Studies on the impact of food web effects on nitrification in aquatic sediments

Nitrification is an important biogeochemical pathway in the upper, oxic layer of aquatic sediments and is predominantly accomplished by two groups of chemolithotrophic nitrifying bacteria. While these bacteria have been subject to numerous autecological studies before, they have rarely been regarded as part of food webs, in which they have to compete with other organisms for nutrients and substrates and in which they are prey to other organisms. The impact of ciliates as important bacterial grazers in sediments on nitrification and nitrifying bacteria was investigated, both in marine and freshwater systems. In vitro experiments were conducted to detect and evaluate a possible grazing selectivity of ciliates for or against nitrifying bacteria. Furthermore, the effect of a bioturbating omnivorous polychaete (Nereis diversicolor) in marine sediments was studied. Natural sediments in laboratory flumes were used to detect quantitative effects of ciliates or Nereis. The results indicate that increased ciliate abundances led to higher nitrification potentials, higher abundances of nitrifying bacteria and higher nitrate concentrations. The effects were strongest in marine sediments. Selective feeding could be ruled out as a cause. Increased bacterial abundances in the presence of the Nereis and changes in the community composition of nitrifying bacteria were found. The long-known and widely used settling method for the enrichment of fixed ciliate cells from plankton samples was critically reviewed in an additional study. The settling times commonly used are based on experience or estimation, but not on experimentally derived data. Settling times for seven ciliate cultures were determined empirically and theoretical sinking velocities were calculated. A method for the determination of the ciliate density (in terms of g ml-1) had to be developed for the theoretical approach. The results revealed that the sinking velocity is much higher than assumed, which allows to save up to 95 % of settling time and improves the quality of the results.