Human Activities Reshape Greenhouse Gas Emissions From Inland Waters.

Inland waters are significant sources of greenhouse gases (GHGs) in an increasingly human-dominated world, yet the mechanisms by which human activities reshape GHG emissions from these systems remain poorly understood. Here, we synthesized research from three human-dominated landscapes-agricultural, urban, and impounded river systems-to demonstrate that inland waters within these systems exhibit significantly higher GHG emissions compared to their natural or seminatural counterparts. This is particularly evident for CH4 and N2O emissions, which show median enhancement ratios of 2.0-10 and 2.4-13 across the systems, respectively. In contrast, CO2 emissions exhibit overall lower enhancement (median enhancement ratios of < 2.0-3.1), largely due to simultaneously increased photosynthetic uptake from aquatic eutrophication. These observations underscore a clear human footprint on aquatic GHG emissions and the underlying biogeochemical processes. The observed changes in GHG emissions are driven by increased inputs of sediments, carbon, and nutrients from human-disturbed landscapes, coupled with the expansion of aquatic anoxia resulting from increased aquatic metabolism, fine sediment deposition, and eutrophication. Beyond altering emission rates, human activities also modify the abundance and distribution of inland waters, potentially exerting substantial, yet unquantified, effects on landscape-scale GHG emissions. We highlight the importance of understanding these processes for accurately quantifying and mitigating the human footprint on aquatic GHG emissions. Future research and mitigation efforts should account for the variability and mechanisms discussed in this review to effectively address human-induced GHG emissions from inland waters.