Estuarine sediments are important sites for the interception, processing, and retention of organic matter, prior to its export to the coastal oceans. Stimulated microbial co-metabolism (priming) potentially increases export of refractory organic matter through increased production of hydrolytic enzymes. Using the microphytobenthos community to directly introduce a pulse of labile carbon into sediment, we traced a priming effect and assessed the decomposition and export of preexisting organic matter. We show enhanced efflux of preexisting carbon from intertidal sediments enriched with water column nutrients. Nutrient enrichment increased production of labile microphytobenthos carbon, which stimulated degradation of previously unavailable organic matter and led to increased liberation of “old” (6855 ± 120 years BP) refractory carbon as dissolved organic carbon (DOC). These enhanced DOC effluxes occurred at a scale that decreases estimates for global organic carbon burial in coastal systems and should be considered as an impact of eutrophication on estuarine carbon budgets.
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