Dissolved organic matter (DOM) rather than warming and eutrophication directly affects partial pressure of CO₂ (pCO₂)...

Environmental warming and eutrophication pose significant challenges to shallow lake systems, where dissolved organic matter (DOM) serves as a diverse and intricate mixture of organic macromolecules, playing a pivotal role in aquatic ecosystems. Despite its complexity, comprehending the interplay between environmental changes and DOM composition alterations and their subsequent impacts on aqueous CO₂ partial pressure (pCO₂) is essential for a better understanding of carbon cycling. Yet, our current understanding in this realm remains limited. To address this gap, mesocosm systems were established to investigate how elevated water temperature and eutrophication, alongside changes in DOM composition, influence pCO₂ dynamics. Results indicate that while temperature and nutrient levels do not directly influence pCO₂ fluctuations, they indirectly affect aqueous pCO₂ through their modulation of DOM composition. Elevated temperature and nutrient concentrations notably enhance both the production and degradation of indigenous protein-like organic matter and increase the accumulation of humic-like organic compounds, with phosphorus released from sediment playing a particularly significant role. Furthermore, the degradation rate of protein-like organic matter significantly exceeds its accumulation rate. On the other hand, the impact of water eutrophication on DOM composition surpasses that of temporal temperature variations, with a 2∼4 °C temperature rise showing minimal effects on DOM composition. Notably, the degradation of protein-like organic matter markedly increases aqueous pCO₂, while the rise in humic-like organic matter in water exerts minimal influence on pCO₂ concentrations. A comprehensive understanding of carbon cycling processes under environmental changes will facilitate effective management of lake ecosystems and the advancement of carbon mitigation technologies.