Marine phytoplankton forms the basis of the food web in the oceans. Phytoplankton, although small enough to be invisible to the naked eye, can under favourable conditions actually be seen from a space satellite, because it occurs in such huge quantities. According to NASA, phytoplankton produces between 50% and 90% of the oxygen in the air that we breathe, depending on the season. Conversely, phytoplankton consumes most of the atmospheric CO2, and so contributes greatly to maintaining a balanced ecosystem which is essential for all life and a healthy planet. More than 99% of all creatures inhabiting the oceans depend, either directly or indirectly, on phytoplankton for their survival. This book provides a key to determine almost 300 phytoplankton species from the North Sea around Helgoland and Sylt, documenting them with close to 1100 images and 70 line drawings on 85 plates. This book is an important contribution to our unterstanding of marine phytoplankton of North Sea ecosystems.