The planktonic diatom diversity of ancient Lake Hovsgol, Mongolia

Lake Hovsgol (51degreesN, 101degreesE) is Mongolia's largest lake, over 1.6 million years old, and the southernmost extant lake in the 25 million years old Baikal Rift Zone. In spite of their proximity and geological relationship, Lake Baikal and Lake Hovsgol share no common offshore planktonic diatoms, though their planktonic communities are both characterized by species-poor richness and probable endemism. Lake Hovsgol's phytoplankton has been studied sparsely for nearly 100 years; however, a modern taxonomic and systematic treatment has not been undertaken. Vertical net-tow plankton samples collected in summer 1998 from throughout the lake showed that the phytoplankton is dominated by chlorococcalean green algae, nanoplanktonic cyanobacteria, and six predominant diatom species: a large-celled race of Asterionella formosa, the epizooic Synedra cyclopum, two new Stephanodiscus species, Cyclotella sibirica, and a complex of forms identified as C. ocellata. Some taxa may be endemic to Lake Hovsgol or restricted in distribution to lakes in and near the Baikal Rift Zone. Stephanodiscus mongolicus Edlund, Soninkhishig & R.M. Williams sp. nov. is characterized by a spine and marginal fultoportula at each interfascicle, one rimoportula, and no central fultoportulae. Stephanodiscus jamsranii Edlund, Soninkhishig & R.M. Williams sp. nov. has a spine on each interfascicle, one rimoportula, and heterovalvy expressed in valve form and placement of central fultoportulae. Cyclotella ocellata contains a complex of forms. Evidence of near species-level separation is a polymodal distribution of postsexual initial cell sizes, a larger size range of initial and vegetative cells than is known for C ocellata, notably at the taxon's upper size spectrum, and persistence of these size distributions. The complex may represent a species flock that has developed reproductive separation but has not yet clearly separated along morphological lines. Alternatively, there may be a life history mechanism that allows stepwise auxosporulation or multiple vegetative cell sizes to enter sexuality and produce size-restored cell lines containing a wider range of cell sizes than previously reported for C. ocellata.