Rainstorm-induced organic matter pulses: A key driver of carbon emissions from inland waters

Numerous rivers and lakes in the monsoon climate zone are heavily influenced by frequent rainstorms that mobilize dissolved organic matter (DOM) from pristine or urbanized environments into downstream lakes. Of particular concern is the mobilization of DOM from anthropogenic effluents, which are commonly enriched in aliphatic compounds that can be easily degraded by microorganisms. Rapid degradation of highly biodegradable DOM, in turn, may cause significant depletion of dissolved oxygen in the water, which, by creating anoxic conditions at the bottom water-sediment interface, promotes microbial production of CO2 and CH4. Further investigations based on high-frequency monitoring and novel techniques such as ultra-high-resolution mass spectrometry and isotopic measurements, are needed to elucidate the processes and mechanisms by which pulsed aliphatic inputs impact lake carbon emissions.

Background and importance

Lakes and other inland waters are hotspots for transforming terrestrial organic matter as they receive, actively process, and transport up to 5.1 petagrams of carbon (PgC) annually1 despite covering less than 3.7% of the Earth’s non-glaciated land surface. Global estimates of lake carbon cycling have been revised several times, but recent ones suggest that carbon dioxide (CO2) fluxes from lakes and other inland waters to the atmosphere can reach up to 3.9 PgC year−1,1 while lake methane (CH4) emissions contribute about 0.4 Pg CH4 year−1, accounting for nearly half of the annual global CH4 emissions.2 Carbon emissions result from the transformation of organic matter in terrestrial and aquatic ecosystems. In aquatic ecosystems, organic matter mainly consists of dissolved organic carbon (DOC), which can constitute up to 90% of the total organic carbon pool in rivers and lakes, and it plays a crucial role in the carbon cycle of inland waters.3 The degradability of lake organic matter is largely determined by its source and chemical composition. Terrestrial organic matter, usually the primary contributor to the lake organic carbon pool, undergoes significant microbial and photochemical degradation after entering lakes (Figure 1). Freshwater lakes are typically in a state of CO2 and CH4 supersaturation, and mesocosm and laboratory experiments have shown that adding fresh DOC and organic matter may significantly increase CO2 and CH4 production,4 emphasizing that investigating the biogeochemical cycling of terrestrial organic matter is crucial for understanding the dynamics of lake carbon emissions.