- Charophytes grow attached to soft bottoms in ponds, streams, lakes and estuaries and are highly threatened throughout Europe according to the national Red Lists. We used Danish studies on freshwater charophyte distributions conducted around 1940 and repeated measurements during recent years to evaluate the historical development of species richness and dominance patterns. We also tested to what extent historical changes of species abundance in 29 waterbodies were related to landscape features, water quality and species traits.
- We found that three species of freshwater charophytes (Chara filiformis, Tolypella intricata and Nitella gracilis) have apparently disappeared from Denmark while one species (Chara connivens) has immigrated. National species richness has thus declined from 21 to 19 species.
- Species abundance based on occurrence in many waterbodies followed a linear rank–log abundance relationship both in the historical and the recent studies. The dominance structure was stronger today than historically as common species have become relatively more abundant and uncommon species relatively rarer.
- Among species traits, perenniality and preference for alkaline waters typical of deep-growing species in large alkaline lakes, a rare contemporary habitat, were significantly related to the historic species decline. Species increasing in abundance had wide tolerances to alkalinity and water nitrogen content.
- Twenty-nine lakes and ponds studied repeatedly showed a significant decline of mean species richness from 3.4 to 2.4 during the 70 years. A small increase in species richness has taken place during the recent 15–20 years in several lakes experiencing reduced nutrient loading. However, many species survive today in relict populations and may find it difficult to recolonise lakes in which water quality has improved.
- The historical decline of species richness was significantly related to higher nutrient concentrations, higher phytoplankton biomass and lower transparency of eutrophied waterbodies. In contrast, the amount of wetlands and openness of the landscape close to a waterbody did not predict the historical development perhaps because local processes or long-distance dispersal determine species richness.
- Considering together the loss of former freshwater habitats and the deterioration of surviving habitats, we conclude that charophyte occurrence has declined by about 56% in Denmark during the last 70 years. Species reductions determined from the reinvestigated waterbodies (29%) and the national species list (10%) are much lower and are less suitable measures of the developmental status of charophytes.