SummaryIn aquatic hazard assessment used for extrapolations over entire ecosystems or water bodies, the environmental gradients which greatly modify the fate and effects of chemical substances in aquatic communities (marine, estuarine, brackish-water, as well as limnic) must be known. The Baltic Sea, in spite of being a geographically very limited water basin, is characterized by a great variety of hydrographical and biological gradients. Thus this cmc1osed, brackishwater sea is used as an overall example of the problems that arise when interpreting ecotoxicological data, from the "eco" point of view, in a variable environment. The aim is to elucidate problems in identifying species and subsystems at risk, as well as differences along the natural gradients, in patterns of response to exposure at population and ecosystem level. Consequently, toxicity gradients are formed from the south to the north, and, in a permanently stratified body of water, vertical differences in relative toxicity (toxiclines) of e. g., heavy metals can be determined. Possible test strategies, and in particular their field validation, are discussed in relation to the environmental gradients described, and to different concepts of community stress and recovery.
IntroductionAmong aquatic ecotoxicologists the general difficulties in applying the results of small-scale laboratory testing of harmful substances to an "ecological reality" have been repeatedly discussed. The purpose of this paper is to visualize how some of these difficulties are manifested within a single: body of water, the Baltic Sea. The same problem is also valid for other bodies of water, e. g., inland waters. Thus many of the viewpoints presented couldl be illustrated with examples from lakes (e. g., along the oligotrophy-eutrophy gradient, or in relation to continuing acidification) or running waters in northern latitudes. We have selected the Baltic Sea as an example of this complex problem, for several reasons, however. Firstly, it represents an enclosed sea surrounded by seven nations with very different effiuents to the Baltic, and secondly, it is one of the best-studied large aquatic ecosystems, covering gradients of vital importance to the understanding of toxicity both in marine and limnic environments.The Baltic Sea covers no more than 0.1% of the total area of the world ocean, but it is large and complicated enough to offer a good framework for a L. Landner (ed.), Chemicals in the Aquatic Environment