So far lake management is found on criteria derived from the knowledge of how lake ecosystems respond to the impact of single pressures. However, today many lakes suffer from the impact of several pressures, which act simultaneously and interact in a complex manner influencing all levels of the ecosystem. In such cases, the ecological status is varying according to the sensitivity of lake ecosystems and the combination of pressures.

Lake management is further complicated because the reduction/increase of one pressure may alter the fate of the other. Thus, it is necessary to understand the interactive mechanisms between the pressures and the internal ecosystem factors that may influence these mechanisms, e.g., in the case of eutrophication and chemical contamination the mechanisms that may alter bioavailability and the fate of contaminants need to be considered. In this example, the implementation of measures to reduce nutrients would potentially improve the trophic status but could also lead to an increase of contaminant concentrations in the system and higher bioaccumulation of toxicants in the pelagic biota. Improved oxygen conditions at the bottom may lead to recolonization of sediments by benthic fauna but as a result of bioturbation and geochemical processes, contaminants that were previously trapped in the sediments may be mobilized. The level of contaminants may, vice versa, influence the primary production and indirectly the status of eutrophication.

The impact from combined pressures is generally assessed through the response of biological groups such as fish or macroinvertebrates that are sensitive to several pressures, e.g., benthic invertebrates respond sensitively to a number of human impacts (hydrological, climatological, morphological, navigational, recreational, and others) and could potentially be used for a holistic indication of lake ecosystem health. In general, single biological group multimetric indices composed of several metrics sensitive to different pressures are combined in a unique index integrating the response to a wide range of alterations are but of limited use in management.

In summary, the response of freshwater organism groups to single stressors has been described in numerous studies but there are only a limited number of studies on the response of organism groups to multiple-stress situations. Thus, there is a general need to quantify these relationships for the improvement of the current lake ecological models and to support lake management.

In the absence of such tools and to resolve the problem, managers may need to define a hierarchy among the pressures to identify priority actions in the appropriate geographical scale.

The appropriate unit of management of water quality is at watershed level. It is within this context that indicator's criteria relating to single pressures and general pressure indicator's criteria in lakes should be incorporated into a decision support system to allow interpretation regarding their influence on the structure and functioning of lake ecosystems. The efficacy of any restorative or conservative management measures to achieve the goal of sustainable catchment and lake use should be reviewed periodically. The usefulness of criteria should be reevaluated both within context of their ability to reflect pressure and also their relevance to lake biota.

See also: Effects of Climate Change on Lakes; Mixing Dynamics in Lakes Across Climatic Zones.

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