The salinity index is computed from diatom assemblages. In order to clarify the correlation between salinity and salinity index, data on salinity measurements and diatom assemblages of 51 bottom sediments in five brackish lakes are investigated: Kamoko in Niigata, Matsukawaura in Fukushima, Takahokonuma and Obuchinuma in Aomori, and Akkeshiko in Hokkaido. A correlation coefficient of 0.74 and regression equation of y=0.0645x+1.907 (x: salinity, y: salinity index) are acquired for the bottom sediments. In 41 bottom sediments except outliers, a correlation coefficient of 0.98 and regression equation of y=0.0923x+1.1421 are acquired. These expressions indicate 1.94 to 3.84 and 1.19 to 3.91 in salinity index are equivalent to brackish water (salinity: 0.5–30‰), respectively.

bottom sediments, brackish lake, diatom assemblages, salinity, salinity index

We conducted a geo-slicer survey and taxonomic study of fossil diatoms in the Toberi River mouth area, Hokkaido, Japan, to clarify fossil diatom assemblages during the last millennium as background data for paleoenvironmental studies. We collected sediment samples and identified 185 diatom taxa belonging to 67 genera, though we were not able to identify 30 taxa at the species level. In the study area, two major diatom assemblage changes during the last millennium, were recognized. One occurred after the 19th century and could be attributed to anthropogenic factors, and the other may have been caused by tectonic movements associated with great earthquakes that occurred along the Kuril Trench in the 12–13th and 17th centuries.

The lower Pliocene interval of the marine Tentokuji Formation distributed near Nangai Dam, Daisen City, Akita Prefecture, northeastern Japan, was found to contain fossil lacustrine diatom Tertiariopsis costata along with an unknown Tertiariopsis taxon which in all probability was transported from inland lakes to marine environments. This paper describes this unknown taxon as Tertiariopsis undulata H.Tanaka & Yanagisawa sp. nov. The species exhibits the main characteristics of the genus Tertiariopsis: a sessile rimoportula on the mantle, mantle fultoportulae with three satellite pores, mantle fultoportulae and rimoportula covered by a marginal lamina, and distinct sections of mantle separated by hyaline strips. The new species, however, differs from other Tertiariopsis species in having a transversely undulate valve face. The species resembles T. costata as both species possess ridge-like thin mantle costae located on the inner mantle between the valve rim and mantle fultoportulae, but differ in valve face features: T. undulata has an undulated valve face, while T. costata is typified by an almost flat valve face.

Actinocyclus setanensis sp. nov., is described from an early Miocene freshwater sediment of the Futoro Formation located in Setana Town southwestern Hokkaido, Japan. The species has a relatively small round valve face with radial to vague fasciculate areolae rows, pseudonodulus located on valve face/mantle boundary and two to three (rare cases four) stalked rimoportulae, thick and bulged externally with small round openings. Within, we compare the differences between other related Miocene freshwater Actinocyclus taxa and propose it as a new species. The taxon is described with accompanying light and scanning electronic microscope photographs.

A fossil nonmarine Actinocyclus species, A. krebsii sp. nov., is described from a lower to middle Miocene diatomite in the Chojabaru Formation, Nagasaki Prefecture, western Japan. The new species is characterized by having a valve face with concentric undulation, a shallow mantle, non- or vaguely fasciculate areolae, hyaline stripes extending from the valve face to the mantle, rimoportulae born from distal ends of the hyaline stripes, and a single pseudonodulus. Size-dependent variations are concentric undulation, the development of internal labia of rimoportulae, and the number of hyaline stripes and rimoportulae. Size-independent variations occur at the valve face/mantle junction: external expression of a hyaline area and internal expression of the pseudonodulus. Among nonmarine species of Actinocyclus, A. krebsii belongs to the non-fasciculate group that is characterized by having relatively few large and closely packed (low density) areolae on the valve face. Within this group, A. krebsii is most similar to A. venenosus because they both have long hyaline stripes that extend from the valve face to the mantle. Actinocyclus krebsii, however, has a higher density of areolae on the valve face and longer hyaline stripes than A. venenosus. Additionally, A. krebsii does not have the well-developed hyaline valve/mantle junction and spines that characterize A. venenosus.