Diatoms and their consumer copepods obtained monthly through a time series of plankton tows in the Tsugaru Strait were analyzed. Counts of relative species abundance were conducted on diatom and copepod assemblages, and scores of non-metric multidimensional scaling (NMDS) were calculated for the assemblages using a matrix based on these counts. Three diatom assemblages corresponding to the seasonal blooms of Chaetoceros socialis, Chaetoceros debilis and Chaetoceros radicans; one diverse assemblage with common occurrences of Bacteriastrum hyalinum and Chaetoceros lorenzianus; and one assemblage formed mostly of large cell-size diatoms including a giant centric diatom Coscinodiscus wailesii are discerned in the NMDS. Consumers of diatoms, predominantly herbivorous copepods of Pseudocalanus and Clausocalanus species, commonly co-occurred with the last assemblage. Interpretations for the occurrence of this assemblage include a biased increase of C. wailesii in the water column and/or selective grazing by copepods on small cell-size diatoms leading to the assemblage being relatively dominated by large forms.
Stephanodiscus hitaensis H.Tanaka & Nagumo sp. nov., is characterized by one valve face fultoportula located in the middle of the valve center surrounded by other fultoportulae arranged in a circular pattern. Two, sometimes three, rimoportulae with external tubes are situated on the valve face/mantle junction at the end of interfascicles. Spines are thick, bent almost in half with the upper portion flat toward the valve. The new species is shown with LM and SEM photographs. The material was taken from the Lower to Middle Pleistocene lacustrine sediment of the Oyama Formation, Hita City, Oita Prefecture, Kyushu, Japan.
The Shirasawa Formation, one of three formations of the Akyu Group located in western Sendai City, Miyagi Prefecture, Japan, is a Late Miocene freshwater deposit. Samples from tuffacious mudstone of the Shirasawa Formation were collected from the northern Akyu site and the Imosawa River of the Okubushi site. We observed 39 diatom taxa belonging to 26 genera which were composed of 88–100% of centric diatoms and 0–12% pennate ones. The most abundant diatoms of the Akyu site were Aulacoseira distans (86–89%), followed by Mesodictyon yanagisawae (6–9%) and Aulacoseira praegranulata var. praeislandica f. praeislandica (2–4%). At the Okubushi site, Mesodictyon yanagisawae (1–88%), Aulacoseira subarctica (1–85%), A. distans (2–31%), A. praegranulata var. praeislandica f. praeislandica (0–26%), Staurosilla lapponica (1–7%), Aulacoseira pusilla (0–2%) and Gomphosphenia grovei (0–2%) were abundant or frequent. The five aboundant taxa from both sites are reported herein with their morphology and related information.
Lake Koyama-ike, located in Tottori Prefecture, western Japan, is a coastal brackish lake facing the Japan Sea. Since 1983, this lake has been characterized by artificial fluctuation of its lacustrine water quality. We collected lake bed surface sediments from 25 sites of the lake on July 10, 2010, corresponding to a period when the water salinity had been controlled to be in the range 150–1,000 mg/l. The diatom assemblages in the samples comprised a total 137 diatom species belonging to 38 genera. Among them, the major 11 species belonging to 8 genera, i.e., Aulacoseira granulata, A. ambigua, Cyclostephanos dubius, Cyclotella atomus complex (C. atomus var. atomus and C. atomus var. gracilis), C. meneghiniana, Cyclotella sp., Diploneis sp., Fallacia florinae, Lindavia praetermissa, Thalassiocyclus lucens, and Thalassiosira lacustris, occupied more than 70% of all samples. Results of the clustering analysis indicate that similar diatom assemblages were found at most sampling sites and their distribution pattern was unclear. We interpreted the axes of the principal component analysis as water salinity (axis 1) and distance from the lake's shore (axis 2), respectively. Totally, seven of the predominant diatom species, except for L. praetermissa, F. florinae, and two unidentified species (Cyclotella sp. and Diploneis sp.), were common in the periphytic diatom flora near the lake's shore.