Benthic filter feeders (BFF) can reduce phytoplankton concentration (abundance) thereby controlling eutrophication in several ecosystems, including rivers. However, experiments suggest warming can alter the relationship between BFF grazing rate and the growth rate of (heterotrophic) planktonic prey. To investigate how eutrophication control by grazers is altered with temperature under the influence of other important abiotic (water depth, and speed, light, and turbidity) and biotic factors (initial phytoplankton concentration [hereafter: Pin value], BFF density and spatial BFF distribution), we developed a spatially-explicit computer simulation model. This model simulates the dynamics of a phytoplankton population traveling through a simplified river channel while being grazed by BFF. Our model includes the thermal responses of BFF grazing and phytoplankton growth. The results show that BFF grazing can qualitatively alter and, in some circumstances, even reverse the response of phytoplankton to warming. Moreover, the response of grazer-controlled phytoplankton to warming, water depth and Pin value is non-linear and phytoplankton can increase steeply with slight changes within some ranges of these variables. In addition, these variables can interact causing their combined effects on eutrophication to differ from what is expected considering their isolated effects. Generally, the effect of most variables, including temperature, Pin value and BFF density and spatial distribution, is larger at shallow waters. Moreover, our study shows that phytoplankton control can be substantially improved by heterogeneous BFF distributions where the BFF are located at the extremes of the river either upstream or downstream instead of homogenously distributed along the whole river. However, warming can cause a switch between these two optimal distributions or even can cause differences among the spatial distributions to disappear. In general, the homogeneous BFF distribution can be used as conservative estimate of eutrophication control. In conclusion, this work shows that trophic control can qualitatively alter the response of eutrophication to warming, supporting previous studies suggesting that the prediction of global warming effects requires considering not only the thermal responses of organisms but also their trophic interactions. In addition to these biotic variables, this thesis reveals that considering the interactions between abiotic and biotic variables and including their spatial distribution are important for eutrophication control. Especially, the detection of thresholds in the response of grazer-controlled phytoplankton to temperature, water depth, Pin value, and spatial BFF distribution indicates that one should be careful with predictions because of potential abrupt changes. Although further studies are needed to make specific recommendations for water quality management, our work provides preliminary suggestions on the conditions where grazers or Pin reductions can be more efficient to control eutrophication.

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This cumulative dissertation presents an environmental economic geography approach to the study of waste and recycling. Thereby, it introduces the notion of ‘waste economies’, which describes the conjunction of the production of waste with the societal handling as well as the valorisation of waste. Two distinct regional case studies serve to illustrate different aspects of waste economies. The first case investigates the valorisation of surplus manure from intensive livestock farming through biogas production in a highly industrialized rural region in north-western Germany – the example of manure and digestate in the Oldenburger Münsterland. The second case focuses on the recovery and revalorisation of wasted materials in the labour-intensive urban informal economy of a metropolitan area in eastern India – the example of informal plastic recycling networks in Kolkata. On a theoretical level, the conceptualization of waste economies is located at the intersection of environmental economic geography and the interdisciplinary field of waste studies. It draws on the global production networks approach, social metabolism and Marxist political economy to analyse waste as a form of ‘hybrid’ socio-nature. Following a qualitative research methodology, the analysis of the two cases depicts the close entanglement of economic and environmental processes in the production, societal handling and economic valorisation of waste, and reveals how this intersection is conducive for capital accumulation. Three different economic processes and dynamics serve as central analytical dimensions to delineate the characteristics of waste economies with regard to the expanded reproduction of capital accumulation, that is, the continued growth of capitalist economies: processes of externalisation as well as dynamics of expansion and intensification. Through the combined up-scaled analysis of two empirical cases on a higher level of theoretical abstraction, this dissertation offers a better understanding of the economic function of waste in growth-oriented capitalist economies. In this way, it contributes to the global recycling network and global destruction network approaches within economic geography and relates them to scholarly concerns about global environmental change.