Perturbation increases source-dependent organic matter degradation rates in estuarine sediments

Abstract. Despite a relatively small surface area on Earth, estuaries play a disproportionally important role in the global carbon cycle due to their relatively high primary production and rapid organic carbon processing. Estuarine sediments are highly efficient in preserving organic carbon and thus often rich in organic matter (OM), highlighting them as important reservoirs of global blue carbon. Currently, estuaries are facing intensified human disturbance, one of which is sediment dredging. To understand estuarine carbon dynamics and the impact of perturbations, insights into sediment OM sources, composition, and degradability are required. We characterized the sediment OM properties and oxidation rates in one of the world's largest ports, the Port of Rotterdam, located in a major European estuary. Using a combination of OM source proxies and end-member modeling analysis, we quantified the contributions of marine (10 %–65 %), riverine (10 %–60 %), and terrestrial (10 %–65 %) OM inputs across the investigated transect, with salinity ranging from 32 (marine) to almost 0 (riverine). Incubating intact sediment cores from two contrasting sites (marine versus riverine) suggested that OM degradation rates in marine sediments were about four times higher than those in riverine sediments, which was also observed during a 35 d subaerial bottle incubation experiment with mixed surface sediment. Moreover, subaerial incubation of mixed sediment showed a two- to three-fold increase in OM degradation rates compared to intact core incubation, highlighting that perturbation and subsequent enhanced oxygen availability can substantially boost OM degradation. By combining detailed quantitative characterization of estuarine OM properties with degradation experiments under varying conditions, the results further our understanding of the factors that govern OM degradation rates in (perturbed) estuarine systems. Ultimately, this contributes to constraining the impact of human perturbation on OM cycling in estuaries and its role in the carbon cycle.