Organic carbon burial and degradation in estuarine sediments in Europe's largest port area (Port of Rotterdam, The Netherlands)

Estuaries are highly active biogeochemical environments at the land-sea interface. They release approximately 0.25 Pg C y⁻¹ on a global scale, which is equivalent to 17% of the total oceanic uptake despite occupying an area that is only 0.03% of the global oceans (Li et al., 2023). This disproportionate impact underlines the importance of understanding the processing of riverine and coastal carbon in estuarine systems. This processing, particularly the breakdown to form the greenhouse gases CO₂ and CH₄, is controlled by the properties (e.g. source and composition) of organic matter (OM) and the depositional conditions. In this respect, harbors are profoundly human-impacted estuaries that continuously supply vast quantities of organic-rich dredged sediment, the environmental footprint of which is of prime concern for sustainable coastal and port management. While sources and composition are essential parameters with respect to CO₂ (and CH₄) generation, these are challenging to determine in the dynamic setting of a harbor with strong tidal influence. Here, we use detailed organic chemical analyses to investigate how OM composition and depositional conditions control the release of greenhouse gases from (dredged) Port of Rotterdam sediment.

The Port of Rotterdam (PoR) is located in the Rhine-Meuse estuary, with sediment and OM composition controlled by the interaction between river, sea, and human activities. During a sampling campaign in 2021, both bulk surface sediments and intact sediment cores were collected at different geographical locations throughout the harbor. A general west-to-east gradient of marine influence was presented, which coincided with the changes of organic carbon and nitrogen content and their isotope abundance. The macromolecular organic matter (MOM) was isolated and analyzed with pyrolysis-GC-MS, revealing it to be of mixed terrestrial, marine, and potential anthropogenic origins. Particularly, the abundance of terrestrial pyrolytic biomolecules (e.g. guaiacols, syringols, polysaccharides) decreased as the depositional environment became increasingly marine. Despite of OM composition changes along the salinity gradient, similar organic matter degradation rates were measured in short-term (8-hour) whole-core incubations at two sites with contrasting bulk OM signatures. This was likely attributed to the rapid degradation of fresh OM at the sediment surface. In comparison, 6-week aerobic incubation suggested marine sediments possessed a larger labile carbon pool than riverine sediments. Our results indicated that PoR sediments are characterized by large spatial variability in OM quantity and quality, further determining the carbon stock and stability. OM source seems to play a crucial role in influencing the carbon stability. Considerable attention still needs to be given to link OM characterization and degradability. However, OM degradation results from OM properties governing degradability in combination with environmental conditions (e.g. electron acceptors, microbial activities, and temperature). This was witnessed by a significantly larger benthic methane efflux in riverine sediment than marine sediment. Besides, the relative significance of OM composition influencing on degradation also depends on the timescale of interest. Nevertheless, the spatial heterogeneity in OM stability between different depositional environments highlights the need for applying ‘carbon-sensitive’ management of sediments in relation to their reactive carbon fraction when exposed to human pressure and climate change.