Marine Chemistry最新文献

This study investigated the redox speciation and mobility of V in the acid-extractable fraction of surface sediments from the Krka River estuary using an optimized IC-UV/Vis analytical method. The separation of V(IV) and V(V) redox species was done using anion-exchange based chromatographic method, while pseudo-total V concentrations were measured using HR ICP-MS analytical instrumentation. Extracted V concentrations from the sediment fraction (pH = 5, HCl) and determined pseudo-total V concentrations were used to calculate the Enrichment Factor (EF) and Risk Assessment Code (RAC), indicating potential anthropogenic influence and environmental risk. A simple PHREEQC model was developed to asses V speciation in the oxic bottom seawater layer simulating possible remobilization of the leached sediment phase. The results of the study show that minor fraction of V is present in the acid-extractable phase across the surface sediment of Krka River estuary. Higher V mobility is mostly observed at locations rich with clay minerals, terrigenous input, and carbonates. Anthropogenic influence was linked to higher enrichment but lower mobility, suggesting binding to less mobile sediment phases (reducible, organic and residual fractions). The predominance of reduced V(IV) species in the acid-extractable sediment fraction indicates a potentially low V toxicity risk in the sediments of Krka River estuary, even in cases of high potential remobilization of V. However, the model predicted complete oxidation of V(IV) to V(V) upon remobilization into the oxic bottom water layer. This highlights the complexity of V behavior in natural estuarine systems, where the toxicity risks of possible V remobilization still remain unclear. Results of this study demonstrate the need for the strengthening efforts in speciation of V in the mobile sediment phase to obtain a cohesive outlook on its potential toxicity and biogeochemical cycling.

Monthly water samples were collected from the lower Mississippi and Pearl Rivers between January 2009 and August 2011 to investigate the heterogeneity in the dynamic variations of dissolved organic carbon (DOC), colloidal organic carbon, chromophoric and fluorescence dissolved organic matter (CDOM and FDOM), PARAFAC-derived fluorescent components, and other optical properties including spectral slope, specific UV absorbance (SUVA), and fluorescence indices between the two contrasting river systems. The lower Mississippi River exhibits relatively lower concentrations of DOC (306 ± 41 μM C) and CDOM (27.9 ± 5.7 m⁻¹ at 254 nm), featuring lower aromaticity (indicated by SUVA₂₅₄) and apparent molecular weight (or higher spectral slope) with weak seasonal variability. The Pearl River, in contrast, has elevated levels of DOC (537 ± 212 μM C) and CDOM (66.4 ± 31.4 m⁻¹), characterized by higher aromaticity, higher molecular weight, and significant seasonality, primarily originating from soil-derived allochthonous sources. The abundance of the >1 kDa colloidal DOC was, on average, 58 ± 3 % of the bulk DOC in the lower Mississippi River and 68 ± 6 % in the lower Pearl River. The >1 kDa high-molecular weight DOM (HMW-DOM) consistently had lower spectral slope and biological index (BIX) values, but higher humification index (HIX) values compared to both bulk DOM and low-molecular-weight DOM (LMW-DOM) counterparts. These trends could be representative of other similar large and small rivers. Four PARAFAC-derived fluorescent components (three humic-like and one protein-like) were identified for both rivers. A positive correlation between discharge and terrestrial humic-like fluorescent components indicated their dominant allochthonous sources, while the protein-like component decreased with increasing discharge, consistent with its autochthonic source and a dilution effect during high flow seasons. The occurrence of large flood events during the sampling period contributed to large DOC pulses, with DOM of higher aromaticity and HMW-DOM. This has important implications for coastal ocean ecosystems, which are increasingly impacted by river flooding events under changing climate conditions. Our results also provide an improved understanding of DOM dynamics in two representative rivers and establish a baseline dataset for future studies to assess changes in sources and composition of DOM and their impacts on the coastal ocean in response to climate, hydrological, and anthropogenic influences.

Dissolved inorganic carbon (DIC) and particulate organic carbon (POC) dynamics in the world's estuaries have been studied extensively at monthly, seasonal, and annual time scales with particular focus on their concentrations and export fluxes to the coastal oceans. However, given the dynamic nature of the estuaries, the effect of tidal and diel cycles on the processes modulating DIC and POC dynamics remains obscure. To decipher the biogeochemical processes at tidal scale, DIC and POC concentrations and their carbon and nitrogen isotopic compositions were measured across the salinity gradient at every high and low tide for nine consecutive days (14–23 October 2019) in the Mahanadi estuary, a tropical estuary at the east coast of India. Showing contrasting differences across salinity gradient in DIC, POC and their isotopic compositions, DIC and POC were significantly different during high and low tide in the mixing zone only during spring duration. This showed the effect of spring-neap tidal cycle owing to water level fluctuations and mixing intensity in the estuarine mixing zone. Linear least-squares regression models indicated carbonate and/or silicate weathering by biogenic CO₂ to be the probable DIC source in the freshwater region of the estuary. Deviations of observed DIC concentrations and δ¹³C_DIC from the conservative mixing values suggested pronounced alteration of DIC source signature in the mixing zone. A process-based model approach aimed at delineating possible biogeochemical processes affecting DIC dynamics indicated calcite dissolution during low tide and calcite precipitation during high tide to be dominant processes in the mixing zone. Additionally, signatures of more than one simultaneous biogeochemical process modulating the DIC dynamics were also observed. POC pool in the mixing zone was largely influenced by its removal through rapid remineralization during both high and low tides. Graphical plots also showed that POC in the mixing zone and at the saline location was significantly affected by processes such as degradation, whereas it was only slightly affected in the freshwater region of the estuary. δ¹³C_POC, along with the C/N ratio of POM, indicated that C3 plants and/or their derived soil were the major source of POM in the freshwater, whereas the higher contribution of riverine POM and marine phytoplankton was observed in the mixing zone and saline location, respectively.