Lake organic and inorganic carbon cycle decoupling in response to historical watershed activities in Yunnan, China

As anthropogenic impacts to both the climate system and freshwater resources continue unabated and are expected to intensify in coming decades, an increasing number of lakes will experience carbon cycle perturbations. Lakes that have been experiencing such perturbations for millennia can clarify the nature and severity of carbon cycle disturbances as well as recoveries. In lakes with authigenic carbonate material, the use of both inorganic and organic carbon isotopes to detect the decoupling of the inorganic and organic carbon cycles has been underutilized. We summarize here the application of these methods to three lakes in Yunnan, China, which have been impacted by human activities for the last 1,500 years.  Further we compare the results from this time period to the middle and late Holocene, both periods characterized by minimal anthropogenic influence. Decreased precipitation, increased evaporation, and changes in landscape vegetation drive changes observed in sediment carbon isotope compositions from 5,500 to 3,500 years BP. Stabilization of these factors from 3,500 to 1,500 years BP resulted in fairly consistent within-lake nutrient cycling. Following anthropogenic manipulation of lake levels after 1,500 years BP and despite differences in the magnitude of such activities, a pervasive feature in all of these lakes is the decoupling of the inorganic and organic carbon cycles, primarily driven by an influx of oxidized organic carbon from the watershed and/or the respiration of lake sediment organic matter. Carbon cycle decoupling persists into present-day for some lakes, illustrating the importance of considering historical, legacy activities.