Florian Zander

The presence of clay-organic flocs in cohesive mud results in a complex rheological behavior of mud, including viscoelasticity, shear-thinning, thixotropy and two-step yielding. In this study, the effect of microbial degradation of organic matter on the rheological properties of mud samples, collected from different ports, was examined. The mud samples were collected from five different European ports (Port of Antwerp (PoA), Port of Bremerhaven (PoB), Port of Emden (PoE), Port of Hamburg (PoH) and Port of Rotterdam (PoR)), displaying varying sediment properties. The rheological analysis of fresh and degraded mud samples was performed with the help of several tests, including stress ramp-up tests, amplitude sweep tests, frequency sweep tests, time-dependent tests and structural recovery tests. The results showed: (i) a significant decrease in yield stresses and complex modulus after organic matter degradation for mud samples from PoA, PoH and PoR, (ii) a negligible change in rheological properties (yield stresses, crossover amplitude and complex modulus) for mud samples from PoB, and (iii) a significant increase in rheological properties for mud samples from PoE. For time-dependent tests, mud samples from PoB showed a substantial increase in hysteresis (~50% mean value) as compared to the changes in yield stresses and crossover amplitude. The analysis of gas production during degradation of organic matter showed a (i) significant release of carbon per g dry matter for mud samples from PoA, PoH and PoR, (ii) lower carbon release per g dry matter for mud samples from PoB, and (iii) a negligible carbon release per g dry matter for mud samples from PoE, which corresponded well with the change in rheological properties.

Purpose: Sediment organic matter (SOM) influences settling and thus the rheological behavior of suspended particles by enhancing flocculation or reducing surface charges by forming organo-mineral complexes that facilitate particle–particle interactions in consolidating sediments. It was, therefore, assumed that the microbial degradation of SOM and its spatio-temporal variability would affect sediment rheological properties and enhance port maintenance dredging and navigability of ports and waterways. Methods: To investigate this effect, samples were taken at six locations along a transect of 30 river kilometers through the Port of Hamburg, Germany, during nine sampling campaigns within two years. The collected samples were divided into different layers based on the differences in visual consistency and strength. For analysis of SOM degradability, the samples were incubated in the laboratory for 250 days in glass bottles under aerobic and anaerobic conditions following the evolution of gas composition (CH4, CO2) and pressure in the bottle headspace over time. Yield stress was analyzed before and after the dissolved organic matter (DOM) decay using a rheometer with Couette geometry. Standard properties of solids and pore water were also analyzed. Results: Shear strength decreased upon SOM decay under both anaerobic and aerobic conditions. Under anaerobic conditions, organic matter decay reduced static and fluidic yield stresses to an average of 74% and 79% of the fresh sample values. Consolidated layers at lower depths showed the highest absolute decrease in fluidic yield stress of up to –110 Pa due to a larger absolute amount of degradable organic matter in these layers in connection to higher bulk density. Pronounced spatial trends with higher changes in yield stress at upstream locations and lower yield stress changes at downstream locations coincided with a decreasing gradient of SOM degradability from upstream to downstream. Seasonal trends indicated that the investigation area is impacted by temporally changing factors. Conclusion: The availability of easily degradable organic matter significantly affects sediment strength, especially under the anaerobic conditions, even when the mass loss of organic matter mass loss is small. Seasonal variability in yield stress changes upon SOM decay indicate that the site-specific responses were modulated by overarching seasonal effects impacting the entire investigation area. It was assumed that during an anaerobic decay, the formation of gas bubbles added an additional physical component to the effect of biological SOM decay.

Under anaerobic conditions, degradation of organic matter in river sediments leads to gas formation, with organic carbon being released mainly as CH4 and CO2. Gas bubbles reduce sediment density, viscosity and shear strength, impede sonic depth finding and are suspected to affect the sediments’ rheological properties. Moreover, methane (CH4) is a potent greenhouse gas with a global warming potential (GWP100) of 28-36. Therefore, the climate impact may vary greatly depending on the way sediments are managed (for example, type and frequency of dredging and relocation in the water body or treatment on land). The objective of this paper is therefore to quantify the time-dependent stability, or inversely, the lability of sediment organic matter (SOM) as a basis for prediction of effects on sediment mechanical properties and on the release of greenhouse gases.Within two years, over 200 samples of predominantly fine-grained sediment were collected from nine locations within a 30 km transect through the Port of Hamburg. All samples were, amongst other analyses, subjected to long-term (> 250 days) aerobic and anaerobic incubation for measurement of SOM degradation, yielding a comprehensive data set on the time-dependent change in degradation rates and the corresponding size of differently degradable SOM pools. SOM degradability exhibited a pronounced spatial variability with an approximately tenfold higher anaerobic and a roughly fivefold higher aerobic degradability of upstream SOM compared to downstream SOM. Lower δ13C values, higher DNA concentrations and a higher share of organic carbon in the light density fraction as well as elevated chlorophyll concentrations in the water phase support the hypothesis of increased biological sources of SOM at upstream locations and increased SOM degradability in shallow compared to deep layers (Zander et al., 2020).First statistical and time series analyses indicate thatLong-term SOM lability appears to be predictable from short-term measurements. The relationship between short-term and long-term SOM degradation is site-specific and also differs for layers of different age (depth). This supports the above-mentioned variability between sites regarding the size of differently degradable carbon pools, as well as for the depth profile at any one site. The relevance of the available electron acceptors (redox conditions) for SOM degradation, i.e. the ratio between carbon release under aerobic and anaerobic conditions, differs less by site but more so by layers of different age (depth). This is plausible as especially the top layers are exposed to more variability in redox conditions than the deeper layers that are always under reducing conditions. Zander, F., Heimovaara, T., Gebert, J. (2020): Spatial variability of organic matter degradability in tidal Elbe sediments. Journal of Soils and Sediments, accepted for publication.

The project BIOMUD, part of the scientific network MUDNET (www.tudelft.nl/mudnet), investigates the decomposition of sediment organic matter (SOM) in the Port of Hamburg. The microbial turnover of sediment organic matter under reducing conditions leads to the formation of methane, carbon dioxide and others gases causing a change in the sediment rheological parameters. BIOMUD is aiming to explain the effect of organic matter lability on the rheological properties impacting the navigable depth of the harbour.Samples of freshly deposited material were taken in 2018 and 2019 at nine locations in a transect of 30 km through the Port of Hamburg. Analyses included abiotic parameters (among others grain size distribution, standard pore water properties, standard solid properties, stable isotopes, mineral composition) and biotic parameters (among others anaerobic and aerobic organic matter degradation, DNA, protein and lipid content, microbial population). At four locations, physical density fractions and chemical organic matter fractions were analysed.The quality of organic matter was described by normalising carbon released from microbial degradation under both aerobic and anaerobic conditions to the share of total organic carbon (mg C/g TOC). Organic matter pools with different degradation rates were used to quantify the lability of organic matter. The share of faster degradable (more labile) pools correlated strongly with the size of the hydrophilic DOC fraction, confirming results of Straathof et al. (2014) who investigated dissolved organic carbon pools in compost. The hydrophilic DOC fraction was closely correlated to the polysaccharide concentration, explaining the input of easily degradable organic matter. Moreover, the amount of organic carbon present in the sediment’s light density fraction < 1.4 g/cm3 strongly correlated with the hydrophilic DOC fraction and, less strongly, with organic matter lability. High organic matter quality, i.e. the labile, easily degradable fraction, was further related to the chlorophyll concentration in the water column but also the ammonium concentration in the sediment’s pore water.It was hypothesised that the observed toposequence of decreasing organic matter quality from upstream to downstream could be explained by a chronosequence of increasing degradation and therefore ageing of organic matter as the sediment passes through the harbour area. Further, it was hypothesized that the harbour received organic matter of higher degradability, originating from phytoplankton biomass, from the upstream part of the Elbe river, whereas the input from the tidal downstream area provided organic matter of lower quality (degradability).This study was funded by Hamburg Port Authority.

Purpose: The microbial turnover of sediment organic matter (OM) in ports and waterways impacts water quality, sonic depth finding and presumably also rheological properties as well as greenhouse gas emissions, especially if organic carbon is released as methane. As a consequence, sediment management practices as a whole are affected. This study aimed to discern spatial OM degradability patterns in the Port of Hamburg and investigated correlations with standard analytical properties as a basis for future predictive modelling. Materials and methods: Sediments in the Port of Hamburg were repeatedly sampled at nine locations along an east-west transect using a 1-m corer. In a stratified sampling approach, layers of suspended particulate matter (SPM), fluid mud (FM), pre-consolidated sediment (PS) and consolidated sediment (CS) were identified and individually analysed for long-term aerobic and anaerobic degradation of organic matter, DNA concentration, stable carbon isotope signature, density fractions and standard solids and pore water properties. Results and discussion: The investigation area was characterised by a distinct gradient with a 10-fold higher OM degradability in upstream areas and lower degradability in downstream areas. Concomitantly, upstream locations showed higher DNA concentrations and more negative δ¹³C values. The share of bulk sediment in the heavy density fraction as well as the proportion and absolute amount of organic carbon were significantly larger at downstream locations. A depth and hence age-related gradient was found at individual locations, showing higher degradability of the upper, younger material, concomitant with higher DNA concentration, and lower OM turnover in the deeper, older and more consolidated material. Deeper layers were also characterised by higher concentrations of pore water ammonium, indicative of anaerobic nitrogen mineralisation. Conclusions: Organic matter lability is inversely linked to its stabilisation in organo-mineral complexes. The observed degradability gradient is likely due to the different OM quality in relation to its origin. Downstream OM enters the system with the tidal flood current from the direction of the North Sea whereas upstream locations receive OM originating from the catchment, containing more autochthonous, plankton-derived and more easily degradable components. At individual sampling points, depth-related degradability gradients reflect an age gradient, with easily degradable material in top layers and increasing stabilisation of OM in organo-mineral compounds with depth.