A Comparative Study on the Diurnal Variation of Carbon Emissions in the Water Retention Zone–Reservoir–Drainage Channel Continuum of a Typical Karst Underground River Outlet in Southwest China

<p indent="0mm"> Groundwater is an important water source in karst areas and a significant medium for storing atmospheric carbon dioxide. Dams built at the underground river outlets in karst regions benefit agricultural irrigation and the assurance of drinking water, but limited research has analyzed how carbon emission fluxes in the water body change after such human interventions. To investigate the characteristics and controlling factors of water carbon emission fluxes in the context of dam construction at underground river outlets in karst areas across diurnal and spatial scales, a study was conducted on the water continuum of the Longdongtang underground river outlet retention area‒reservoir‒drainage channel in the Longtan karst valley of the Wuling Mountain region in southwestern China. Water temperature, dissolved oxygen, pH, Ca²⁺, and HCO^–₃ were monitored every <sc>1 hour</sc> during the day and night, and <italic>δ</italic>¹³C_DIC was monitored every 2 hours during the day and night (from October 14 to 15, 2023). The CO₂ flux at the water-air interface (<italic>F</italic>CO) and net ecosystem productivity (NEP) were calculated and compared. The results show that <italic>F</italic>CO exhibits a characteristic of being low during the daytime and high at nighttime. During the day, <italic>F</italic>CO accounts for 46.4%–48.8% of the total <italic>F</italic>CO, while at night, it accounts for 51.2%–53.6%. The diurnal variation reached a maximum of 22.21×10³ g C/(d·m²). <italic>F</italic>CO displayed significant spatial differences, with specific values of <italic>F</italic>CO being lowest at the underground river outlet retention area <sc>(0.61 g C/(d</sc>·m²)), higher in the reservoir karst pool <sc>(0.75 g C/(d</sc>·m²)), and highest at the main drainage channel <sc>(0.81 g C/(d</sc>·m²)). The study found that changes in biological metabolic processes and water flow patterns were the main factors controlling the diurnal and spatial variations in <italic>F</italic>CO at the karst underground river dam, with biological metabolism having the most significant impact. The diurnal variation of NEP was opposite to that of <italic>F</italic>CO, and its spatial variation showed an increasing trend from the dam inlet to the dam outlet: <sc>0.34 g C/(d</sc>·m²) at the underground river outlet retention area,<sc> 0.31 g C/(d</sc>·m²) in the reservoir karst pool, and <sc>0.26 g C/(d</sc>·m²)at the main drainage channel This indicates that spatial differences in biological metabolism may shape the increasing trend of <italic>F</italic>CO along the flow path. The results of this study show that dam construction at the underground river outlet altered the hydraulic conditions, creating a favorable environment for aquatic plant growth. This, in turn, promoted the biological carbon pump effect while reducing CO₂ flux emissions to the atmosphere. This finding provides new insights for future management of carbon emissions in water bodies in karst areas.