Spatial Distribution and Drivers of pCO<sub>2</sub> in Coastal Reservoir

<p indent="0mm"> Carbon dioxide (CO₂) is one of the important greenhouse gases that causes global warming. Reservoirs play a crucial role in terrestrial aquatic ecosystems, serving as both sources and sinks of greenhouse gases. Due to the influence of many factors (e.g., surface physical-chemical properties, carbonate system), the carbon (C) cycling processes in reservoirs are complex and have significant regional differences. Water salinization is a common issue in coastal reservoirs and has significant impact on the C cycle. To further analyze the C cycle process in coastal reservoirs, this study focuses on the Tianjin Beidagang Reservoir as the research object. The spatial distribution of the partial pressure of carbon dioxide (<italic>p</italic>CO₂) in the water of the coastal reservoir was investigated and the influencing factors were analyzed through the hydrochemical characteristics and other relevant indicators. The results indicated that, in the horizontal direction, the maximum <italic>p</italic>CO₂ was observed on the upstream of reservoir. In the vertical direction, the overall <italic>p</italic>CO₂ exhibited a trend of being higher at the bottom than at the surface. <italic>p</italic>CO₂ demonstrated a significant negative correlation with total dissolved solids (TDS) and a positive correlation with Ca²⁺ and <italic>p</italic>CO₂ in the middle and deep layers, but showed a negative correlation between Ca²⁺ and <italic>p</italic>CO₂ in the surface layer. These results indicate that mineral dissolution and precipitation in coastal reservoirs, as well as plant photosynthesis, have significant impact on <italic>p</italic>CO₂, and thus affect C biogeochemical cycle in coastal reservoirs. This study will provide important fundamental data and scientific references for elucidating the ″C source-sink″ function and clarifying the C biogeochemical cycle mechanism in coastal reservoirs.