Hydrological isolation affected the chemo-diversity of dissolved organic matter in a large river-connected lake (Poyang Lake, China)

The transportation processes during aquatic systems regulate the ultimate chemistry of dissolved organic matter (DOM), and in recent years, climate changes and human activities have altered the hydrological patterns of many rivers and lakes, which generated some severe issues, such as hydrological isolation. However, how hydrological isolation affects variations of DOM chemistry in large lake systems is still poorly understood. Here, optical properties and molecular compositions of DOM samples derived from a large river-connected lake (Poyang Lake, China) and its nearby seasonal sub-lakes (formed by hydrological isolation) were characterized using Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS) and ultraviolet-visible (UV-Vis) spectroscopy. The results revealed more abundance of organic matter in sub-lakes than that in the main lake according to high dissolved organic carbon (DOC) concentrations and absorption coefficients (a254 and a280). Large proportions of CHOS formulas were identified by FT ICR MS in sub-lakes DOM, which were produced through Kraft reactions (sulfide/bisulfide + lignin CHO -> CHOS) in the interface of sediment/water, and greatly contributed to aliphatic compounds. In addition, obvious variations of compounds (such as polyphenols, highly unsaturated and aliphatic compounds) and lability of DOM were observed between sub-lakes and main lakes, which were mainly caused by the different degradation pathways of DOM (photodegradation in sub-lakes while biodegradation in the main lake). Our results demonstrated that hydro-logical isolation has significant impacts on DOM chemistry, and provides an improved understanding of the DOM bio-geochemistry process in Poyang Lake and supports the management of the large lake systems.