Composition, Source, and Photodegradation Exposure Control Dissolved Organic Matter Biodegradability in a Freshwater Wetland Landscape

Freshwater wetlands are rich in dissolved organic matter (DOM) and are often considered sources of downstream riverine DOM. There is limited knowledge about the biodegradability of this DOM and thus the extent to which it could fuel downstream metabolism, but evidence varies and is often site-specific. Further, this material is often simply called terrestrial, eliding the specifics of its formation and transformation from plant and soil precursors to DOM-rich wetland water. Focusing on temperate depressional wetlands in Maryland (USA), we investigated the potential origins and biodegradability of wetland DOM and potential precursor materials from the surrounding terrestrial landscape using microcosm incubation experiments. We found that DOM in wetland surface water had very limited direct biodegradation potential, which was a function of its composition and not nutrient limitations or pH barriers to microbial growth. Photodegradation altered wetland surface water DOM composition and improved its biodegradability. Plant material from the wetland landscape produced highly biodegradable leachates, while water-exchangeable DOM from soils was more biodegradable than DOM in wetland surface water but less so than plant leachates. Finally, fluorescence characteristics were tied to biodegradability patterns and confirmed that bacteria fundamentally shape DOM optical properties and chemical composition. Our results suggest that while wetlands are rich sources of DOM to downstream waters, it may be poorly degradable leftovers from microbial activity in the wetland environment unless light exposure during transport renders it more reactive.