Abstract: Inland water bodies are crucial to global carbon cycle and greenhouse gas budget estimations, with dissolved inorganic carbon (DIC) playing a central role in aquatic carbon dynamics. We investigated the characteristics and influencing factors of DIC concentrations based on year-long field observations on three functionally different ponds (aquaculture pond, sewage pond, and natural pond) of the Tongyang River Basin, a typical agricultural basin in eastern China. The results showed that there were significant differences in DIC concentration among the ponds with functional differences. Aquaculture pond had the highest DIC, with an annual mean value of 22.04±5.44 mg·L^-1, followed by sewage pond (17.84±2.50 mg·L^-1). The lowest DIC concentrations occurred in natural pond (14.56±1.74 mg·L^-1). Seasonal fluctuations in DIC were found in these ponds. A notable feature was that natural pond showed higher concentrations in the cold season and lower in the warm mostly due to the seasonal changes in temperature and precipitation. In contrast, DIC concentrations in aquaculture and sewage ponds showed different seasonal trends, which were influenced mainly by water body nutrient loads (e.g. NH₃-N and TP). In summary, the significant spatiotemporal variations of DIC in these small ponds of agricultural watersheds were related to the inter-connected physical, chemical, and biological factors. More attention should be paid on aquatic carbon cycling in small ponds of agricultural watersheds.

Key words: water carbon cycle, small water body, agricultural activity, spatiotemporal variation