AbstractThe earliest records of marine macroalgae from Helgoland (German Bight, North Sea) are known from the middle of the 19th century. Since then 274 marine macroalgal species have been reported: 77 species of Chlorophycota; 100 species of Phaeophycota; and 97 species of Rhodophycota. Additionally 11 species were only recorded as drift and 51 species as doubtful for Helgoland. Remains of the herbarium of Paul Kuckuck, the first curator for botany at the Helgoland Biological Station between 1892 and 1914, are still located there and consist of 173 macroalgal species from Helgoland. On comparing this 100 year-old herbarium and other old sources with recent macroalgal records it became clear that changes in species composition have occurred. After the II. World War several species were not found again such as Arthrocladia villosa, Corynophlaea crispa, Cutleria multifida, Eudesme virescens, Mesogloia vermiculata, Sporochnus pedunculatus, Antithamnion cruciatum, Apoglossum ruscifolium, Chondria dasyphylla, Helminthora divaricata, Jania rubens and Osmundea ramosissima. Other species became very rare such as Dictyota dichotoma, Leathesia difformis, Stictyosiphon soriferus, Helminthocladia calvadosii, Scinaia furcellata. Significantly, perhaps, most of these species comprise a heteromorphic life history with the appearance of the macroscopic phase restricted to (spring and) summer. Many new species of green algae were recorded for Helgoland after 1959 due to new substrata and the research activites of Peter Kornmann, curator for botany after 1959 and Paul-Heinz Sahling his technical assistant. Introductions of species during the considered time period were: Bonnemaisonia hamifera, Codium fragile, Mastocarpus stellatus and Sargassum muticum. Type material of the following species is located at The Marine Biological Station at Helgoland: Mikrosyphar porphyrae, Porphyra insolita, Ulva tenera.