Accumulation of Terrestrial Dissolved Organic Matter Potentially Enhances Dissolved Methane Levels in Eutrophic Lake Taihu, China

Inland waters play an important role for the storage of chromophoric dissolved organic matter (CDOM) and outgassing of methane (CH 4 ). However, to date, linkages between the optical dynamics of CDOM and dissolved CH 4 levels remain largely unknown. We used multi-year (2012−2014) seasonal data series collected from Lake Taihu and 51 connecting channels to investigate how CDOM optical dynamics may impact dissolved CH 4 levels in the lake. High dissolved CH 4 in the northwestern inflowing river mouths coincided with high underwater UV–vis light availability, dissolved organic carbon (DOC), chemical oxygen demand (COD), DOM aromaticity, terrestrial humic-rich fluorescence, in situ measured terrestrial CDOM, depleted dissolved oxygen (DO), stable isotopic δ 2 H, and δ 18 O compared with other lake regions. Our results further revealed positive relationships between dissolved CH 4 and CDOM absorption at 350 nm, i.e. a (350), COD, DOC, terrestrial humic-rich fluorophores, and DOM aromaticity, and negative relationships between dissolved CH 4 and DO, δ 2 H, and δ 18 O. The central lake samples showed a major contribution of terrestrial-sourced molecular formulas to the ultrahigh resolution mass spectrometry data, suggesting the presence of allochthonous DOM sources even here. We conclude that an elevated terrestrial CDOM input likely enhances dissolved CH 4 levels in Lake Taihu.