Fragmentation and separation of diatom valves in modern shallow lake sediments: a case study in Lake Kitaura, Japan

We investigated diatom-valve fragmentation and frustule separation in surface sediments in Lake Kitaura, a shallow lake in Japan, to clarify how sedimentation processes affect diatom assemblages. Twenty-two surface-sediment samples were collected from Lake Kitaura in 2014. Aulacoseira spp. were characteristic planktonic taxa, and Cocconeis placentula and Planothidium lanceolatum were characteristic benthic taxa, both with monoraphid valves. The fragmentation ratios of Aulacoseira spp., the most abundant taxon, were higher nearshore than in the central part of the lake. Cocconeis placentula, an epiphytic species, was widely distributed in Lake Kitaura, and its fragmentation ratios increased with distance from the lakeshore. The coexistence ratios of the two valve types of this species decreased with distance from the lakeshore and from macrophytic vegetation. Planothidium lanceolatum, a fluvial benthic species, was abundant at the three northernmost sites near a river mouth, and the frustules of this species were seldom separated. These results suggest that long-distance transport promotes fragmentation and separation of benthic diatom valves in shallow lakes.