A key role of environmental management is to direct time and effort towards large environmental problems rather than towards small ones. To do this, it is crucial to have a system of criteria to define, structure, analyse and rank the problems. Subjective viewpoints are, evidently, not sufficient. Fully objective scientific methods and results are generally only applicable for specific substances in specific contexts, generally at finer scales (the cell, specific organs, and the like) but rarely at the ecosystem scale, which is the focus of this approach. The Potential Ecological Risk (PER) approach presented here is generic and could be applied to most types of ecosystems in most regions. The elements of the approach are: (1) definition of ecosystem boundaries, (2) definition of operational effect variables related to a defined chemical threat, (3) ecometric analysis (effect-load-sensitivity analysis), (4) integration (or summation) over impact area, and (5) integration (or summation) over impact time. Effect-load-sensitivity analysis is an important part of the PER approach. Case studies are used to provide examples for the major chemical threats to Swedish lakes and coastal ecosystems, acidification, eutrophication and toxic contamination of mercury, radiocaesium and chlorinated organics. Based on relative values, the ranking gives: Acidification of freshwater ecosystems (700) > eutrophication of coastal ecosystems (560) > eutrophication of lakes (300) > mercury contamination of lakes (240) > contamination of chlorinated organics of Swedish coastal areas in the Baltic (220) > radiocaesium contamination of lakes (70).

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