Species and synonym list of the German marine macroalgae based on historical and recent records

A summary of all macroalgal records to occur in the German water bodies of the North Sea and the Baltic Sea during the last 190 years are presented, with their synonymes. All available literature and herbarium collections were revised and a potential species list was compiled. Therefore, the list reflects the sum of all records between 1819 and 2008 and not the actual presence of macroalgae along the German coast. 469 species have been recorded since 1819 including insecure registrations and single observations. Species richness decreases rapidly from the subtidal areas and the rocky shore of Helgoland to the brackish waters of the Baltic Sea with its often instable sediments. 322 species have been registered for Helgoland whereas the Western and Eastern part of the German Baltic Sea does not contain more than 211 and 181 species respectively. For the North Frisian Wadden Sea and the East Frisian Wadden Sea, 113 and 112 species have been registered, respectively. In relation to species number red algae are most abundant around the island of Helgoland, followed by brown and green algae. In the Frisian Wadden Sea green algae are most common, whereas in the Baltic Sea filamentous brown algae gain in importance. The presence of neophytes increased during the last decades, but total number does not exceed 10 species. The red algae Callithamnion teragonum, Halurus flosculosus and Neosiphonia harveyi are the latest new species records from Helgoland whereas Gracilaria vermiculophylla is the latest new species recorded in the German Wadden Sea and the Baltic Sea since 2004.