Heterogeneity of molecular-level and photochemical of dissolved organic matter derived from decomposing submerged macrophyte and algae.

Aquatic macrophytes and algae are the most important sources of autochthonous dissolved organic matter (DOM), and their transformation and reuse significantly affect aquatic ecosystem health. In this study, Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) was used to identify the molecular features between submerged macrophyte-derived DOM (SMDOM) and algae-derived DOM (ADOM). The photochemical heterogeneity between SMDOM and ADOM by UV254-irradiation and their molecular mechanism were also discussed. The results showed that the molecular abundance of SMDOM was dominated by lignin/CRAM-like structures, tannins, and concentrated aromatic structures (sum of 91.79%), while that of ADOM was dominated by lipids, proteins, and unsaturated hydrocarbons (sum of 60.30%). UV254-radiation resulted in a net reduction of tyrosine-like, tryptophan-like and terrestrial humic-like, and conversely a net production of marine humic-like. The light decay rate constants obtained by the multiple exponential function model fitting revealed that both tyrosine-like and tryptophan-like components of SMDOM could be rapidly and directly photodegraded, while the photodegradation of tryptophan-like in ADOM depended on the production of photosensitizers. The photo-refractory fractions of both SMDOM and ADOM were as follows: humic-like > tyrosine-like > tryptophan-like. Our results provide new insights into the fate of autochthonous DOM in aquatic ecosystems where "grass-algae" coexist or evolve.

Responses of dissolved organic matter (DOM) characteristics in eutrophic lake to water diversion from external watershed.

Eutrophication is an important water environment issue facing global lakes. Diversion of water from external watersheds into lakes is considered as effective in ameliorating eutrophication and reducing algal blooms. Nevertheless, the changes in lake water environment caused by external water diversion, especially the influence of water diversion on the characteristics of dissolved organic matters (DOM), are still poorly understood. We therefore used a combination of EEM-PARAFAC, Principal Component Analysis (PCA), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) to investigate the effects of water diversion from the Niulan River on DOM characteristics in Lake Dianchi. The results showed that the water diversion from the Niulan River significantly improved the water quality of Lake Dianchi, the concentrations of TN, TP, COD and Chla decreased rapidly, and the degree of humification of dissolved organic matter (DOM) increased, which was in sharp contrast with that of pre-implementation. Firstly, the diversion of water from the Niulan River mainly led to changes in the structure of pollution sources. The load of influent rivers and sewage treatment plants rich in lignin and tannins increased, and the input of terrestrial humus increased. Second, the improved water quality reduced algal enrichment and frequency of blooms, and reduced the release of lipid- and protein-riched algal-derived DOM. Finally, the hydraulic retention time of Lake Dianchi caused by water diversion was shortened, the hydrodynamic conditions were significantly improved, and the dissolved oxygen (DO) level gradually recovered, which played a positive role in improving the humification degree of DOM. Our findings provide new insights for exploring the improvement of eutrophic lake eco-environmental quality caused by water diversion projects.