High carbon fixation during thermal stratification period in a subtropical periodic stratified reservoir: Evidences from RDOC conversion efficiency driven by MCP

Recalcitrant dissolved organic carbon (RDOC) generated by microbial carbon pumps (MCP) significantly influences terrestrial waters and may contribute to the formation of a long-lasting carbon sink. However, there remains a notable lack of research on the carbon fixation processes and efficiencies of MCP in response to changes in thermal structure within subtropical reservoirs. In this study, we examined the effectiveness of transforming dissolved inorganic carbon (DIC) into dissolved organic carbon (DOC) and subsequently into RDOC through the influence of MCP at various water depths during both Thermal stratification (TS) periods and Mixing (MX) period in the Dalongdong (DLD) Reservoir, a representative subtropical reservoir. The findings indicate that the conversion efficiency of microbiologically recalcitrant dissolved organic carbon (MRDOC) was typically four times greater during the TS periods compared to the MX period. This increase can be attributed to a higher abundance of bacteria involved in carbon fixation, as well as elevated levels of external semi-labile dissolved organic carbon (SLDOC) and labile dissolved organic carbon (LDOC), along with the accumulation of organic matter. Notably, the conversion efficiency peaked in the thermocline during the Obvious thermal stratification (OTS) period. During the TS periods, heterotrophic and chemoautotrophic bacteria played a significant role in carbon fixation in the epilimnion and thermocline, while fewer bacteria were engaged in carbon fixation in the hypolimnion. Conversely, throughout the MX period, the effects of water temperature and pH result in a diminished role of autotrophic bacteria in carbon fixation, leading to a decline in MRDOC conversion efficiency at all water layers. These results enhance our understanding of the carbon cycling processes influenced by the MCP effect in terrestrial waters experiencing changes in thermal stratification.