Composition, distribution, and source of organic carbon in surface sediments of Erhai Lake, China.

Lake sediment is an important organic carbon (OC) sink. Nevertheless, few studies have been conducted on sediment organic carbon (SOC) in lakes, and the effects of environmental variables on SOC pools remain poorly understood. We combined physicochemical and spectroscopic analyses to investigate the composition, distribution, and source of OC in surface sediments of Erhai Lake, southwest China, and explored the relationships between environmental variables and its SOC pool. The SOC pool consists of relatively high proportions of labile organic carbon fractions, mainly from algal production, which are rapidly decomposed and exhibit high turnover rates. The relative content of humus carbon ranges from 13.5 % to 20.5 %, with fulvic acid carbon predominating (average 52.95 %), indicating weak humification and a relatively active humus carbon pool. The dissolved organic matter in water column and sediments of Erhai Lake is largely influenced by endogenous production, with a great contribution from phytoplankton. Surface sediments contained more protein-like components than overlying waters (80.0 % vs. 63.0 %), attributed mainly to abundant algal deposition and intense bacterial metabolism. Among environmental variables, sediment chlorophyll a showed the strongest relationship with the SOC pool, and was associated with rapid decomposition and promotion of the humification process, which supported the conclusion that algae had an important influence on the SOC pool. The SOC pool in the southern region of the lake is mainly contributed by algae, other microorganisms, and sewage, exhibiting a greater potential to release organic matters into the water column. The center and northern SOC pools show relatively stable characteristics and stronger OC sink capacity, mainly because of the input of terrestrial refractory organic matters from runoff. Our data shed light on the OC storage mechanisms in the surface sediments of Erhai Lake and provide theoretical bases for enhancing the OC sink of sediments in the lake.

Relative contributions of different sources to DOM in Erhai Lake as revealed by PLS-PM.

Dissolved organic matter (DOM) is a complex mixture of organic molecules that plays an important role in freshwater lake ecosystems. Excitation emission matrix-parallel factor analysis (EEM-PARAFAC) is an important tool for the identification and source analysis of DOM but is still inadequate for studying the differences and quantifying the contributions of different sources. Here, based on the maximum fluorescence intensities (Fmax) of the four fluorescent components (e.g., protein-like component C1, and the humic-like components C2, C3, C4) identified by EEM-PARAFAC, combined with large-scale data obtained from observations at Erhai Lake, we used partial least squares path modeling (PLS-PM) to improve the understanding of the migration and transformation mechanisms of DOM. The results showed that the phytoplankton and trophic state had greater impacts on DOM, while the impacts of sediment and inflow rivers were less significant. The results of the models that used nitrogen nutrients (N) and phosphorus nutrients (P) to separately indicate the trophic state suggested that the driving force of P on DOM was stronger than that of N in Erhai Lake. Among the four fluorescent components, the protein-like component with the largest relative proportion (41.98%) was mainly affected by phytoplankton, which was consistent with the autogenic characteristics obtained through spontaneous source index (BIX). The duality of the humic-like components was consistent with the duality of DOM through fluorescent index (FI). C3 had a higher sensitivity to the trophic state than the other components, and C2 received the greatest positive contribution from the rivers entering the lake. These results provide an improved insight into the influence of different sources on the behavior of DOM and demonstrate the potential of using PLS-PM to study the complex driving mechanism of aquatic biogeochemical parameters.

Adsorption and degradation of 14C-bisphenol A in a soil trench.

Bisphenol A (BPA) has caused widespread concern among scholars as a result of its estrogenic toxicity. It exists mainly in natural waters, sediments, and soil, as well as sewage and wastewater sludge. Considering that BPA is a common environmental pollutant that is removed along with chemical oxygen demand (COD), nitrogen, and phosphorus in drainage treatment systems, it is important to research the fate of BPA in sewage treatment systems. In this research, laboratory batch experiments on soil degradation and adsorption were conducted with 14C-BPA, aiming to discuss the transport and degradation characteristics of BPA in both simulated facilities and a soil trench. Based on the experimental results, the Freundlich model could be applied to fit the isothermal adsorption curve of the BPA in soil. A low mobility characteristic of BPA was discovered. The mineralization rate of BPA was fast and that of the reaction showed small fluctuations. After degradation, 21.3 and 17.7% of the BPA groups (the experimental group treated with ammonia oxidase (AMO) inhibitor and the control group) were converted into 14CO2, respectively. This indicates that the nitrification and degradation of BPA had a certain competitive relationship. Besides, nitrification did not significantly affect the soil residue of BPA. Through the soil trench test, the average removal rate of BPA in the soil trench was 85.5%. 14CO2 was discharged via the mineralization of BPA, accounting for 2.5% of the initial input. BPA easily accumulated in the bottom soil of the soil trench. BPA and its metabolites in the effluent accounted for 14.5% of the initial dosage. The residual extractable BPA and its metabolites in the soil accounted for 51.3%, and the remaining part of the unextractable residue represented 19.8% of the initial radioactive dosage.