The various problems of freshwater pollution can be reduced by a wide spectrum of ecological engineering methods. Using the examples eutrophication and acidification, it was shown that water quality can, in principle, be controlled by (1) the reduction of external load and (2) lake-internal ecological processes and structures. The coupling of both strategies may accelerate the recovery of a lake or reservoir and reduce the costs. External techniques aim at the reduction of sources from the catchment by reduction of emissions and the establishment or reactivation of buffer systems, as well as chemical and biological purification of inflows. Internal measures reduce undesired symptoms or optimize the biological, chemical, or physical structure to make the system more resistant against pollution. Measures of ecological engineering can be defined as the large-scale utilization of ecological principles via controlled intervention that changes the ecosystem towards a certain management target. The nutrient concentration of eutrophied lakes can be decreased by chemical inactivation and increasing export of nutrients. Alternatively, the biomass of algae can be reduced by food web manipulation and by artificial destratification controlling the underwater light climate. Abatement of acidification is realized by the application of chemical and biological in-lake measures. Before selecting a lake-restoration measure, the main problems of the lake need to be identified, the goal of the measure should be defined, and the probability of success must be estimated. The development of sustainable targets for lakes and reservoirs should consider the present anthropogenic use and colonization ofthe landscape, the current knowledge of ecology, and ongoing technical progress.

See also: Acidification; Anthropospheric and Anthropogenic Impact on the Biosphere; Atmospheric Deposition; Bifurcation; Biological Wastewater Treatment Systems; Food Chains and Food Webs; Hysteresis; Constructed Wetlands, Subsurface Flow; Lake Models; Resilience; Sediment Retention and Release.

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