Book excerpt: Aquatic Ecosystems explains the interplay between various movements of matter and energy through ecosystems mediated by Dissolved Organic Matter. This book provides information on how much DOM there is in a particular aquatic ecosystem and where it originates. It explains whether the DOM composition varies from time to time and place to place. It also details how DOM becomes incorporated into microbial food webs, and gives a better, clarifying, understanding to its significance of DOM. There are many ways to study DOM and this book focuses on several central questions: How much DOM is there in a particular aquatic ecosytem? Where does it come from? Does the composition of the DOM vary from time to time and place to palce? How does DOM become incorporated into microbial food webs, which are the basis of plant, invertebrate and vertebrate food webs? How can the answers to these and other questions about DOM be considered together so that a better understanding of the significance of DOM can emerge?