Land-use and climate controls on aquatic carbon cycling and phototrophs in karst lakes of southwest China

Highlights: • Recent deforestation significantly decreased lake-water TOC and carbonate precipitation since the 1950s. • Cultural eutrophication increased algal production and lake-water TOC in a pre-industrial deforested catchment. • There exists a strong link between deforestation history and aquatic carbon storage in karst landscapes. • Different land-use types are associated with heterogeneous changes in lake phototrophs. • Regional warming altered diatom community structure favoring small-sized cells in more thermally stratified lakes. Land-use and climate changes have been repeatedly identified as important factors affecting terrestrial carbon budgets, however little is known about how deforestation and catchment development affect aquatic systems in carbonate-rich regions. Multi-proxy analyses of ²¹⁰Pb-dated sediment cores from two hard-water lakes with different land-use histories were applied for assessing carbon cycling and limnological changes in response to land-use changes over the past century in southwest China. Logging of primary forests in the catchment of Lugu Lake, starting in the 1950s, led to a significant increase of catchment erosion, as well as a consistent decline in inferred lake-water total organic carbon (TOC) levels and sediment carbonate accumulation. This process of recent deforestation may significantly reduce the role of lake systems to act as carbon sinks through hampering of both the soil organic carbon flux and the dissolution of catchment carbonate. The decline in lake-water TOC in Lugu Lake further increased algal production (i.e. tracked through sediment trends in chlorophyll a and its main diagenetic products) and changes in diatom composition. In comparison, there was little variation of sediment carbonate content in Chenghai Lake, which has a long history of catchment deforestation, while both primary production and lake-water TOC increased following cultural eutrophication during the last three decades. Furthermore, regional warming was associated with an increase in small-sized diatoms in both deep lakes, likely due to enhanced thermal stability. This study highlights the significant role of vegetation cover and land use in driving aquatic carbon cycling and phototrophs, revealing that deforestation can strongly reduce both inorganic and organic carbon export to lakes and thus aquatic carbon storage in karst landscapes.