Extreme carbon fluxes may result from autochthonous particulate organic carbon regulated by the interactions between picophytoplankton and heterotrophic bacteria in river-reservoir systems

Abstract. Freshwater is a significant natural source of atmospheric methane (CH₄) and carbon dioxide (CO₂) while also receiving significant amounts of particulate organic carbon (POC) from various origins. The variation in carbon (CH₄ and CO₂) fluxes in freshwater systems is heavily influenced by the sources of POC. The trophic interaction between picophytoplankton (PP) and heterotrophic bacteria (HB) plays a vital role in the carbon cycle within the aquatic system. However, the contributions of different sources of POC to the concentrations and fluxes of CH₄ and CO₂ are still unclear. Here, we explored the contribution of POC from different sources to extreme carbon emission and the interaction between PP and HB. The evidence from isotope analysis further proved that the extreme carbon fluxes were strongly influenced by autochthonous POC rather than allochthonous POC. Network analysis showed that the positive interaction strength between phytoplankton and bacterioplankton in extreme carbon groups was higher than in normal carbon groups. The results of the structure equation modeling analysis also highlighted that the PP-HB interaction strongly drove the extreme carbon values. This study first introduced the probability statistics method to identify and classify high or low extreme carbon values. These findings also highlight the importance of PP and HB in carbon extreme emissions, and we hope our study can provide an important implication for integrating PP-HB interaction into predicting extreme carbon emissions in the river-reservoir ecosystem.