Large Variations of Dissolved Carbon Occurred in Small Ponds within An Agricultural Watershed

Abstract

Dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) significantly affect the aquatic carbon budget and ecosystem functions. Small ponds are abundant globally and widely distributed especially in agricultural watersheds, however, the variability of DOC and DIC and their driving factors remain poorly understood, likely leading to large uncertainty of aquatic carbon budget estimates. The presented study was designed to fill the knowledge gap based on a detailed year-long field investigation via examining DOC and DIC concentrations across ponds with differing functionalities (e.g. sewage ponds, irrigation ponds, and natural ponds) of a typical agricultural watershed in eastern China. Our results found a pronounced impact of human activities on pond DOC and DIC, with higher DOC occurring in sewage ponds (10.84 ± 2.83 mg L-1) and irrigation ponds (9.09 ± 2.57 mg L-1) and peak DIC in irrigation ponds (20.36±2.49 mg L-1) compared to that at natural ponds (DOC: 7.54 ± 2.55 mg L-1; DIC: 11.16 ± 3.85 mg L-1) with less human activity. The positive correlations between DOC/DIC and key environmental variables (e.g. nutrients and chlorophyll-a) further demonstrated that human activity can either directly increase the carbon concentrations via pollutant discharge, or indirectly increase DOC concentration via stimulating primary production. Meanwhile, field measurements also found precipitation and temperature play roles in determining the carbon variability. Specifically, precipitation increased the DOC of these ponds via enhancing land-based carbon inputs, and decrease the DIC of irrigation ponds via diluting. Temperature can influence the carbon dynamics through increasing primary productivity and metabolism. Our study underscores the roles of human and natural influences in determining the large variations of DOC and DIC in small ponds, which should be considered in carbon budget estimates of human-impacted small aquatic systems.

Keywords: inland water, carbon cycle, agricultural watershed, DOC variability, DIC variability, Human activity