Records of freshwater diatom fossils over several tens of thousands of years are incomparable because the time interval is much longer than the life span of common lakes. Sediment cores containing such long diatom records have been taken by the International Continental Scientific Drilling Program which incidentally suggests that global freshwater diatom biogeography became synchronized with glacial-interglacial cycles. In a local diatom community, abrupt extinction and arbitrary immigration instead of gradual and repetitive changes were observed. Diatoms changed their dominance in a lake not only as a result of changes in local environmental factors, but their geographic distribution controlled by regional community structure and limited dispersal of each species. Argument against ubiquitous dispersal arose in the last few decades based on comprehensive researches during the 1990s on diatoms as environmental indicators. Insights into diatom biogeography are required to interpret fossil diatom flora, especially those with large geographic distributions and longtime ranges such as glacial-interglacial cycles.

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This paper reviewed diatom-based environmental reconstructions in coastal areas and upgraded so-called “environmental indicators.” After the definition of diatom species as environmental indicators by Kosugi (1988), many papers have referred to them when reconstructing paleo-environments by fossil diatom assemblages. However, in contrast, no one has upgraded them in the subsequent decades. Here we reorganized the environmental indicators following the recent flora work around the Japanese coast. The environmental indicators are upgraded as follows: common brackish species and “brackish water algae or plants (C2)” were removed from the environmental indicators. Eight taxa in “open sea (A)”, five taxa in “inner bay (B)”, two taxa in “marine water algae or plants (C1)”, four taxa in “marine water sand flat (D1)”, eight taxa in “marine water mud flat (E1), one taxon in “brackish water mud flat (E2)”, nine taxa in “lake (M)” and fourteen taxa in “high moor (P)” were added to the new list of environmental indicators.

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The establishment of sea-level index points is essential to reconstruct sea-level changes. Diatom analysis of sediments has been applied to Holocene sea-level studies. In Japan, changes in diatom assemblages during a transition in sedimentary facies from marine to freshwater have been recognized to be the upper limit of marine facies, which provides direct evidence for the marine limit. In this paper, indicative meaning of the marine limit was reassessed, and was interpreted as a sea-level index point formed at a tidal level of mean high water spring. The horizon, at which autochthonous intertidal diatoms are present in great abundance, can yield a sea level index point along with its elevation and depositional age. Pseudopodosira kosugii is known to be a useful indicator for identifying past sea levels during the Holocene. Along most of the Japanese coast, this species occurs abundantly in the successive diatom assemblages of a sedimentary sequence formed as a result of the Holocene marine transgression (‘Jomon’ transgression). Past sea-level reconstructions using intertidal diatom indicators (P. kosugii and Terpsinoë americana) were made for the Tofutsu core collected in Hokkaido. A brief account of a diatom-inferred sea-level change during the last 8,000 years was also shown for the Harimanada coast on the border of the northeastern Seto Inland Sea.

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Studies on diatoms as indicators of the past ocean environment are reviewed from two points of view. One is from diatom temperature indices derived from the dependence of diatom production on SSTs (sea surface temperatures), and the other is from the latitudinal shift of surface-water fronts inferred from changes in the biogeography of diatom species. By taking mainly two indices Td′ and Twt ratios as examples, their usefulness for paleo-SSTs reconstructions from Pliocene-Holocene cores is discussed, and by taking Oligocene-Holocene biogeography of a cosmopolitan diatom species Thalassionema nitzschioides sensu lato as an example, its usefulness as a proxy for delineating oceanic fronts is suggested.

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This paper reviews the recent usage of diatom fossil analysis as an aid for paleoseismology. In tsunami geology, preliminary identification of tsunami deposits is often based on finding anomalous sand deposits in peaty or muddy deposits. The anomalous layers are then diagnosed using several criteria such as regional-scale inundation, grading of the particle size, and floral and faunal fossils within the depos­its. During the interpretation of the deposits, marine diatoms are utilized to eliminate other processes such as flood. To understand the diatom assemblages within the anomalous deposits, many papers have described modern, historical, and prehistoric tsunami deposits. In the early stage of such a study, diatoms within tsunami deposits were regarded as chaotic assemblages that had been composed of broken valves. However, in recent studies such as those of the 2004 Sumatra and 2011 Tohoku tsunamis, diatom assemblages in the deposits did not follow such a pattern. As a result, we can recognize various types of assemblages and realize there is no specific criterion for evaluating event deposits based on diatom analysis. In reconstructing crustal deformation accompanied with earthquakes, diatoms are used mainly for the amount of vertical coastal deformation. For example, diatom micropaleontology was used at southwestern Washington sites to estimate subsidence ranging from 0.5 to at least 1.0 m during six of the seven great earthquakes. In another example, detailed land-level reconstructions using diatoms especially just before the earthquake had occurred have been done in southern Alaska. In the reconstructions, a few decimeters of land-level changes were attributed to preseimic deformation in earthquake cycles. Despite these successful examples, the research area still has the problem of lacking information on the early stage of taphonomy and diagenesis via modern analogues.

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Diatom assemblages in the Kameyama Formation of the Mid-Pliocene Tokai Group and distributed in Tsu City, Mie Prefecture, central Japan are investigated. They consist of 109 taxa in 49 genera, and are characterized by an mixture of both freshwater and marine diatoms. Among them, freshwater planktonic species Aulacoseira praeislandica (including A. praeislandica f. curvata) is predominant, and the marine species Schuettia annulata is subordinate. The mixed flora may suggest that the Kameyama Formation was deposited in a lake-like condition, which was invaded intermittently by seawater in the Mid-Pliocene. This marine invasion may be caused by both local and global settings: this area was located at the southernmost end of the Tokai Group sedimentary basin, and the global climate at this time was warm and the sea level was high.

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Diatom analysis has been commonly used as a tool for reconstructing coastal paleoenvironments, however there still remain some fundamental problems. One major problem is the incorporation of allochthonous diatom frustules in early stages of the taphonomic process, which changes the composition of the living and fossil diatom assemblages. Another is chemical dissolution and physical breakage of diatom frustules during the taphonomic process, which alters the taxonomic composition of dead or fossil assemblages. In order to clarify the taphonomic process, it is necessary to study crystallography and physical mineralogy, and also to understand sedimentology from the viewpoint of diatom frustules as fine silica sediment particles. In addition, to identify soluble species and allochthonous components, it is necessary to study the taphonomic process at each stage and the relationship between the dissolution of diatom frustules and changes in groundwater pH or Eh in particular. On the other hand, we should present a list of total taxa used in the study for reconstructing paleo-coastal environments by diatom assemblages in order to understand the selective preservation of each taxon.

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Time-series variation of diatom sidocoenosis and sinking flux was studied at Station K2 (47°N, 167°E, 5,280 m in water depth) in the western Subarctic Pacific during two study periods (March 2005–June 2006, and November 2009–June 2010).During the first period, the level of diatom flux was similar to those previously observed on the pelagic side of the study area, and the dominant species was Neodenticula seminae, a representative pelagic diatom in the Subarctic Pacific. In the second period, the diatom flux often became much higher than that in the first period, which was due to high dominance of coastal diatoms such as Chaetoceros spp., Thalassiosira nordenskioeldii, and Fragilariopsis spp. Ocean color remote sensing detected a broad water area with high chlorophyll concentration along the east coast of the Kamchatka Peninsula reaching close to Station K2 in May 2009 and 2010. Accordingly, the maximum diatom flux was detected in mid-June 2009. A similar correspondence was previously observed at Station KNOT in 1999.

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This is the first comprehensive report on Japanese Gyrosigma spp. with special reference to their taxonomy and distribution. They are very conspicuous in permanent slides because usually they have large valves with characteristic shapes. However they have not been well studied probably because of their paucity in most samples. In total 30 taxa were identified from samples collected from various localities throughout Japan. The following five taxa are newly reported from Japan: G. acuminatum var. curta, G. acuminatum var. lacustre, G. plagiostomum, G. simile, and G. spencerii var. curvula.

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淡水生Ellerbeckia属珪藻のうち，筆者らにより日本から得られた3分類群について報告する。E. arenaria f. arenaria は山中湖のみから見出された(現生)。他との識別点は，殻面全体に放射状の模様があること，管状突起は2列のスジにまたがっており，2個の小孔が存在することである。E. arenaria f. teresは現生としては阿寒湖から，化石としては中新世から更新世にかけての12の地層から見出された。熊本県の人吉層では密集して塊状の産状を示す層準が見出された。f. arenaria とは放射状の模様が殻面の縁辺部のみに存在すること，管状突起は4列のスジにまたがっており頂部近くに数個の小孔が存在することで識別できる。E. sp. は六角沢層から見出された。本分類群は管状突起が欠如することが多く，存在する場合は4–5列のスジにまたがることで他と区別される。

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新潟県東蒲原郡阿賀町の津川地域に分布する海成上部中新統の野村層からMesodictyon 属の1新種Mesodictyon japonicum を記載した。本種は胞紋の内側に多孔師板があること，殻面に有基突起がな いこと，そして殻套に1個の唇状突起が存在 するという形質を持つ。これらの形質はMesodictyon 属の主要な形質である。しかし，本種は殻套有基突起が3個の付随孔を持つこと，また胞紋の外側縁に肉趾状師板の 基部に類似した小さな突起を持つことで，これまでに知られているMesodictyon属の他の種とは区別される。本種は後期中新世において 限定された生存期間を持つ。すなわち，本種は860万前頃に出現し，820万年前に増加して，550万年前頃には絶滅した。ただし，それ以新の堆積物にも本種が産することがあるが，これは再堆積である可能性が高い。この珪藻 は現在のところ海成層からのみ見つかっているが，海成層中で共産する淡水生のAulacoseira属珪藻と産出パターンが共通であることから，淡水生珪藻であると推察される。

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岐阜県に分布している前期中新世の淡水成堆積物である平牧層からActinocyclus hiramakiensis sp. nov. を記載した。本種の殻面は円形で，ふつう殻中心では点紋が不規則に少なくなる放射状の点紋列が分布する。胞紋は外側，内側とも師板がある。トゲは無い。偽節は殻面／殻套境界にあり，柄の長い唇状突起は円弧状の裂け目が柄の伸長方向に向かって開いている。本種はActinocyclus属に所属するのが適当であるが，本属の現存の種類とは一致しない。形態を示す光学顕微鏡，走査電子顕微鏡による写真を提示し，本種を新種として記載した。

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フワイザ湿原は中東で生態学的に最も重要な湿地の1つである。この湿原で優占的な抽水・沈水植物に付着する植物表生性珪藻の群集構造を，本研究で初めて明らかにした。全体で38属74種の珪藻が記録された。優占種はCocconeis placentula var. euglypta とFragilaria pulchellaであった。フワイザ湿原の植物表生性珪藻群集は淡水生種(貧塩性：39%)，汽水生種(中塩性：33%)，および海生種(真塩性：12%)から構成されていた。海生種はおそらく海から流されてきたものである。

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Morphological characteristics of a planktonic diatom Chaetoceros minimus were described from a brackish-water lake, Lake Nakaumi, Shimane Prefecture, Japan. This taxon is small in size with a single plastid, and has weakly silicified valves. The cell body is almost cylindrical in shape; 4.5–12 µm long in pervalvar axis and 2.0–4.0 µm in diameter. The valves of the frustule are similar, asymmetrically conical in shape with tops located axisymmetrically or asymmetrically to the pervalvar axis. A single long seta (0.5–50 µm) arises from the top of the valve face margin, and runs almost parallel (weakly bent) to the pervalvar axis. Ellipsoidal resting spores were observed, which are 3.5–7.5 µm long in pervalvar axis and 2.2–3.0 µm in diameter. Two types of cell sizes of Chaetoceros minimus are distinguishable (large and small),which suggests a relationship between cell size and water quality (salinity and/or water temperature). In this study, cells from Lake Nakaumi belong to the small type. Moreover, thin and thick cell-walled types are recognized from Lake Nakaumi. It is presumed that this dimorphism in shape corresponds to water temperature and/or cell cycle (vegetative or resting cell).

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