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DNA metabarcoding (DNA-MB) targeting the whole plankton community is a promising approach in studies of sediment samples from water bodies, but its effectiveness in ancient material is not well demonstrated. We applied DNA-MB of plankton in a sediment core to reconstruct the paleo-environment of Lake Shinji, Japan, through a marine lagoon/freshwater lake transition during the past 2300 years. We interpreted core-sample plankton taxonomy and habitat by reference to the modern plankton community in water samples. OTUs (operational taxonomic units) belonging to Dictyochophyceae were 81.05% of the total reads in sediments. However, Ciliophora, Copepoda and Labyrinthulea formed the majority of plankton taxa in the water samples, suggesting that they are under-represented in sediment. A drastic change in plankton composition correlated with a large decrease in sediment sulfur concentration, implying the change of aquatic environment from marine lagoon to freshwater lake. This event took place ca. 1200 CE in Lake Shinji. A 250 year-long transitional period followed, during which the total DNA sequence reads were very low. This suggests that salinity fluctuations created a hostile environment for both marine and freshwater plankton species. Our results show that DNA-MB of the whole plankton community is effective in reconstructing paleo-environments. [ABSTRACT FROM AUTHOR]

Annual-scale environmental and ecosystem changes since the mid-19th century were reconstructed in a seasonally frozen lagoon, Lake Mokoto, located along the Okhotsk Sea coast of Hokkaido, northern Japan. X-ray fluorescence (XRF) and diatom analyses of laminated sediment were conducted to determine the impact of short-term climate oscillations and anthropogenic eutrophication on the lagoon ecosystem. In this lagoon, eutrophication has progressed since the early 20th century due to deforestation and human activity in the catchment area, such as development of the cattle industry and field agriculture. In particular, the sedimentation rate of Lake Mokoto and potassium and phosphorus contents of lake sediment have increased rapidly since 1970, in association with an increase in the number of cattle. In addition, the Lake Mokoto ecosystem shows interannual and decadal cyclic fluctuations that are separate from the anthropogenic effects. Prior to anthropogenic eutrophication, the nutrient condition of Lake Mokoto showed a close relationship with the Pacific decadal oscillation (PDO). The growth of seagrass, inferred from epiphytic diatom abundance, showed a clear decadal cycle following variation in solar irradiance. [ABSTRACT FROM AUTHOR]

Abstract: Diatom assemblages of the surface and in core sediment samples from Lake Saroma (Japan) were examined for the purpose of evaluating anthropogenic effect on the coastal environmental changes. Before the first inlet excavation, the lake''s water quality and ecology were controlled by water exchange with the Okhotsk Sea as well as lake-level variation. However, large-scale ecological modification occurred, mainly due to artificial excavation and shellfish industrial farms. A distinct record of the succession of the dominant diatom taxa was preserved in core sediments. Low-oxygen water was prevalent in the lake in 1929, before the first inlet excavation. Immediately after the first inlet excavation, the low-oxygen water in the western basin of the lake began to disappear, in a trend that became increasing transparent, which has been attributed to an increasing rate of water exchange. However, the lacustrine environment of bottom sediments resumes deterioration 20years after since the first artificial excavation: the resultant deposition of river-mouth materials into the deep basin caused eutrophication and environmental disturbance of the lake bottom. At the same time, the eutrophication of surface water became intensified with the onset of intense scallop culturing beginning in 1966. Increasing organic loads deposited onto the bottom layer in the form of excreta from the scallop nursery led to more oxygen deficiency and the elution of nitrogen and phosphorus from the sediment, which again brought about eutrophication of the surface layer. Such environmental change was reflected in a decrease of benthic diatom taxa and an increase of planktonic taxa, trends which have continued until today. Particularly, the numbers of diatom assemblage have been decreasing all over the lake during the last 10years, which suggests that Lake Saroma''s present-day deterioration and eutrophication will continue or become even worse. [Copyright &y& Elsevier]

Paleoenvironment and paleoclimate changes during the Little Ice Age (LIA) were investigated using sediments from Lake Hamana. This lagoon in central Japan is under a complicated climate system influenced by the East Asian summer monsoon (EASM), East Asian winter monsoon (EAWM), and strong typhoon events. Diatom assemblages and chemical components of a 1.3 m sediment core suggest historical events of rapid salination and cyclical ecosystem shifts accompanied by monsoon condition changes. Most research has indicated that Lake Hamana changed from a freshwater lake to a brackish lagoon because of the Meio earthquake in 1498 CE. However, based on results from the diatom assemblage and chemical components, this salination occurred because of changes in the Hamana paleoriver flow, and this change happened before the earthquake, around the mid-15th century. Furthermore, it is clear that Lake Hamana was an oligohaline lagoon before salination. The lithology of the lake sediments and the diatom assemblage changed during the LIA; sand, water, and total diatom contents decreased. However, the relative abundance of marine diatom taxa increased during the LIA. These changes were introduced by a weakening EASM coupled with low precipitation under low solar activity during the LIA. Several flood events represented by freshwater diatom taxa and magnetic susceptibility peaks co-occurred with strong El Niño events during the LIA. It is inferred that this strong El Niño during the LIA induced frequent typhoons causing floods in central Japan. [ABSTRACT FROM AUTHOR]