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TU Berlin

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Initial project plan

Doctoral student: Clara Romero

Supervisors: Prof. Mark Gessner, Dr. Gabriel Singer, Prof. Matthias Barjenbruch, Prof. Birgit Kleinschmit

State of the art and preliminary work

High nutrient and organic carbon loads provide scope for fuelling an intense ecosystemmetabolism of urban water bodies (Bernot et al. 2010, Beaulieu et al. 2013). Heterotrophic processes dominate whenever light is a limiting factor and external organic carbon is the main resource base (Bernot et al. 2010, Mulholland et al. 2001, Iwata et al. 2007). This is true for both the extensive technical networks of the urban water system and many natural or semi-natural surface waters. The metabolic rates in surface waters are expected to range between those of contrasting technical systems, such as drinking water supply pipes and sewers. However, it is unknown to what extent total system metabolism varies between the various parts of the urban water system, to what extent rates differ from those in natural ecosystems, and which role interfaces have in determining rates. The dominance and intensity of heterotrophic activity cause many natural surface waters to be CO2 oversaturated and thus leads to notable gas emissions to the atmosphere (Butman & Raymond 2011, Duarte & Prairie 2005). This situation will be exacerbated in urban water bodies, unless toxic substances suppress biological activity (Schäfer et al. 2012). However, since neither concentrations nor rates have been determined, except for isolated local measurements (Prasad et al. 2013), the contributions to the total gas fluxes of the various natural and technical parts of the urban water system are unknown, as is the role of urban waters for carbon fluxes at the regional and global scale (Regnier et al. 2013).

Aims and work steps

The aim of the doctoral thesis is to (i) adopt a long-established concept of ecosystem ecology to quantify the whole-system metabolism (Odum 1956, Marzolf et al. 1994, Jeppesen et al. 2010) in natural and technical components of the urban water system, (ii) identify the key drivers of metabolism in these settings, (iii) evaluate the importance of interfaces, (iv) estimate the CO2 emissions into the atmosphere from surface waters and technical parts of the urban water system, and (v) ultimately project estimates of the whole-system metabolism and the net CO2 emissions across the water-atmosphere interface to the metropolitan area of Berlin. Major types of water bodies (e.g. Lake Tegel, Wannsee, Panke, Erpe, Teltow channel) will be covered. The metabolism will be derived from continuous oxygen records measured with optical sensors deployed in the field at many locations. Oxygen concentrations will be converted to CO2 concentrations based on respiratory quotients. If the balance of the in-situ generation and consumption of CO2 by primary production and respiration and external CO2 inputs is positive, the water becomes oversaturated and emits CO2 into the atmosphere. The emission rates will be estimated (i) by measuring concentrations and computing fluxes based on modelled gas exchange efficiency (rapid method for high spatial coverage; Butman & Raymond 2011), and (ii) by directly measuring fluxes in the field with custom-built flux chambers and portable infrared gas analysers (reference method, low spatial coverage, but high temporal resolution; Sand-Jensen & Staehr 2012). Gas exchange efficiency can be modelled using diel oxygen records (Hornberger & Kelly 1975) and information on the controlling variables, particularly wind speed and hydrodynamic properties of the water bodies (Genereux & Hemond 1992, Crusius & Wanninkhoff 2003). These variables will be assessed together with environmental variables and pollutants which are potential controls of the whole-system metabolism. The GIS will be used to extrapolate the measured rates to the scale of the City of Berlin based on site characteristics and mapped information on the spatial extent of natural and technical urban waters (Cierjacks et al. 2011). These data lead to – initially coarse – estimates of total CO2 net production and release from water bodies as a component of total urban metabolism (Kennedy et al. 2007).

Connections to interfaces and other doctoral theses

This doctoral thesis will profit primarily from close collaboration with N3 (shared general conceptual framework, sampling sites, and equipment). In addition, assessments of pollutant effects 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 profit from collaboration with N2. Finally, the methodology for gas exchange measurements between water and air will be coordinated with T2. 



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