Carbon, nutrient and metal dynamics in sediments of a major European harbor: Where river meets sea

Estuaries play a key role in regional and global carbon, nutrient, and metal cycling, yet they are increasingly impacted by human activities. Among these, dredging, followed by sediment reuse, is particularly disruptive to sediment biogeochemistry because it exposes large volumes of previously buried, anoxic sediment to oxic conditions. This process strongly alters sediment biogeochemistry, stimulating greenhouse gas emissions and potential contaminant release. Meanwhile, climate-driven changes such as global warming and saltwater intrusion are expected to impose additional, longer-term changes to estuaries. This thesis investigates how the anthropogenic perturbations under changing environmental conditions impact sediment biogeochemistry in the Port of Rotterdam, located in the Rhine–Meuse estuary. An integrated approach combining field observations, laboratory experiments and numerical modeling is used to investigate in-situ and dredged sediments along a salinity gradient from marine to riverine environments. A central focus is the reactivity of sedimentary organic matter and its role in CO₂ production. Organic matter composition varies strongly along the estuary: marine sediments are enriched in labile algal material, while riverine sediments are dominated by more recalcitrant terrestrial inputs. These differences are reflected in the greenhouse gas generation potential. Incubation experiments further demonstrate that organic matter preserved under anoxic conditions can be rapidly oxidized upon exposure to atmospheric oxygen, resulting in carbon emission rates that are faster than those observed in many topsoil incubations from a global database. Besides CO₂ emissions, dredged sediments in the Port of Rotterdam contain elevated heavy-metal concentrations due to historical industrial activity. Wet chemical extractions show clusters of metals with distinct reactivity patterns due to their association with different metal pools (e.g. carbonates, oxides, organic-matter-bound fractions). This is also reflected by their mobility behaviors in the pH-dependent leaching experiments, suggesting that the risk of metal release may vary among clusters when deposition conditions change during dredged sediment processing. Characterized by the dynamic depositional conditions, coupled Fe-S-CH₄-P cycling shows large spatial and temporal dynamics across the harbor. Porewater measurements suggest that dredging can strongly disrupt Fe-S-CH₄-P dynamics in the short term, in particular, leading to an increase in CH₄ emissions. However, reduced CH₄ emissions are expected in the long term due to saltwater intrusion and the associated increase in sulfate availability for anaerobic CH₄ oxidation. At the same time, elevated sulfate availability stimulates sulfide production, which promotes dissolution of iron minerals (e.g. vivianite, (hydr)oxides). This process may intensify phosphorus remobilization and impact coastal water quality. Overall, this thesis shows that dredging represents a major short-term perturbation of estuarine sediment biogeochemistry, whereas climate-driven changes exert longer-term impacts. By identifying key controls such as oxygen, salinity, and temperature, this work provides process-based insights to support sustainable sediment management.