Lakes are of importance for their heritage, ecological, and aesthetical value; however, current management strategies regard lakes primarily as a resource. They have been and are at present mainly used for drinking water supply, fishing, irrigation, waste disposal, industrial processes and cooling, transportation, hydroelectric power generation, and for recreation. These uses and further anthropogenic activities in the catchments of lakes have caused an enormous impact on these ecosystems. Consequently, many lakes worldwide are currently threatened by many types of pollution and suffer from several ecological disturbances.

Effective management and protection of lakes are therefore of critical importance for their preservation as both a heritage asset and also as a beneficial resource for humans. Management of water quality in lakes has been an important issue worldwide during the past decades and important improvements have been reached. Yet, the goal of sustainable management is still not being achieved for many lakes. In part, this is explained by the complex nature of lakes requiring consideration of multiple interacting factors by managers, such as climate, catchment characteristics, lake morphometry, stratification patterns as well as the biota present. Currently, it is recognized that sustainable management is possible only as part of a comprehensive ecosystem river basin management strategy.

Management Principles

Management as prescribed by legislation was in the past founded on the regulation of emissions at their source through the establishment of emission limit values or by the setting of chemical quality standards for particular uses of water (Table 1 ). In both cases the aim was to ensure the continued use of lakes as a resource. However, these approaches have proved not to be sufficient for the protection of ecological integrity.

More recently, attention has turned to the establishment of ecological objectives for these ecosystems. There are several approaches to ecosystem management, but generally the common goal is to restore and sustain ecosystem functions based on the assumption that provided the ecosystem is protected most uses are also protected. The core concepts of ecosystem management are as follows:

1. assessment of status expressed as quality of composition, structure and functioning of the ecosystem components;

2. definition of reference baseline conditions for ecosystem functioning against which a measured deviation can be evaluated.

The European Water Framework Directive is an example of a recent legislation on water management that is based on these two core concepts.

Pressures and Drivers

Industrial emissions, sewage, agricultural run-off, hydrological modifications, and human induced climate changes are the major drivers of pressure on lakes. These are responsible for the six major pressures affecting lakes today:

1. excessive inputs of nutrients leading to eutrophi-cation

2. hydromorphological modifications

3. acidification

4. alien species

5. hazardous substances

6. climate changes.

Effective management requires a focus on criteria indicative of the ecosystem response to such pressures. Having a comprehensive suite of criteria at a lake manager's disposal will allow the evaluation of the extent of pressure affecting a lake and its influence on the ecosystem's structure and functioning. Such criteria form an important input to the river basin management plan.

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