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TU Berlin

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Abstract

Urban lakes are aquatic ecosystems that are influenced by a water management system and by a possibly strongly sealed urban catchment. The water management aims to fulfill multiple objectives: providing drinking water, sanitation and recreation for the people as well as keeping the ecosystem in a sustainable and resilient state. Urban lakes are facing several stressors. For instance, climate change will affect thermal characteristics of lakes, frequency and duration of rainfall events, inflow discharges as well as atmospheric circulation patterns (especially wind speed and wind direction). Urbanization will cause further morphological degradation and an increasing pressure to utilize urban freshwater resources. Consequences of these stressors could be eutrophication of the urban lake ecosystem causing the formation of harmful algal blooms, the settling of invasive species or the development of a legacy contamination in the sediment. An adaptive and sophisticated urban lake management system has the potential to mitigate the impacts of these stressors on the urban lake ecosystem to keep them in a sustainable and resilient state.

The aim of this thesis is to explore the feedback mechanisms between Lake Tegel in Berlin, Germany, and its respective water management system. Lake Tegel's water management consists of a phosphorus elimination plant (PEP), which reduces the inflow's phosphate concentrations, a lake pipeline, which bypasses additional discharges to the PEP, and groundwater abstraction wells for bank filtration. This thesis covers a spectrum from past, present to future management measures by (1) analyzing the impact of past management measures by investigating the sediment composition, (2) current management measures by monitoring the lake water quality, and (3) future management measures by using numerical models. The sediment composition of Lake Tegel was examined by taking sediment cores at different sites and by comparing them with sediment cores from reference lakes. The cores were analyzed using X-ray fluorescence spectroscopy with high spatial resolution. Data were statistically evaluated by using principal component analysis (PCA), k-means clustering and self-organizing maps. To get frequent field data, a logger chain monitoring electrical conductivity, water temperature and dissolved oxygen was deployed at Lake Tegel's deepest site. Monitored data were used to explore limnophysical conditions of the stratification periods from 2017 until 2018. To project the impact of climate change on the lake system and to analyze alternative management measures, numerical models were calibrated and validated to Lake Tegel. A vertical 1D model (GLM-AED2) was used to explore the impact of climate change under different management setups on Lake Tegel's stratification stability, onset and offset as well as on the dynamics of dissolved oxygen and phosphate concentrations. A depth-averaged 2D model (open TELEMAC-MASCARET with EUTRO for water quality) was applied to compute water exchange times in dependence of the wind and to project the formation of phytoplankton blooms after short-duration heavy rainfall events under different management setups and wind directions.

The results confirmed that Lake Tegel's sediment composition was spatially heterogeneous probably due to the management system and the flow dynamics. The strong impact of flow dynamics on potential sedimentation processes was also confirmed by the application of the depth-averaged 2D model. The past management measures were successful in reducing the abundance of heavy metals making Lake Tegel's recent sediment layers similar to the ones of a reference lake with low urban impact. The recent monitoring of Lake Tegel revealed the existence of vertical seiches in the periods 12 and 8 h, a weak stratification period in winter 2017/2018 and the existence of a proposed density current that originated from the PEP during the winter season and affected the bottom water heat and dissolved oxygen budget. The vertical 1D model projected that future water temperatures will increase and that the summer stratification will extend with earlier onset and later offset. Further, the additional water discharges by the PEP with low phosphorus loadings can buffer high nutrient loadings from the River Havel inflow and weaken the summer stratification stability, which could otherwise benefit the formation of harmful cyanobacteria blooms. Therefore, the PEP discharges and the corresponding low nutrient loadings are a positive factor to mitigate the impact of climate change on the Lake Tegel ecosystem. Computing the influence time distributions in dependence of wind direction using the depth-averaged 2D model clarified that under east-wind conditions, which intensify river intrusion into Lake Tegel, the influence times were the shortest. From the evaluation of the short-duration heavy rainfall scenarios it was recommended to reduce external nutrient loadings and to increase external discharges to mitigate the formation of phytoplankton blooms.

The water quality of Lake Tegel is controlled by the wind stress and external inflows. Therefore, the hydrodynamics are quite complicated and result in heterogeneity regarding the sediment composition. Such 'run-of-the-river' systems can be managed by modifying external discharges even further to reduce the lake hydraulic residence time, to weaken summer stratification stability as well as to mitigate the formation of phytoplankton blooms. Nonetheless, an elimination of external nutrient loadings in the PEP is the most important management measure to keep Lake Tegel in a clear-water state. The existing management system structure at Lake Tegel seems to be crucial in sustaining the lake's good ecological state and water quality especially regarding the projected impact of climate change. The numerical simulations showed that deactivating the management measures would affect Lake Tegel in manifold ways. Without the nutrient-low PEP loadings and additional discharges by the lake pipeline phytoplankton blooms will occur more often during the summer period, which will affect turbidity and water temperatures. Eventually, the thermal stratification is modified promoting the formation of cyanobacteria blooms. Also, short-duration heavy rainfall events will promote phytoplankton growth when nutrient-low PEP discharges were absent. An active and adaptive lake management that utilizes the PEP as well as the lake pipeline can keep the Lake Tegel ecosystem in a good and sustainable state and prevent a regime shift towards a turbid phytoplankton-dominated state.

Urban lakes are vulnerable aquatic ecosystems that have to be analyzed more thoroughly in the future to keep them in a sustainable and resilient state. Available and additional water quality and sediment quality data should be used to set up a sophisticated urban lake model solving the 3D Navier-Stokes equations coupled to a sediment diagenesis model. This will benefit the formulation of water management scenarios with focus on external nutrient loadings and their (potentially short) retention time in the lake system. Such a sophisticated urban lake model could then be used as part of an urban catchment model, which further incorporates river, wastewater treatment plant, groundwater, soil and urban runoff sub-models and can investigate future applied research questions regarding the urban 'Integrated Water Resources Management'.

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