Temporal changes in hydrochemistry and DOM characteristics of the Brahmaputra River: implication to the seasonality of water quality

Seasonal and diurnal changes of pCO2 in the lower Brahmaputra River, Bangladesh

Monthly 24-h real-time high-resolution monitoring was conducted for 1 year to investigate the carbon dioxide (CO2) dynamics in the lower Brahmaputra River. The partial pressure of CO2 (pCO2) was observed between 136 and 1213 µatm. The pCO2 was almost 1.5 times higher during the wet season (May-October) than during the dry season (November-April). Diurnal variation in pCO2 was pronounced during the dry season. In contrast, no clear diurnal pattern was observed during the wet season. The combined measurements of O2-CO2 in river water provide insights into net chemical and microbial activity. In January and March, there was a bidirectional exchange of CO2, occurring from air to water (sink) and from water to air (source). However, in April, the CO2 exchange was unidirectional, taking place solely from air to water, whereas in all other months, it occurred from water to air. The carbon dioxide flux (FCO2) in the river ranged from - 31.12 to 137.74 mmol m-2 day-1, with an average of 25.26 mmol m-2 day-1 during the dry season and 78.56 mmol m-2 day-1 in the wet season. This flux significantly contributes to the regional net carbon budget of the world's largest delta.

The deciphering of microplastics-derived fluorescent dissolved organic matter in urban lakes, canals, and rivers using parallel factor analysis modeling and mimic experiment

The aim of the study is to investigate the leaching of fluorescent dissolved organic matter (fDOM) from microplastics. In addition, this study identifies the connection between fDOM and microplastics in the aquatic environment. Three-dimensional excitation-emission matrix identified five fluorophores, that is, peak A, M, T, Tuv, and Wuv, and the parallel factor analysis modeling identified five components, that is, tryptophan-like, p-hydroxy acetophenone, humic acid (C-like), detergent-like, and fulvic acid (M-like) in the urban surface water. Mimic experiments using commonly used synthetic plastic (like microplastics) in Mili-Q water under solar radiation and dark environments demonstrate the release of fDOM from plastic. Two fluorophore peaks were observed at Ex/Em = 250/302 nm and Ex/Em = 260/333 nm for the expanded polystyrene plastic polymer and one fluorophore peak at Ex/Em = 260/333 nm for the low-density polyethylene. Fluorophore and component intensity exhibited notable associations with microplastics in the aquatic environment. These findings indicated that the characteristics and dynamics of fDOM in urban surface water are influenced by microplastics. PRACTITIONER POINTS: Fluorescent dissolved organic matters were identified in urban surface waters. Expanded polystyrene (EPS) had shown two fluorophores at Em/Ex = 250/302 and Em/Ex = 260/333. Low-density polyethylene (LDPE) had one fluorophore at Em/Ex = 260/333. Fluorophore and component intensity in the aquatic settings exhibited associations with microplastics.

Effects of acid mine drainage on photochemical and biological degradation of dissolved organic matter in karst river water

Dissolved organic matter (DOM) can be removed or transformed by photochemical and biological processes, producing the negative effect of transforming organic carbon into inorganic carbon, which plays a vital role in the karst carbon cycle. However, acid mine drainage (AMD) will affect this process, so the degradation of DOM in karst river water (KRW) needs to be studied in this context. In this study, to reveal the evolution processes of DOM under photochemical and biological conditions in AMD-impacted KRW, AMD and KRW were mixed in different ratios under conditions of visible light irradiation (VL), biodegradation (BD), ultraviolet irradiation (UV) and ultraviolet irradiation + biodegradation (UV+BD). The average DOC concentrations in samples after mixing AMD and KRW in different proportions decreased significantly (by 23%) in UV+BD, which was 1.2-1.4 times higher than under the other conditions and would lead to a significant release of inorganic carbon. Further analysis of the fluorescence parameters via parallel factor analysis (PARAFAC) revealed that the DOM fluorescence components in AMD comprised mainly protein-like substances derived from autochthonous components, while the DOM fluorescence components in KRW were mainly humic-like substances with both autochthonous and allochthonous sources. Therefore, AMD could promote both the photochemical and biological degradation of DOM in karst receiving streams, resulting in the conversion of DOC to inorganic carbon. The results showed that the synergistic effects of UV+BD and AMD accelerated the degradation of DOM and the release of inorganic carbon in KRW, thus affecting the stability of the karst carbon cycle.

Distribution, characteristics and fate of fluorescent dissolved organic matter (FDOM) in the Bay of Bengal

The Bay of Bengal (BoB) is the largest sink to retain discharges from major rivers and the Sundarbans Mangrove Forest in Bangladesh and upholds significant ecological and resource diversity. This study aims to characterize, and identify sources, spatial dynamics, and the fate of the principal ecological web driver that is fluorescent dissolved organic matter (FDOM) in the BoB using advanced techniques of excitation-emission matrix (EEM) fluorescence spectroscopy and multivariate parallel factor (PARAFAC) analyses. The identified four protein-, two humic- and one detergent-like FDOM components mostly showed higher abundance in the shallow water than deep unlike a protein-like component. Such exceptional protein-like component was identified to form colloidal structure under elevated salinity in deep water. Autochthonous humic-like FDOM originated from primary production and water temperature counteracted microbial polymerization in shallow and deep water, respectively. The annual mass deposition indicated the influx of anthropogenic pollutants from both terrestrial and internal marine systems.

Differential responses of hydrochemical factors and LULC changes on the spatial and temporal hydrogeochemistry of the eco-sensitive Baraila wetland, Bihar, India

Freshwater scarcity, deterioration and associated water management remain to be one of the most challenging aspects of high-population density economies especially in subtropical/tropical regions. The present research deals with hydrogeochemical analysis of the eco-sensitive Baraila wetland and possible sources of anthropogenic pollution. The hydrochemical characterization was performed in GIS environment, considering meteorologically induced parameters and spatial variability. Temporal variations were assessed through different seasons, namely pre-monsoon, monsoon and post-monsoon with the help of multivariate statistics. The changes in water depth across the seasons showed significant hydrochemical variations in the vertical profile of the wetland apart from thermal demarcation. Seasonal variations in the hydrogeochemistry were induced by multiple physicochemical parameters, geochemical processes, geomorphology of the surrounding area and land use and land cover (LULC) changes. The pre-post flooding changes revealed that aquatic vegetation was increased by 15.36% whereas a major decrease in water bodies (- 73.2%) occurred. Dissolved oxygen (DO), pH, temperature and carbonates are fundamental towards establishing wetland's water chemistry. The water type is primarily of Ca-HCO3 type, mostly derived from rock-water interactions and cation exchange processes. Irrigational quality of water was assessed through multiple indices (sodium adsorption ratio (SAR), %Na, residual sodium carbonate (RSC), magnesium hazard (MH) and total hardness (TH)) and plots. The heavy aquatic vegetation abundance and eutrophication because of agricultural run-off is currently the major issue with the Baraila wetland and may be playing a simultaneous role in regulating the water chemistry to a large extent apart from other geochemical processes. The hydrogeochemical interactions between sediment and overlying water have created distinct effects on biota and land use/land cover changes. Their role in the landscape is prominent in this respect and may be utilized for environmental management, eco-tourism and employment boost. Serious lack of hydrogeochemical studies in this important floodplain wetland and its rapid deterioration deems it necessary to focus on the comprehensive research and wetland management options for its conservation and sustainable usage in future.

Biogeochemistry of the dissolved organic matter (DOM) in the estuarine rivers of Bangladesh-Sundarbans under different anthropogenic influences

The Bangladesh-Sundarbans is the Outstanding Universal Value (OUV) articulated by UNESCO, is under different anthropogenic stress. The present study focused on the status of estuarine biogeochemistry of the dissolved organic matter (DOM) of the Bangladesh-Sundarbans using different optical methods. Four fluorophores: Peak A (230-265/408-488 nm), Peak M (290/414 nm), Peak C (365/488 nm), and Peak W (320/410 nm), and three fluorescent dissolved organic matter (fDOM) components (two humic-like, one detergent-like) were identified in the Sundarban mangrove Rivers by Excitation-Emission Matrix (EEM) and Parallel Factor (PARAFAC) analyses. Among the three components, the terrestrial-derived humic-like Component had a high intensity in five samples among six in the Bangladesh-Sundarbans. The total fluorescent intensity and calculated dissolved organic carbon (DOC) concentration were maximum in Harbaria and minimum in Kotka and Dublar char, respectively. Synchronous fluorescence spectroscopy (SFS) identified protein-like component besides humic-like DOM. The optical indices described that natural fDOM components were from terrestrial sources, were matured, and autochthonous fDOM production was low. The DOM components were relatively lower in molecular size and aromaticity in Harbaria. However, water samples in Harbaria contained organometallic compounds that had much absorbance at 254 nm wavelength. DOM components had low energy and more π-conjugated molecules in structure in the Dublar char and Kotka. Components in Dublar char had comparatively higher molecular size and weight than other sampling stations. The Harbaria and Mongla port contained more hydrophobic and less polar substances than other stations. This study will firmly add diversified notions to future research regarding mangrove forest.

The Distribution of DOM in the Wanggang River Flowing into the East China Sea

Dissolved organic matter (DOM) is a central component in the biogeochemical cycles of marine and terrestrial carbon pools, and its structural features greatly impact the function and behavior of ecosystems. In this study, the Wanggang River, which is a seagoing river that passes through Yancheng City, was selected as the research object. Three-dimensional (3D) fluorescence spectral data and UV−visible spectral data were used for component identification and source analysis of DOM based on the PARAFAC model. The results showed that the DOM content of the Wanggang River during the dry season was significantly higher than during the wet season; the DOM content increased gradually from the upper to lower reaches; the proportion of terrigenous components was higher during the wet season than during the dry. UV−Vis spectral data a280 and a355 indicated that the relative concentrations of protein-like components in the DOM of the Wanggang River were higher than those of humic-like components, and the ratio of aromatic substances in the DOM of the Wanggang River water was higher during the wet season. The DOM in the Wanggang River was dominated by protein-like components (>60%), and the protein-like components were dominated by tryptophan proteins (>40%). This study showed that the temporal and spatial distributions of DOM in rivers can be accurately determined using 3D fluorescence spectroscopy combined with the PARAFAC model. This provides useful insight into the biogeochemical process of DOM in rivers of coastal areas.