In-situ deep ocean monitoring reveals rapid kelp degradation limits marine biomass-based carbon sequestration potential and alters benthic ecosystems

Abstract Sinking macroalgal biomass carbon to the deep seafloor has been proposed as a means of removing atmospheric carbon dioxide, but the persistence of this carbon and its ecological impacts remain poorly constrained. Here, we present results from a year-long in-situ experiment in the Northeast Pacific oxygen minimum zone, where kelp was deployed and monitored with high-resolution time-lapse imaging. Over 90% of the biomass decomposed and disappeared within approximately 100 days, primarily via microbial and faunal activity. Some fraction of the carbon likely entered dissolved pools with longer residence times, but overall carbon sequestration efficiency may be limited under these conditions. Kelp emplacement also altered benthic community structure, based on >5000 annotated faunal observations from video imagery spanning 13 species, underscoring important ecological trade-offs. These results highlight the need for site-specific, process-based monitoring to inform environmental, ecological, and carbon storage verification frameworks for responsible biomass-based carbon dioxide removal.