Succession Of Diatom Assemblages Research Articles

Sedimentary Diatom Records Reveal the Succession of Ecosystem in Lake Xihu, Dali over the Past 50 Years

In recent decades, intense human activities have caused a decline in many lake ecosystems in Yunnan Province, rendering the transformation of the lake from a clear macrophyte-dominated state to a turbid phytoplankton-dominated state. Improved understanding of the ecological changes in lake ecosystem has significant implications for management. In this study, a small lake in Dali Prefecture of Yunnan Province, i.e., Lake Xihu, was selected. Combined with diatom records and physicochemical proxies from the lake sediments, this paper focuses on the long-term ecological changes in Lake Xihu, Dali since the mid-1960s. The results show that the Lake Xihu, Dali has undergone a significant shift in stable states over the past 50 years. Prior to 2000, the benthic-epiphytic species (i.e., Cocconeis placentula, Staurosira construens, Gomphonema angustum, and Achnanthidium minutissimum) dominated in diatom assemblages, indicating oligotrophic conditions; since 2000, benthic-epiphytic species (i.e., Encyonopsis microcephala and Navicula cryptocephala) and planktonic species (i.e., Cyclotella atomus, Cyclotella meneghiniana, Stephanodiscus hantzschii, and Aulacoseira granulata) dominated successively, indicating mesotrophic to eutrophic conditions. Principal component analysis based on the diatom assemblages in temporal scale showed the response of diatoms succession to nutrients. Redundancy analysis also confirmed that nutrient enrichment was the main driving force for the succession of diatom assemblages in Lake Xihu, Dali. In the past 50 years, climate change and human activities (i.e., agricultural reclamation, fertilization, animal husbandry, and fishery) have enhanced the accumulation of nutrients in the lake. The continuous loading of nutrients promoted the propagation of planktonic algae, and also the productivity of the lake, rendering the transformation to a turbid phytoplankton-dominated state.

Environmental changes at final warming of Middle Pleistocene (MIS 7) in South Kurils

Entirely new data have been obtained on the South Kuril environments at the time of the Middle Pleistocene warming. These data are correlatable with MIS 7. The data show climatic rhythms, and shed new light on paleo-oceanologic conditions and the evolution of environments at that time. Paleo-environmental reconstructions are based on multidisciplinary studies of the marine deposit sequence exposed in a sea cliff in the eastern part of Tanfiliev Island, including a buried oyster reef. In the course of the studies, analyses of mollusc fauna, diatoms, and pollen were performed. Benthic foraminifera were first found in the Middle Pleistocene sediments of the South Kurils. The sediments were dated using the 230Th/U method, and by the presence of the zonal diatom species Thalassiosira nidulus. The warming process was not unidirectional: two warmer intervals were separated by a slight cooling. A complicated process of climatic fluctuation is reflected in the succession of diatom assemblages, the latter also being influenced by the warm current and the solar heating of coastal water. In the first stage of warming, there existed an oyster reef composed of densely packed shells of Crassostrea gigas. Nineteen associated species were found, 11 percent of them extra-limital warm water species. The presence of these species suggests the summer surface sea water temperature was 6°C – 8°C above that of today, on the Tanfiliev coast, at the time of the reef growth. The warming coincided with marine transgression: the latter was once interrupted with a limited regression which coincided with a short period of cooling. The sequence of deposition environments is reconstructed as follows: a shallow bay – a coastal lake – an open bay. The adjacent land was covered with cool-temperate broadleaf and coniferous-broadleaf forests, the coniferous species gaining in importance during cooling. During the second warm phase, the proportion of coniferous and small-leaved species in plant communities was drastically reduced.

Late Holocene diatom biostratigraphy and sea-level changes in the southeastern Beaufort Sea

Late Holocene sediments from the Atkinson Point area were analysed to provide quantitative reconstructions of recent sea-level changes in the southeastern Beaufort Sea. The succession of diatom assemblages in five cores revealed paleoenvironmental changes induced by the transgression of the Beaufort Sea during successive periods of lacustrine conditions, breaching and flooding of thermokarst lakes by the sea, and the landward migration of sandy spits. Based on radiocarbon dates and quantitative paleodepth determinations, a relative sea-level curve for the late Holocene has been developed. Despite a loss of temporal precision due to old carbon contamination, an envelope of sea-level change has been defined for the last 2 ka BP, suggesting a sea-level rise in the order of 1.1 mm a-1 for the last millennium. This paper presents the first sea-level reconstruction inferred from a diatom-based transfer function. It represents an improvement over traditional methods which were limited to qualitative estimates of past sea levels.

過酸化水素水処理‐比濁法による硫黄分析と堆積環境

Sedimentary sulfur mainly of FeS2 (pyrite) was oxidized to sulfate with 30% H2O2 and the concentration of sulfate was determined by the turbidimetric method.Treatment procedures were as follows.1) Wet sediment (approximately 0.3g) was washed twice with distilled water followed by centrifugation (3, 000rpm, 15mins), and dried at 80°C.2) The dry sample (0.1g) was reacted with 10ml 30% H2O2 for 30mins at room temperature and then for 2mins at 90-100°C.3) When oxidation reaction ceased (usually after 15hrs), 50ml distilled water was added and the sample filtered through a 0.45μm membrane filter to separate extract from residue. Filtrate was rinsed and brought to 100ml volume with distilled water for the turbidimetric method.The content of sedimentary sulfur determined by this method was much higher in the marine and brackish water sediments than in the freshwater sediments. The vertical variation in sulfur content was in good agreement with the succession of diatom assemblages. As a sulfur analysis indicative of paleoenvironmental changes, this method is considered useful for its simplicity and for the small amount of sample used.

高度に汚染された溜池から得られた堆積物中の珪藻群集とその遷移

A recent sediment core was collected from a heavily polluted irrigation pond, “Niinomi-ike”. Vertical distribution of species composition of diatom assemblages was examined to evaluate their succession in relation to phosphorus concentration in the sediments. It was found that the species composition of planktonic and periphytic assemblages in the lower part of the core was replaced by different species in accordance with the increase in total-phosphorus concentration in the sediment. Thus in the planktonic assemblages, Cyclotella comta, C. stelligera, Melosira ambigua and M. distans were replaced by C. meneghiniana, C. pseudostelligera and M. granulate. In the periphytic assemblages, Achnanthes minutissima was also replaced by Navicula frugalis and Nitzschia palea. Additionally, there is the decrease in frustules of acidophilous taxa. The succession process observed in the planktonic assemblages was characterised by the replacement of the related species in the same genus. Based on the succession of diatom assemblages, it was concluded that this pond has changed from a eutrophic one to a hypereutrophic one, and the saprobity in the periphytic environment has changed from a oligo or β-mesosaprobic status to an α-mesosaprobic one.

The succession of diatom assemblages and Holocene sea-level changes during the last 6,000 years at Sado Island, central Japan: The Holocene development of Lake Kamo-ko II.

Based on the ecological spectra of the diatoms grouped into marine, brackish and freshwater, sediments from Loc. 2 were classified into four diatom zones : (1) Transitional Zone 3 (Tr-3), characterized by the alternation of marine and freshwater species as dominant. (2) Marine Diatom Zone 2 (MD-2) reflecting the small transgression at c.a. 3, 000 yBP is dominated by marine species. (3) Transitional Zone 4 (Tr-4) including remarkable changes in sedimentary environment is subdivided into Tr-4a and Tr-4b. (4) Freshwater Diatom Zone 3 FD 3) is dominated by freshwater species and may reflect the lowering stage of sea level during the Yayoi Age. Each zone is considered to be closely related with the development of Lake Kamo-ko caused by Holocene sea-level changes.

The succession of diatom assemblages and Holocene sea-level changes during the last 6,000 years at Sado Island, central Japan: the holocoene development of Lake Kamo-ko I.

The diatoms in core sediments from Loc. 3 were analyzed to clarify the paleoenvironmental changes of Lake Kamo-ko during the last 6, 000 years. Diatoms were grouped into three ecological categories; marine, brackish and freshwater. Based on the ecological spectra of the diatoms, the sediments were divided into five diatom zones. The results are discussed with reference to Holocene sea-level changes as follows : Marine Diatom Zone 1 (MD-1) is characterized by marine species Diploneis novae seelandiae indicating that this site might have had a wide channel with the sea at the time of Holocene transgression. Transitional Zone 2 (Tr-2) is dominated by marine diatoms and freshwater ones alternately. Freshwater Diatom Zone 2 (FD-2) is characterized by freshwater planktonic species Melosira italica suggesting that sedimentation took place under standing-water conditions such as a freshwater lake. This may reflect the small regression at c.a. 4, 500 yBP. Marine Diatom Zone 2 (MD-2), first reported here, is dominated by marine species Diploneis suborbicularis and Nitzschia granulata, and may reflect the small transgression at c.a. 3, 000 yBP. Transitional Zone 4 (Tr-4) is considered to reflect the process of emergence at this site.