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Lakes are known to be strongly affected by climate change as a result of their sensitivity to atmospheric forcing. The combined response of urban lakes to climatic changes and to urbanization of the catchment may be further altered by water quality management measures. We studied Lake Tegel in Berlin, Germany as a representative urban lake profoundly influenced by intense water management measures and a resulting complex hydrodynamic situation: Lake Tegel is fed by nutrient-rich river discharges and effluents from a phosphorus elimination plant (PEP). We estimated changes in water temperatures, the Wedderburn number, and the buoyancy frequency, as well as concentrations of dissolved oxygen and phosphate under climate change using a one-dimensional vertical hydrodynamic model coupled to a water quality model. Further, we investigated how four management scenarios with varying discharges of the PEP could affect the lake system. In all simulations, water temperatures increased and summer stratification extended. The modeling results demonstrated that the water management system buffers the high nutrient supply from the river inflow and can effectively mitigate negative effects of climate change on Lake Tegel, thus highlighting its importance for the lake ecosystem. [ABSTRACT FROM AUTHOR]

Climate forecasts project a global increase in extreme weather events, but information on the consequences for ecosystems is scarce. Of particular significance for lakes are severe storms that can influence biogeochemical processes and biological communities by disrupting the vertical thermal structure during periods of stratification. An exceptional storm passing over northern Germany in July 2011 provided an opportunity to assess the consequences and underlying mechanisms of such extreme events on the interplay between the physics and ecological characteristics of a deep, nutrient-poor lake. Wind speeds were among the most extreme on record. A suite of variables measured throughout the event consistently indicates that a cascade of processes pushed the clear-water lake into an exceptionally turbid state. Specifically, thermocline deepening by the storm-entrained cyanobacteria of a deep chlorophyll maximum located at about 8 m depth into the surface mixed layer. Released from light limitation, intense photosynthesis of the cyanobacteria boosted primary production, increased algal biomass, raised the pH and thus induced massive calcite precipitation to a level never observed within three decades of lake monitoring. As a consequence, water transparency dropped from 6.5 to 2.1 m, the minimum on record for 40 years, and the euphotic zone shrank by about 8 m for several weeks. These results show that cyanobacterial blooms not only are promoted by climate warming, but can also be triggered by extreme storms. Clear-water lakes developing a deep chlorophyll maximum appear to be particularly at risk in the future, if such events become more intense or frequent. [ABSTRACT FROM AUTHOR]

The aim was to disclose the seasonality in the internal seiches during summer stratification in 2010 within the small, 3 km long, ellipse-shaped, temperate Lake Arendsee (Germany) with simple morphometry. We used observations of temperatures from two thermistor chains (09 April to 24 September), conductivity and temperature profiles, and wind speeds and directions (January to December). To analyze the seasonality in the periods, appearance and modal structure of the seiches, and the wind forcing, we applied spectral analysis on monthly time series of isotherms and wind speeds, and wavelet analysis on 6 month long time series of integrated potential energy and wind speeds. To determine the effect of earth's rotation on seiches, we applied two analytical models based on three-layer density stratification. Earth's rotation affected internal seiches significantly even in a small lake. That was shown by the models and the Burger number being < 1 during stratification. Vertical mode 1 Kelvin-type seiches dominated the internal seiche weather in Lake Arendsee, with their periods changing because of seasonal variations in stratification. Weak, free seiches of vertical mode 2 were amplified during three episodes: (1) in mid-summer, when their periods coincided with the inertial period at the lake's latitude, (2) in late spring, and (3) in autumn, both when their periods approached 24 h. The amplifications were caused by resonance between free seiches and wind-forced oscillations at diurnal and inertial periods. Such short-term resonance events, associated with enhanced mixing, are inherent for the seasonal stratification cycle in the most temperate lakes. [ABSTRACT FROM AUTHOR]

Abstract: Lake Tegel (Berlin, Germany) is controlled by two main inflows: inflow #1 (River Havel) is heavily phosphorus-laden, whereas inflow #2 is an artificial confluence that includes discharge from a municipal wastewater treatment plant distinguished by high levels of phosphorus and pharmaceuticals. To reduce the phosphorus load on the lake, a phosphorus elimination plant (PEP) is situated at inflow #2. Moreover, the two inflows are short-circuited by a pipeline that transfers part of the inflow #1 water to the PEP and finally releases it into inflow #2. The pipeline and the PEP have contributed to a continuous reduction in the total phosphorus concentration of Lake Tegel in the past 25 years. We investigate the question of whether the existing lake pipeline can also be used to reduce the amount of pharmaceuticals in Lake Tegel originating from inflow #2 by dilution with water from River Havel, by diverting part of inflow #2 around the lake, or by a combination of both strategies. The circulation pattern of Lake Tegel is complicated by complex bathymetry and numerous islands and is therefore highly sensitive to winds. We tested seven different management scenarios by hydrodynamic modeling for a period of 16 years with the two-dimensional version of the Princeton Ocean Model (POM). None of the scenarios provided a strategy optimal for both pharmaceuticals and phosphorus. Nonetheless, compound regimes, such as alternating the pipe flow direction or adding another pipeline, allowed the most abundant pharmaceutical (carbamazepine) to be reduced while maintaining the current phosphorus level. This study demonstrates the ability of immediate lake regulation measures to maintain water quality. In the case of Lake Tegel, the pipeline can be fully effective with regard to pharmaceuticals only in combination with additional efforts such as advanced pharmaceutical treatment of wastewater and/or phosphorus reduction in the River Havel catchment. [Copyright &y& Elsevier]

Prediction of mixing intensity of water masses in riverine Lake Tegel (Berlin, Germany) can be used to trace the fate of pollutants that enter the lake through several inflows. Because the contributions of each inflow have not yet been quantified and because the lake features complex bathymetry and numerous islands, a hydrodynamic circulation model with high spatial resolution and dynamic wind forcing is useful. We applied the two-dimensional version of the Princeton Ocean Model to separate the influences of wind and river discharge on the currents and mass transport in Lake Tegel. For model validation, we compared the simulation results with 1 year of electrical conductivity data, which was used as a conservative tracer to distinguish between water from the River Havel and water supplied by a smaller second inflow. Calculation of currents alone is insufficient to investigate water exchanges between rivers and lakes, especially when several islands create multiple pathways for river intrusion. Therefore, mass transport simulations are applied. Our calculations based on archetypical scenarios indicate that the proportion of (polluted) water from the River Havel in the main basin of Lake Tegel fluctuates with river discharge and wind, which either amplify or neutralize each other. [ABSTRACT FROM AUTHOR]

Rising northern hemispheric mean air temperatures reduce the amount of winter lake ice. These changes in lake ice cover must be understood in terms of resulting effects on lake ecosystems. Accurate predictions of lake ice phenology are essential to assess resulting impact. We applied the one-dimensional physical lake model FLake to analyse past variability in ice cover timing, intensity and duration of Berlin-Brandenburg lakes. The observed ice phenology in two lakes in the period 1961-2007 was reconstructed by FLake reasonably well and with higher accuracy than by state-of-the-art linear regression models. Additional modelling results of FLake for 38 Berlin-Brandenburg lakes, observed in the winter of 2008/09, were quite satisfactory and adequately reproduced the effects of varying lake morphology and trophic state. Observations and model results showed that deeper and clearer lakes had more ice-free winters, later ice cover freezing and earlier ice cover thawing dates, resulting in shorter ice-covered periods and fewer ice-covered days than shallow and less clear lakes. The 1947-2007 model hindcasts were implemented using FLake for eight Berlin-Brandenburg lakes without ice phenology observations. Results demonstrated past trends of later ice start and earlier ice end, shorter ice cover duration and an increase in ice-free winters. [ABSTRACT FROM AUTHOR]

Using the carcass sinking rate and density determined in laboratory for several freshwater zooplankton species, we developed a model of zooplankton carcass sinking as affected by turbulence and stratification. The model was subsequently used to estimate the residence time of zooplankton carcasses in the water column of Lake Stechlin, a typical temperate dimictic lake in northeastern Germany. The residence time varied among the different species and was strongly affected by thermal stratification. At the peak of summer stratification, the carcasses stayed up to 5d in the 70m-deep water column before reaching the lake bottom. Residence time was long enough that zooplankton carcasses could serve as an important matter and energy source for bacteria in the lake's pelagic zone and hence have the potential to significantly affect aquatic carbon and nutrient cycling. The proposed model of sinking rates, based on physically sound relationships, can be easily applied to other passively sinking particles, and be integrated into large ecosystem models. [ABSTRACT FROM AUTHOR]