Metabolic processes and their effects on carbon cycling in a karst reservoir, south China.

Metabolic processes of aquatic photosynthetic organisms significantly influence the multi-time scale cycle of dissolved inorganic carbon (DIC) in aquatic environments. Running karst water characterized by high DIC content are becoming lentic water body due to damming activities around the world, which could benefit for aquatic metabolic processes and change initial carbon cycle processes. However, despite the availability of high-resolution monitoring data, there is a lack of comprehensive studies examining the characteristics of the metabolic processes of aquatic photosynthetic organisms, controlling factors, and their relationship with changes in reservoir thermal structure and carbon cycling. Based on this, this study selected the Dalongdong (DLD) reservoir, a typical karst reservoir located in Southwest China, as the research area for multi-parameter high-resolution continuous monitoring conducted in 2021. Seasonal sampling of relevant indicators was conducted in June, August, and December. The results indicate that the average net ecosystem production (NEP) of surface-layer during mixing period was 0.40 mg L-1 d-1, significantly lower than the thermal stratification period (4.65 mg L-1 d-1). The reservoir demonstrated a net autotrophic state, with the estimated carbon sink of approximately 81.54 t C km-2 y-1. Seasonal and diurnal variations in metabolic processes were primarily influenced by photosynthetically active radiation, which subsequently affected the hydrochemical characteristics. During the thermal stratification period, high temperatures and significant DIC fertilization effect sustained high production efficiency among aquatic photosynthetic organisms. In contrast, during the mixing period, biological growth was more dependent on upwelling nutrient inputs from the reservoir bottom. Through physical disturbances and the inhibition of biological photosynthesis, CO2 emissions increased during rainfall, while NEP rose following the rainfall. Combining high-resolution continuous data with seasonal sampling data, the multi-scale effects of aquatic photosynthetic metabolism on the carbon cycle in aquatic systems are elucidated, which is essential for accurately calculating the carbon budget in reservoir basins.