Spatial variability of organic matter degradability in tidal Elbe sediments

Abstract

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.

Rheology of Mud: An Overview for Ports and Waterways Applications

“…However, this technique also possesses some inherent drawbacks which are the facts that (i) the conversion between raw data and particle size is based on the assumption that the particles are spherical and have a homogeneous composition, (ii) the measurements are possible in a limited range of concentrations, and (iii) there can be a serious overestimation of the amount of large particles due to the mathematical smoothing of the PSD's by the manufacturer's software [30]. The total organic carbon (TOC) of mud samples is commonly analyzed by using loss-on-ignition method [31,32], which is based on weighing the sample before and after heating at 430-500°C for 24 h. The total organic carbon is then estimated by loss in weight. All these sediment properties are known to significantly influence the rheological characteristics of mud samples.…”

“…There are two common sources of organic matter in mud: (i) natural and (ii) anthropogenic. The natural sources include erosion of terrestrial topsoils, plant litter, planktonic and pelagic biomass while surface runoff and sewage waste contribute to the anthropogenic source of organic matter [32].…”

“…In addition to organic matter content, its extent of degradation can also significantly affect the rheological properties of mud. Aerobic degradation (in the presence of oxygen) of organic matter usually results in the production of carbon dioxide while anaerobic degradation produces methane in addition to carbon dioxide [32]. For a detailed information about the aerobic and anaerobic degradation of mud, see ref.…”

Seasonal Stratification and Biogeochemical Turnover in the Freshwater Reach of a Partially Mixed Dredged Estuary

“…The three-dimensional resolution of the sedimentary layer would allow for simulation of vertical profiles of oxygen in the sediment and thus a higher spatial diversity of zones of nitrification or denitrification. Implementing a biogeochemical sediment module might thus result in better reproducing the peak ammonium levels observed in the low-oxygen zones of the port area where sediment pore water reveals ammonium concentrations greater than 330 mmol m −3 locally (Zander et al, 2020). The second point has physical and biogeochemical implications.…”

“…Although research has addressed major aspects of the along-channel distribution of nutrients, plankton and oxygen (Goosen et al, 1999;Dähnke et al, 2008;Sanders et al, 2018), little is known regarding the vertical and three-dimensional structure of biogeochemical dynamics and its response to the estuarine physics. Recent studies reveal sensitivity of the organic matter decomposition, carbon, and nutrient cycling to bottom oxygen availability (Zander et al, 2020). The organs of certain fish species caught near the port of Hamburg indicate that the animals experienced moderate hypoxia (Tiedke et al, 2014).…”