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.