Turnover of Suspended and Settled Organic Matter in Ports and Waterways

Abstract

Organic matter plays a major role in global ecosystems and has several functions in terrestrial and marine environments. As organic matter impacts, among others, the rheological behaviour and settling rates of mineral sediment particles, it is of great relevance to the definition and maintenance of the nautical depth in ports and waterways. The microbial decay of organic matter leads to the emission of climate forcing gases like CO2 and CH4. In this theses it will be provided fundamental insight in the behaviour of sediment organic matter in the aquatic river system. It presents analyses of field and laboratory experiments using sediment samples taken during 21 sampling campaigns between 2018 and 2020 in the Port of Hamburg, Germany. The focus lay on sampling locations with high sedimentation rates. It is investigated chemical, physical and biological parameters and their variability in space and over time. It quantifies the share of anaerobically and aerobically degradable sediment organic matter in a depth profile and along a transect of about 30 km within the tidal Elbe river. Sediment organic matter at upstream and downstream locations is mainly allochthonous as it comes from the catchment (upstream) or from North Sea (downstream). Young organic matter, entering the system from upstream, has predominantly biogenic sources. Upstream organic matter originates from the catchment, containing plankton-derived and more easily degradable components. It was shown that the most upstream location was nourished primarily by upstream fluviatile sediments. This location was characterised by the highest concentrations of chlorophyll a, microbial biomass, silicic acid, EPS, humic acids and hydrophilic organic matter, the most negative δ13C signature and by the highest oxygen consumption rate, with decreasing trends towards downstream locations. At downstream locations, organic matter is mainly of allochthonous origin, entering the harbour mainly with the tidal flood current from the direction of the North Sea. The organic matter degradability was the lowest at downstream locations and organic matter was stabilised in organo-mineral associations. It is elucidated that spatial patterns of organic matter degradability can be explained by a source gradient. It was found that sediment organic matter lability is inversely linked to its stabilisation in organo-mineral complexes. The degradability gradient could be explained by different organic matter quality in relation to its origin. A fast, medium, slowly and non-degradable pool (pool 1 to pool 4) were identified based on the measured organic matter lability. Temporal and spatial variabilities (gradient and depth) were observed as well as seasonal changes of degradable organic matter pools. An age gradient was found with easily degradable material in top layers and increasing stabilization of organic matter in organo-mineral compounds with depth. The degradability was larger in upper sediment layers. It was also larger under aerobic conditions but the differences between aerobic and anaerobic decay decreased from upstream to downstream. The investigation area mostly comprised stabilised organic matter. On average around 20 % of TOC was anaerobically degradable and around 30% of TOC was aerobically degradable. Thermometric pyrolysis was shown to serve as a useful proxy to predict organic matter degradability in river sediments, with the Hydrogen-Index (HI) correlating well with degradability. Further, it will be demonstrated that the sediment organic matter decay has a biological, chemical and physical effect on the shear strength. Degradation of organic matter significantly affects sediment strength, especially under the anaerobic conditions. The formation of gas bubbles under anaerobic conditions added an additional physical component to the effect of biological organic matter decay. The susceptibility of the sediment to yield stress changes might depend on the availability of easily XIV degradable organic matter. Pronounced spatial trends were found with higher changes in yield stress at upstream locations and lower yield stress changes at downstream locations. Finally, this thesis will demonstrate the metamorphosis of sediment properties and sediment organic matter from its state in suspension to being part of the settled and consolidated sediment as well as from upstream to downstream. Temporal and spatial gradients were found for aerobic and anaerobic carbon fluxes as well as for potentially degradable organic carbon. A first draft of a carbon flux estimate originating from the microbial decay of organic matter from the investigation area is presented which can be used for future carbon foot printing assessments, for example for port maintenance activities.