Kotaro Hirose

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>Vertical variations in <sup>210</sup>Pb, <sup>134</sup>Cs, and <sup>137</sup>Cs content (Bq/kg) were measured in the uppermost 2 m of the INW2012 sediment core, which was taken from Lake Inawashiro-ko. Analyses of <sup>210</sup>Pb<sub>(ex)</sub> suggest that the top 39 cm were deposited within the last 90 years, while <sup>134</sup>Cs and <sup>137</sup>Cs results indicate that a depth of 29 cm corresponds to the beginning of the 1950s. Combined with lithostratigraphic observations, these dates suggest that the massive sandy silt layer observed in the top 13.5 cm of the core was deposited as a result of the 2011 earthquake in the Pacific Ocean off the coast of Tohoku. In addition, the gray-brown clay layer between 42 and 37.5 cm depth is associated with the Mt. Bandai eruption in AD 1888. On the basis of these event horizons and their ages, sedimentation rates are estimated to be 1.0 and 2.0 mm/yr at depths of 200-42 cm and 37.5-13.5 cm, respectively.</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).

We verified the significance of a simple and easy treatment using ultrasonic cleaner, which was designed to minimize possible modification of original diatom assemblages preserved in sediments. Our ultrasonic treatment demonstrated that diatom valves incurred little damage ; in addition, this treatment was effective to disaggregate the consolidate structure mainly of fecal pellets. Comparing the diatom assemblages in the samples treated by hydrogen peroxide (an established method for diatom slide preparation) with those processed by ultrasonication, we concluded that the ultrasonic treatment provides a better result in preserving micro and fragile valves that otherwise easily flow out while captured in supernatant during the dacantation process of the hydrogen peroxide method. Furthermore, the investigation of the difference between diatom assemblages in aggregate structures (mainly of fecal pellets) and those in sediment matrix suggests the possibility that diatom analysis of community structures in fecal pellets tends to reconstruct the population dynamics of consumers whose fossils are known to be rarely preserved in underwater sediment.

<SUP>210</SUP>Pb and <SUP>137</SUP>Cs age dating, total organic carbon (TOC) and total nitrogen (TN) contents analysis, and diatom analysis were conducted in sediment samples from two short cores collected from Sakoshi Bay (core HNA) and off Aioi City (core HNB) , northern Harima-Nada, southwest Japan. The results of <SUP>210</SUP>Pb and <SUP>137</SUP>Cs dating shows that ages at the bottom of core HNA and that of core HNB were estimated at 1883AD and 1786AD, respectively. Spatio-temporal changes of diatom assemblages and TOC and TN contents were as follows : In coastal areas of northern Harima-Nada, eutrophication occurred earlier than the initiation of the high economic growth period, and diatoms flourished during the 1940s-1960s. Deterioration of marine environments peaked in the 1960s-1970s. In the 1980s, Sakoshi Bay and a site off Aioi City underwent different paleoenvironmental changes. Since the 1990s, water and bottom environments in Sakoshi Bay were deteriorated by oyster farming, while water environments off Aioi City were influenced by the improvement of water quality in northern Harima-Nada.

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.