1. Charophytes are benthic macroalgae that live in fresh and brackish waters. Given the historic deterioration of
their habitats and their competitive inferiority relative to tall rooted plants under eutrophic conditions, it is
hypothesized that charophytes are among the most threatened autotrophs. Also, it is expected that charophytes
with generalist niches and functional traits, such as tolerance of a broad salinity range and large height, should
thrive owing to available habitats and stronger competitive ability.
2. These hypotheses were tested comparing the rarity of charophytes in the Nordic countries with other
autotrophs and analysing the relationship to species niche specialization and functional traits. A value on a
rarity–commonness scale was assigned to each species based on their Red List status. Niche specialization was
evaluated by the Outlying Mean Index using a large Danish dataset while traits were derived from the literature.
3. Supporting the hypotheses, 50–87% of charophyte species were Red Listed, which is much greater than for
vascular aquatic (30–35%) and terrestrial plants (18–28%). Commonness of charophytes decreased significantly
with niche specialization in separate analyses of brackish and freshwater sites. For brackish water species, the most
influential parameter determining species distribution was salinity while for freshwater species, alkalinity, lake size
and chlorophyll concentration played a major role. Four functional traits: shoot height, salinity tolerance, bulbil
production and flexible life cycle duration were significantly positively related to commonness.
4. In conclusion, charophytes contain a larger fraction of threatened species compared with other groups of
autotrophs. The few common charophytes are generalists tolerant of a wide range of conditions, while
specialists often have short stature, restricted life cycle variability and are rare in the disturbed contemporary
landscape. It is thus a conservation priority to delineate accurately the environmental conditions preferred by
the threatened species and protect or restore proper habitats.