In this book, the authors present topical research in the study of aquatic ecosystems of which there are two main types: marine ecosystems and freshwater ecosystems. Topics discussed include a change of biodiversity in subarctic freshwater ecosystems located in the northwest region of Russia; assemblages of zoosporic true fungi, heterotrophic straminipiles and plasmodiophorids in freshwater ecosystems; aquatic ecosystem health and the causes, effects and rehabilitation of the eutrophication of aquatic ecosystems.