Coupled effects of dissolved inorganic carbon dynamics and phytoplankton community succession rates on the biological carbon pump (BCP) in karst reservoirs

Karst reservoirs with high dissolved inorganic carbon (DIC) concentration exhibit an efficient biological carbon pump (BCP) effects driven by phytoplankton photosynthesis; however, the influence of phytoplankton community succession on BCP potential remains unclear. To address this issue, we conducted seasonal surveys across 11 karst reservoirs in the Wujiang River Basin and analyzed phytoplankton species composition, nutrients, DIC and its stable carbon isotopes (δ13CDIC). The results showed that reservoirs dominated by nondiatoms exhibited faster seasonal community succession rates, particularly those with algal bloom such as cyanobacteria or dinoflagellates. Environmental filtering-controlled phytoplankton community succession regulated the BCP, and this regulation was closely linked to dynamic changes in DIC speciation (e.g., CO₂(aq) and HCO₃⁻). Low CO₂(aq) favored nondiatoms with strong CO₂ concentrating mechanisms (CCMs), facilitating efficient carbon fixation through HCO₃⁻ utilization and enhancing primary productivity and BCP. Moreover, reservoirs with low hydraulic loads demonstrated pronounced seasonal vertical stratification, which promoted nondiatom growth and resulted in faster seasonal community succession rates and a stronger BCP effect than reservoirs with high hydraulic loads and were dominated by diatoms year-round. These findings indicate that phytoplankton community succession rate under the influence of DIC dynamic as a key regulator of BCP in karst reservoirs, informing their carbon source-sink functions in the context of global warming and eutrophication causing changes in biological communities.