Diversity and conservation of desmids in Bornholm, Denmark – revisiting after 130 years

Abstract

Desmids are unicellular green algae dominating habitats that are vulnerable and highly threatened, like raised bogs, mires and fens. Here, we studied the desmid diversity in four different localities on the Danish island Bornholm. A total of 186 species, 58 of which not previously recorded from Denmark, were observed. The species composition of the locality ‘Kohullet’ was compared to more than 130 year‐old records. In the 1880s this locality contained 65 species vs 70 in in the present study. The species composition has changed towards more oligo‐mesotrophic and more acidophilic species. The reasons for these changes may be the 1960‒1970s’ increase in atmospheric deposition of nitrous‐ and sulphurous oxides leading to a pH‐decrease, and the possible use of Kohullet in the 1880s for cattle drinking leading to more mesotrophic conditions than present days conditions. We used an empirical system developed by Coesel for classifying the trophic state or ‘conservation value’ of freshwater habitats using desmids as indicator organisms. All localities studied had maximum scores, i.e. represented highly structured and well‐balanced ecosystems. Thirty of the recorded species were potential Red List species, using the Dutch Red List criteria for desmids. We suggest that Kohullet should be considered an Important Plant Area (IPA), as it has a high diversity of desmids, a long history of algal records including desmids and may be considered a threatened habitat of major significance to desmids. Furthermore, it is suggested that several desmids are considered Red List species.