The existence of light-dependent (zooxanthellate) corals at depths between 40 to 150 meters (sometimes up to 300 meters) has been first reported back into the nineteenth century and they are specificyally called mesophotic coral reef (MCR) ecosystems. Their systematic study began about 50 years ago, but just in the last decade, mainly due to the development of new diving techniques, these deep coral reefs have been extensively studied.
At the same time from the Mediterranean Sea new communities in these depths - the twilight zone - are described consistently. For example, a veritable underwater forest of black corals (Antipathella subpinnata) was recently discovered from the Adriatic Sea (Tremiti Islands) between 52 and 80 meters of water depth.
A description of biotic communities in the twilight zone in freshwater systems, such as on steep walls, is searched in vain in the relevant literature. Detailed observations and characterization of mesophotic, almost lightless communities,
e. g. of the steep walls in Lake Constance between 40 and 150 meters, are missing so far.
We present first dives into these area (40–100 meters) using the example of the steep walls of Lake Constance (Überlinger See), describe the necessary diving and sampling techniques and skills.
To our knowledge, these are the first scientific dives into these depth ranges in freshwater.