Microbial activity and transport processes in near-shore, permeable sediments

Abstract

High organic carbon turnover in permeable sands is considered to be fueled by the efficient supply of solutes (e.g., oxygen) and organic matter from the overlying water by pore water advection. In situ quantifications of microbial activity in the presence of pore water advection are still rare. In this thesis, microbial activity and transport processes were investigated in near-shore, permeable sediments. A novel method for the measurement of volumetric oxygen consumption rate (OCR) profiles with high depth resolution in permeable sediments was evaluated in chapter 2. Three technical approaches were presented using either oxygen micro-sensors or planar optodes. For the calculation of areal OCR, the volumetric OCR can be integrated over the oxygen penetration depths measured in situ over extended time periods. These areal OCR reflect the influence of hydrodynamics on oxygen distribution. The method was consistent with established methods (interfacial gradients combined with Fick's first law of diffusion, benthic-chambers). The importance of pore water advection for sediment oxygenation and benthic mineralization was shown at the intertidal sand flat Hausstrand (Sylt/Germany) (chapter 3). Three stations were chosen on a transect from the low - to the high water line. Due to pore water advection, oxygen penetrated deeper and more dynamically during inundation than during exposure of the flat. The oxygen penetration depths were closely coupled to bottom water current velocities, proving the impact of pore water advection on sediment oxygenation. Driven by the advective oxygen supply, benthic OCR were high: 71 - 90% of oxygen consumption took place during inundation and aerobic mineralization was the dominant mineralization process at all stations. Mineralization rates were linked to the inundation time of the stations: Oxygen consumption rates were elevated at the lower flat, sulfate reduction rates decreased sharply from the low- to the high-waterline. At two stations (upper- and lower flat) at the intertidal sand flat Janssand near the island of Spiekeroog (German Wadden Sea), also a deeper oxygen penetration was found during inundation (chapter 4). Similar to the Hausstrand, OCR were high and highest during inundation, and sulfate reduction contributed only between 3 - 25% to total mineralization. However, at the two stations, similar surface mineralization rates were measured. In contrast to this, the concentrations of mineralization end products in the pore water at the low water flat were up to 15 times higher than at the upper flat, and the solute concentrations varied independent of season at this station. It was concluded that two filtration processes influence the distribution of metabolic products: (1) a rapid 'skin filtration' in the upper sediment layer during inundation driven by pore water advection and (2) a slow 'body filtration' through deeper sediment layers during exposure driven by drainage. In the coral reef sediments of Heron Reef (Australia), four sites exhibiting different hydrodynamic regimes were investigated (chapter 5). Oxygen penetration and dynamics as well as oxygen consumption, aerobic mineralization and sulfate reduction rates were highly variable between these sites. The supply of oxygen by pore water advection stimulated aerobic mineralization. A simple estimate of the organic matter supplied to the sediments by pore water advection only explained a fraction of the mineralization rates, indicating the importance of other organic carbon sources like benthic primary production. The microphytobenthos at Heron Reef was dominated by diatoms, dinoflagellates and cyanobacteria (chapter 6). Episammic colonies were highly diverse with respect to cyanobacterial 16S rDNA sequences. Photosynthesis was high per unit chlorophyll-a, indicating an active microphytobenthic community. Estimates on the microphytobenthic photosynthetic production in the entire reef were in the order of magnitude as the estimated production by corals. Photosynthesis stimulated calcification at the four investigated sites. The sediments of at least three stations were net calcifying. Sedimentary N2-fixation (acetylene reduction) was highest in the light, indicating the importance of heterocystous cyanobacteria. In coral fingers no N2-fixation was measurable, which stresses the importance of the sediment compartment for reef nitrogen cycling.