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 particulate organic carbon (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 two-week delay between POC sedimentation and CH4 emissions. Despite its eutrophic status, our data demonstrate that this reservoir acts as a carbon sink by sequestering 4.33 g C m-2 y-1, which accentuates the importance of temporal integration at multiple scales when analyzing carbon budget within reservoirs.