Aquatic ecosystem refers to the ecosystem of a waterbody and encompasses both marine and freshwater ecosystems. Marine ecosystems encompass oceanic ecosystems as well as ecosystems of salt marshes, estuaries, lagoons, mangroves, and coral reefs. It may differ both in terms of salinity and biodiversity from freshwater ecosystems, which encompass wetland, river, lake, and pond ecosystems. Many abiotic factors determine the biodiversity of such ecosystems, such as salinity, nutrient levels, the amount of dissolved oxygen, etc. Moreover, biotic factors such as predation by coastal herbivores, including mammals, snails, and geese as well as mechanisms of competition and mutualism influence the biodiversity of aquatic ecosystems. This book attempts to understand aquatic ecosystems and aquatic biodiversity and how different factors impact them. Different approaches, evaluations, methodologies and advanced studies on these domains have been included herein. This book is a resource guide for experts as well as students.