Plankton are microscopic organisms that drift through the world's oceans, lakes, and rivers, forming the foundation of nearly every aquatic food web on Earth. This book offers a straightforward introduction to what plankton are, how they live, and why they matter. You will learn about the main categories of plankton, including phytoplankton and zooplankton, and discover how these tiny organisms influence global oxygen production, carbon cycling, and the health of marine ecosystems. The book also explores the threats that climate change and pollution pose to plankton communities worldwide. Written and compiled with the assistance of artificial intelligence, this volume draws on established scientific knowledge to give you a clear and reliable overview of one of the most ecologically significant groups of organisms on the planet.