Isotopic indicators of source and fate of particulate organic carbon in a karstic watershed on the Yunnan-Guizhou Plateau

• Almost all POC in SPM in the karstic watershed originate from phytoplankton.
• More than 50% POC in most surface and core sediments come from land-derived sources.
• POC input to karstic riverine-lacustrine system is related to anthropogenic impacts.
• POC exporting out the karstic watershed is dominated by aquatic photosynthesis.
• Bed sediments in the karstic riverine-lacustrine system act as an organic carbon pool.

The studied watershed is a Karst-dominated area on the Yunnan-Guizhou Plateau. The local karstic environment might cause the biogeochemical processes involving POC to be different from those in non-karstic regions. Isotopic composition of particulate organic C (POC) and total N (TN) (δ13CPOC, δ15NTN) and C/N ratios (atomic) were used to identify sources and fates of POC in suspended particulate matter (SPM) and sediments over space and time in the karstic watershed. Distributions of POC in SPM and sediments show great seasonal and spatial variations. The δ13CPOC in SPM ranges from −27.4 to −19.0‰ and −33.4 to −22.3‰ in summer and winter, respectively. The C/N ratios in SPM in both seasons are lower than 12 and most are around 7. The surface and core sediments have lower δ13CPOC but higher C/N ratios than those of SPM. According to source analysis using C/N ratios, δ13CPOC and δ15NTN, the major source for POC in SPM is phytoplankton, while POC in surface sediments are attributed to land-derived and aquatic sources. Core sediments in the region affected by less anthropogenic impact have a similar POC source to surface sediment. However, in regions with intense deforestation and farming, land-derived sources may be major contributors to POC in core sediments. Calculated contributions from these sources to POC in the riverine-lacustrine system confirm the above observation. The flux and export rate of POC in SPM are 7.85 × 108 g a−1 and 492 mg m−2 a−1, respectively. In contrast to some world rivers, POC output from the karstic watershed is mainly from in-stream photosynthesis. Land-derived POC is merely redistributed in river and lake beds and sediments in the karstic riverine-lacustrine system act as an organic C pool. The observed features are likely due to intense anthropogenic impact and particular geomorphology of the karstic environment.