Unraveling the Role of Anthropogenic and Natural Drivers in Shaping the Molecular Composition and Biolability of Dissolved Organic Matter in Non-pristine Lakes

Lakes receive and actively process terrestrial dissolved organic matter (DOM) and play an important role in the global carbon cycle. Urbanization results in elevated inputs of nonpoint-source DOM to headwater streams. Retention of water in lakes allows time for alteration and transformation of the chemical composition of DOM by microbes and UV radiation. Yet, it remains unclear how anthropogenic and natural drivers impact the composition and biolability of DOM in non-pristine lakes. We used optical spectroscopy, Fourier transform ion cyclotron mass spectrometry, stable isotopic measurements, and laboratory bioincubations to investigate the chemical composition and biolability of DOM across two large data sets of lakes associated with a large gradient of urbanization in lowland Eastern China, encompassing a total of 99 lakes. We found that increased urban land use, gross domestic products, and population density in the catchment were associated with an elevated trophic level index, higher chlorophyll-a, higher bacterial abundance, and a higher amount of organic carbon with proportionally higher contribution of aliphatic and peptide-like DOM fractions, which can be highly biolabile. Catchment areas, water depth, lake area: catchment area, gross primary productivity, delta O-18-H2O, and bacterial abundance, however, had comparatively little linkage with DOM composition and biolability. Urban land use is currently intensifying in many developing countries, and our results anticipate an increase in the level of biolabile aliphatic DOM from nonpoint sources and accelerated carbon cycling in lake ecosystems in such regions.

Automated summary

Urbanization leads to increased inputs of nonpoint-source dissolved organic matter to lakes, affecting the chemical composition and biolability of the organic matter. This study found that increased urban land use is associated with higher trophic level, chlorophyll-a concentrations, bacterial abundance, and organic carbon content in lakes, with a higher relative contribution of biolabile aliphatic and peptide-like fractions. However, factors such as catchment area, water depth, and lake area: catchment area ratio had comparatively little influence on DOM composition and biolability. These findings suggest that urban land use intensification in developing countries may lead to an increase in biolabile aliphatic DOM inputs from nonpoint sources and accelerated carbon cycling in lake ecosystems.