An easy language book on Aquatic ecosystem is very necessary to know aquatic ecosystem in easy way. The present book is a preliminary attempt to meet this need in a small way. I tried to make it easy to explain every important part of aquatic ecosystem e.g., different zones, animals and plants, types of aquatic ecosystem, biotic and abiotic factors of aquatic ecosystem. I hope it will be helpful for many student and researchers in whole world to attempt similar keys to understand aquatic environment in an easy way.