Chara baltica Bruzelius 1824 and Chara intermedia A. Braun 1859—Distinct species or habitat specific modifications?

Species separation in charophytes is primarily based on vegetative morphology, but the taxonomic delineation at species level worldwide is inconsistent. Here we compare ecophysiological characteristics of selected Chara species with vegetative morphological traits and genetic data. Four populations of the Chara baltica–Chara intermedia species cluster from locations along a north-south gradient through Europe were investigated. Physiological differences indicate habitat-specific adaptations. The Baltic Sea population from Hiddensee was found to be separated from a freshwater river population close to Munich with respect to light and salinity acclimation capabilities. However, a population from a brackish Mediterranean pool near Montpellier and one from the continental brackish water lake “Salziger See” near Halle, Germany, show intermediate ecophysiological characteristics, suggesting a continuum. Genetic analyses using AFLP (amplified fragment length polymorphism) match the results of the physiological analyses. The individuals of the Mediterranean and the Salziger See neither cluster to the typical C. baltica from the Baltic Sea nor to the C. intermedia from the freshwater habitat, but instead take an intermediate position. These results are stable against the background of a larger AFLP dataset on charophytes. Morphological analysis revealed no distinct groups and we therefore conclude that the C. baltica–C. intermedia cluster forms a physiological, morphological and genetic continuum.