Particulate organic carbon sedimentation triggers lagged methane emissions in a eutrophic reservoir

Reservoirs act as carbon sinks when sedimentation of particulate organic carbon (POC) exceeds CO₂ and CH₄ emissions. Here, we study the poorly explored process where phytoplankton-derived acidic polysaccharides (APs) aggregate into particulate organic matter, promoting carbon export to sediments. This source of POC in sediments can mineralize to CO₂ and CH₄ over various timescales. Our research, centered on a Mediterranean reservoir, elucidates phenological trends of APs and POC sedimentation and identifies their predominant drivers. Our findings present synchronic sedimentation patterns of POC and APs but identify a 2-week delay between POC sedimentation and CH₄ emissions. Despite its eutrophic status, our data demonstrate this reservoir's role as a carbon sink, sequestering 4.33 g C m⁻² yr⁻¹. This highlights the need to consider various time scales when quantifying carbon budgets in reservoirs.