Hazard assessment of a compound (xenobiotic) discharged to the aquatic environment requires data on both exposure and effects to various components of the ecosystem. The multitude of ecological gradients in the Baltic Sea is used as a background example for discussing the complexity of the issue and the need for new approaches. Therefore, this book attempts to go beyond the simplistic, standardized short-term laboratory tests traditionally used as a basis for hazard assessment of chemicals, and gives strong emphasis to the interpretation of ecotoxicological data in their real, ecological context, pointing out the need to consider the natural mortality distribution of the population under study, the role of keystone species and of species with broad ecological niches versus those with narrow, specialized niches.

I Background.- 1. Introduction.- 2. Ecosystem Variability and Gradients. Examples from the Baltic Sea as a Background for Hazard Assessment.- 3 The ESTHER Approach to Environmental Hazard Assessment of Chemicals.- II Special Topics.- 4 Factors Determining the Fate of Organic Chemicals in the Environment: the Role of Bacterial Transformations and Binding to Sediments.- 5 Bioavailability and Uptake of Xenobiotics in Fish.- 6 Bioaccumulation and Biomagnification of Hydrophobic Persistent Compounds as Exemplified by Hexachlorobenzene.- 7. Fish Bile Analysis for Monitoring of Low Concentrations of Polar Xenobiotics in Water.- 8 Ecological Concepts Important for the Interpretation of Effects of Chemicals on Aquatic Systems.- 9 Selected Assays for Health Status in Natural Fish Populations.- 10 Community Testing, Microcosm and Mesocosm Experiments: Ecotoxicological Tools with High Ecological Realism.- III Case Studies.- 11 Advanced Hazard Assessment of Arsenic in the Swedish Environment.- 12 Advanced Hazard Assessment of 4,5,6-Trichloroguaiacol in the Swedish Environment.- 13 A Tentative Hazard Assessment of Hexachlorobenzene in the Aquatic Environment.- IV Conclusions.- 14 Concluding Remarks.- Appendix I.- Species and Genera Index.