TU Berlin

Urban Water InterfacesInitial project plan

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Initial project plan

State of the art and preliminary work

Urban water systems comprise a complex network of lakes and other standing water bodies which are partially connected by canals and slow-flowing rivers. Multiple interfaces develop with technical components of the urban water system, with the atmosphere, and also within the water bodies. High loads of organic carbon and nutrients are often received not only from the catchment but also from technical systems such as sewage plants, as direct runoff from paved surfaces, or from dust deposition (Nehls & Shaw 2010). Therefore, urban water bodies can be highly productive (Gunkel et al. 2009), unless discharge of toxicants curbs biological activity. Eutrophication symptoms are accompanied by changes in the quality and quantity of settling particles (Rychla et al. 2012). In turn, mineralisation of the sedimented organic matter consumes oxygen which becomes depleted in the deep water layers above organically rich sediments in such a way that anaerobic carbon transformation becomes the dominant process (Glissmann et al. 2004; Groß-Wittke et al. 2010, Hoffmann & Gunkel 2011). Anaerobic processes and the resulting gas fluxes across the sediment-water, water-atmosphere interfaces as well as across
the oxycline have not been well studied in urban waters, although they are differ from those in natural aquatic ecosystems (Barros et al. 2011, Bastviken et al. 2011, Casper et al. 2000, 2003).

Two important gases formed in freshwater sediments under anoxic conditions are methane (CH4) and nitrous oxide (N2O) (Conrad et al. 2009). Both are powerful greenhouse gases (GHGs). They dissolve in sediment pore water where supersaturation can lead to significant gas accumulations (Casper et al. 2000). Subsequently, the gases either diffuse across the sedimentwater interface or rise as bubbles (ebullition). In the case of CH4, only a fraction of the gas
released from the sediments reaches the atmosphere because CH4 is effectively oxidised when reaching oxic water layers (Duc et al. 2010, Junier et al. 2010). Both the production of GHGs and CH4 oxidation are microbial processes (Dumont et al. 2011, 2013) which depend on environmental conditions (temperature, electron acceptors, pollutants; Nedwell 1984).

Aims and work steps

The aims of the proposed doctoral thesis are to: (i) evaluate and adapt established methods to be applied to urban aquatic systems; (ii) determine rates of GHG formation in sediments of urban water bodies; (iii) quantify GHG fluxes across the sediment-water interface, oxycline, and waterair interface (Casper et al. 2003); (iv) assess the importance of CH4 oxidation especially at the oxycline; (v) assess the degree of heterogeneity of process rates and fluxes at interfaces within and across water bodies; (vi) relate these rates and fluxes to morphological, hydrological, waterchemical and ecological system characteristics, including carbon sinking fluxes; (vii) identify hotspots; and (viii) provide a first estimate of process rates and GHG emissions for the water bodies of a large city based on a GIS model (Cierjacks et al. 2011).

The GHG formation in the sediments and oxidation at the oxycline will be determined by measuring process rates with standard methods (Casper et al. 2003, 2009) adapted to urban waters. The GHG release from sediments and subsequent emissions into the atmosphere will be measured with custom-built flux chambers (Flury et al. 2010). A spatial sampling strategy will be developed to ensure representative coverage of the major types of water bodies in Berlin (e.g. Lake Tegel, Wannsee, Panke, Erpe, Teltow channel). Site characteristics will be determined concomitantly and used together with information on water body distribution to provide GISbased estimates for the entire city. Thus, the collected data will facilitate assessment of the significance of urban water bodies as GHG sources to the atmosphere.

Connections to interfaces and other doctoral theses

This doctoral thesis will profit primarily from close collaboration with N4 (shared general conceptual framework, sampling sites, and equipment). In addition, assessments of pollutant effects on microbes will provide a linkage to the doctoral theses focusing on pollutants (N1, T4, T6), and the explicit spatial analysis needed to upscale from local measurements and local processes to the regional scale will be shared with N2. Further links are provided with T3, N5, N6
and N7 through a common focus on the sediment-water interface. Finally, the methodology for gas exchange measurements between water and air will be coordinated with T2.


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