Volume 21 of the series "Susswasserflora/Freshwater Flora of Central Europe" describes the aquatic fungi. The present Part 21/1 deals with freshwater lichens (symbiotic associations of a fungus with a photosynthetic partner, usually green algae or cyanobacteria), which can be found in most running waterbodies with clean water and stable bedrocks, but some species are also characteristic for moderately polluted rivers and lake margins. The freshwater lichen flora has a huge potential for bioindication of silting, bedrock stability, constancy of inundation and acidification. Many species are in need for conservation efforts, but due to the missing of appropiate tools which facilitate the determination of these organisms, they have only rarely been part of inventories of freshwater bioma so far. Many freshwater lichens are representatives of genera with many species, which have been considered as taxonomically highly problematic.

Intensive research efforts during the last decades have increased the knowledge on the species circumscriptions in most of these critical genera, but a modern synopsis of these new developments, including the results of molecular studies was still missing for Central Europe. With less than 100 species, freshwater lichens are a rather small but systematically highly heterogenous group of specialized fungi which konvergently adopted an amphibious lifestyle. They are representatives of several non related orders of Ascomycota. The heterogeneous origin of freshwater lichens made their determination even more difficult, due to the fact that the environmentalist had to use keys which include all species of a given area. These are not only the rather few taxa relevant for freshwater inventories but also a plentidud of terrestrial and epiphytic taxa which never will be met in freshwater habitats. Keys exclusively dedicated to the freshwater representatives were available only for selected genera and they are scattered over several publications with little distribution outside of the community of specialized lichenologists. A comprehensive flora of Central European freshwater lichens was missing completely.

This book is intended to fill that gap and to aid both the lichenologists as well as the non specialized environmentalist on his way to efficient and correct identification of the freshwater lichen flora of Central Europe. Detailed descriptions for all better known taxa of amphibious lichens as well as fotographs for most of the species are given and the keys include spezialized freshwater species as well as a selection of taxa with a wide ecological amplitude which are frequently found at the borders of streams and lakes.

In preparation is Part 21/2 covering the real fungi und fungi related organism."