dissolved organic matter (DOM) during transport in estuaries is essential for gaining a comprehensive understanding of its role within the global biogeochemical cycle. In 2020, a catastrophic flood occurred in the Yangtze River basin. It is currently unknown whether differences in hydrologic conditions due to extreme flooding will significantly impact the estuarine to oceanic DOM cycle. We determined the DOM composition in the Yangtze River estuary (YRE) to the East China Sea by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) during the high discharge and the flood period (monthly average discharge was 1.2 times higher) on the same trajectory. Our study found that the composition of DOM is more diverse, and more DOM molecules were introduced to the YRE during the flood, especially in the freshwater end member. The result revealed that the DOM was significantly labile and unstable during the flood period. A total of 1840 unique molecular formulas were identified during the flood period, most of which were CHON, CHONS, and CHOS compounds, most likely resulting from anthropogenic inputs from upstream. Only 194 of these molecules were detected in the seawater end member after transporting to the sea, suggesting that the YRE served as a 'filter' of DOM. However, the flood enhances the transport of a group of terrigenous DOM, that is resistant to photodegradation and biodegradation. As a result, YRE experienced ~1.6 times higher terrigenous DOC flux than high discharge period. Considering the increased frequency of future floods, our study provides a preliminary basis for further research on how floods affect the composition and characteristics of estuarine DOM. With the help of the FT-ICR MS technique, we can now better understand the dynamic of DOM composition and characteristics in large river estuaries.

Humification levels of dissolved organic matter in the eastern plain lakes of China based on long-term satellite observations

Under the influence of intensive human activities and global climate change, the sources and compositions of dissolved organic matter (DOM) in the eastern plain lake (EPL) region in China have fluctuated sharply. It has been successfully proven that the humification index (HIX), which can be derived from three-dimensional excitation-emission matrix fluorescence spectroscopy, can be an effective proxy for the sources and compositions of DOM. Therefore, combined with remote sensing technology, the sources and compositions of DOM can be tracked on a large scale by associating the HIX with optically active components. Here, we proposed a novel HIX remote sensing retrieval (IRHIX) model suitable for Landsat series sensors based on the comprehensive analysis of the covariation mechanism between HIX and optically active components in different water types. The validation results showed that the model runs well on the independent validation dataset and the satellite-ground synchronous sampling dataset, with an uncertainty ranging from 30.85 % to 36.92 % (average ± standard deviation = 33.6 % ± 3.07 %). The image-derived HIX revealed substantial spatiotemporal variations in the sources and compositions of DOM in 474 lakes in the EPL during 1986-2021. Subsequently, we obtained three long-term change modes of the HIX trend, namely, significant decline, gentle change, and significant rise, accounting for 74.68 %, 17.09 %, and 8.23 % of the lake number, respectively. The driving factor analysis showed that human activities had the most extensive influence on the DOM humification level. In addition, we also found that the HIX increased slightly with increasing lake area (R2 = 0.07, P < 0.05) or significantly with decreasing trophic state (R2 = 0.83, P < 0.05). Our results provide a new exploration for the effective acquisition of long-term dynamic information about the sources and compositions of DOM in inland lakes and provide important support for lake water quality management and restoration.

Unraveling the photochemical reactivity of dissolved organic matter in the Yangtze river estuary: Integrating incubations with field observations

Dissolved organic matter (DOM) sustains a substantial part of the organic matter transported seaward in large estuaries, where photochemical reactions significantly influence its transformation and fate. Irradiation experiments can provide valuable information on the photochemical reactivity (photo-labile, photo-resistant, and photo-product) of molecules. However, previous research paid less attention to exploring the controls of the initial DOM chemistry to irradiation experiments and examining the applicability of their further integration with field research. Here, we conducted irradiation experiments for samples from the freshwater and seawater endmember of the Yangtze River Estuary (YRE), which receives organic matter transport from the largest river in China, the Yangtze River. Molecules that occurred before and after irradiation experiments were characterized by the Fourier transform ion cyclotron resonance mass spectrometry. Results show that both post-irradiation samples have the lower aromaticity degree and reduced oxidation state, while the freshwater endmember sample exhibits more dramatic changes, indicating the controls of parent molecules to the effect of irradiation experiments. Integrating with the "molecular matching" approach, we compared the molecules occurring in field samples with the classified molecules (photo-resistant, photo-labile, and photo-product) acquired from performed irradiation experiments and correlated the relative intensity of photochemical reactivity types with salinity. When applying results from different experiments to conduct "molecular matching", the photo-resistant and photo-labile relative intensity possess consistently positive and negative trends with increasing salinity, respectively. This suggests their reliability for molecular matching applications, while the inconsistent trends for the photo-product relative intensity with salinity suggest its uncertainty in assessing the photo-induced effects. Moreover, the molecular composition within the photochemical reactivity types in field samples also evolved along the salinity gradient and showed similar trends with the DOM changes after experimental irradiation. Despite various factors influencing estimations, it is revealed that a fraction of aromatic molecules and majority of carboxyl-rich alicyclic molecules considered with biologically persistent nature in the YRE freshwater zone are simultaneously not susceptible to photochemical transformation to potentially constitute a long-term marine carbon sink. This study emphasizes the importance and limitations of the combination of field research and laboratory-controlled experiments to provide a better understanding of the crucial role of photochemical reactions in affecting carbon cycling in large estuaries.