Some top predators in the sea are affected by the bioavailability of harmful substances (e.g. pesticides, heavy metals, and hydrocarbons) or increased deficiency syndromes that are thought to be caused by certain molecules. These substances are produced by bacteria and phytoplankton and are transferred by zooplankton to fish and birds. The overall aim of this book is to help readers to understand the factors regulating trophic levels in Photic and Aphotic Zones and to explore possible links to ecology and physiology of zooplanktons.