廣瀬 孝太郎

Human activities in coastal areas have intensified over the last 200 years, impacting also high-latitude regions such as the Baltic Sea. Benthic foraminifera, protists often with calcite shells (tests), are typically well preserved in marine sediments and known to record past bottom-water conditions. Morphological analyses of marine shells acquired by microcomputed tomography (µCT) have made significant progress toward a better understanding of recent environmental changes. However, limited access to data processing and a lack of guidelines persist when using open-source software adaptable to different microfossil shapes. This study provides a post-data routine to analyze the entire test parameters: average thickness, calcite volume, calcite surface area, number of pores, pore density, and calcite surface area/volume ratio. A case study was used to illustrate this method: 3D time series (i.e., 4D) of Elphidium clavatum specimens recording environmental conditions in the Baltic Sea entrance from the period early industrial (the 1800s) to present-day (the 2010 s). Long-term morphological trends in the foraminiferal record revealed that modern specimens have ∼28% thinner tests and ∼91% more pores than their historic counterparts. However, morphological variability between specimens and the BFAR (specimens cm⁻² yr⁻¹) in E. clavatum were not always synchronous. While the BFAR remained unchanged, morphological variability was linked to natural environmental fluctuations in the early industrial period and the consequences of anthropogenic climate change in the 21st century. During the period 1940–2000 s, the variations in BFAR were synchronous with morphological variability, revealing both the effects of the increase in human activities and major hydrographic changes. Finally, our interpretations, based on E. clavatum morphological variations, highlight environmental changes in the Baltic Sea area, supporting those documented by the foraminiferal assemblages.

秋季シンポジウム「プランクトンと古環境・古生態系復元」の開催報告

<p>猪苗代湖で掘削されたボーリングコア（INW212）の深度200～0cmを対象に，<sup>210</sup>Pb，<sup>134</sup>Cs，<sup>137</sup>Cs分析を行った．<sup>210</sup>Pb<sub>（ex）</sub>の分析結果から，深度39cm以深が約90年前以前に相当する可能性が示された．次に，<sup>134</sup>Csと<sup>137</sup>Csの分析結果から，深度29cmが1950年代初頭に相当すると推定された．さらに，これらの年代値と岩相層序について検討を行った結果，深度13.5～0cmの塊状の砂質シルトが2011年の東北地方太平洋沖地震に起因する堆積物であり，深度42～37.5cmの灰褐色粘土が1888年の磐梯山の崩壊に関連する堆積物であることが推定された．これらのイベントの深度と年代からINW2012コアの定常的な堆積速度を算出した結果，深度200.0～42.0cmにおいて1.0mm/yr，深度42.0～0cmにおいて約2.0mm/yrとなった．</p>

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).

海洋における一次生産者としてその動態が環境に大きな影響を与える珪藻群集を，堆積物中の遺骸からできるだけ忠実に再現するために，超音波印加による簡便な処理法を考案した．動物の糞粒（fecal pellet）が主体である集塊が容易に泥化し，かつ珪藻殻の破壊による消失が認められないことから，超音波印加は珪藻分析において有効な処理法であることを示した．また，従来の処理方法である過酸化水素処理を行った同一試料の珪藻遺骸群集と比較した結果，過酸化水素処理ではデカンテーションの過程において除去した上澄みとともに流出する可能性のある小型の分類群を，超音波印加処理を用いることで忠実に計数できることが明らかになった．さらに，超音波印加処理を行った試料の集塊（糞粒）と基質部分（細粒分）の珪藻遺骸群集の違いから，化石としては保存されない一次消費者の動態の一部を堆積物から明らかにしうる可能性を指摘した．

兵庫県赤穂市坂越湾と相生市沖で採取した柱状試料を用いて，過去約数百年間の珪藻群集および堆積物の全有機炭素（TOC）・全窒素（TN）濃度の時空間的変化を明らかにした．さらに，珪藻群集と最近の人為汚染・改変との関連性，および1980年代以降の環境保全対策が珪藻群集に与えた影響に関して検討した．各種分析を行った結果によると，播磨灘北部沿岸域では1930年代以降に水域の富栄養化が始まり，その初期段階には珪藻群集が増加しはじめ，1960年代の高度経済成長期において水域の富栄養化がさらに進行した．1980年代以降，坂越湾と相生市沖では，異なる環境変遷をたどったことが示唆され，1990年代以降に坂越湾ではマガキ養殖の影響，相生市沖では播磨灘北部の水質改善の影響を受けたと考えられる．

We investigated the recent diatom assemblages in 30 surface sediment samples from Osaka Bay, Japan, with the identified diatoms divided into five categories [halobious system ; Oligoeuryhaline Polyhalobous (OPH), Euryhaline Polyhalobous (EPH), Mesohalobous (MEH), Euryhaline Oligohalobous (EOH) and Stenohaline Oligohalobous (SOH)] based on their tolerance for salinity. The horizontal distribution pattern of each category suggested that the recent diatom distribution pattern of the bay is mainly controlled by the transportation of brackish/freshwater species from the mouth of the Yodo River through the river-water discharge into the bay. In other words, the distance from the river mouth controls the relative abundance of brackish/freshwater species at each site, which is a major part of the total floral composition and could be a useful paleoecological indicator for river-water discharge. The distribution pattern of <i>Chaetoceros</i> resting spores suggests they have potential as a paleoecological indicator for anthropogenic environmental degradation in exploited embayments. Taphonomic processes of recent diatoms are also discussed.