This is a research-focused book which presents theoretical and scientific researches in the field of marine ecosystems. The study of marine ecosystems is a vast subject of discussion, which includes various processes, groups of organisms and geographical peculiarities. The objective of this book is to present various significant topics which help in better understanding of the marine ecosystems, what they are, how they work and how we can model them in order to predict their behavior under changing conditions. The various topics covered in this book have been thoroughly reviewed and approved for publication. This book discusses a wide range of subjects ranging from threats to ultra-oligotrophic marine ecosystems and modelling the pelagic ecosystem dynamics in Northwest Mediterranean to the marine ecosystem of the Sub-Antarctic, Prince Edward Islands and meiofauna as a means for marine ecosystem biomonitoring. Other important topics covered in this book include chemical interactions in marine ecosystems, hydrocarbon contamination and the swimming nature of the estuarine copepod Eurytemora affinis, interactions between marine ecosystems and tourism on the Adriatic and Mediterranean. Also, several case studies on important issues like coral reef protection and seabed mapping have been covered in the book.