Search

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]

Sub-annual-scale environmental and ecosystem changes since the mid-18th century were reconstructed in a semi-closed lagoon, Lake Hiruga, located along the Sea of Japan in central Japan. Lake Hiruga is one of the Mikata Five Lakes and is connected to Lake Suigetsu through an artificial tunnel, the Saga Tunnel. Analyses of siliceous phytoplankton (diatoms, silicoflagellates, and chrysophyte cysts) and elemental composition (carbon, nitrogen, sulfur) of laminated sediments were conducted to determine the impacts of irregular hazards and anthropogenic tunnel construction on the lagoon ecosystem. In this lagoon, sediment laminae were deposited after the first Saga Tunnel re-excavation in 1844–1848, and the dominant phytoplankton taxa have changed with each re-excavation. The behaviors of total nitrogen, total organic carbon, freshwater diatom taxa, and chrysophyte cysts in Lake Hiruga are strongly affected by tunnel excavation and water gate construction. In addition to anthropogenic effects, the deposits show extreme hazard impacts of the Lake Hiruga ecosystem. The peaks of marine diatom taxa, silicoflagellate abundance, and sulfur content of Lake Hiruga showed close relationships with the local earthquake event. A relatively high sea level in the coast of Sea of Japan by local subsidence induced active seawater exchange with Lake Hiruga around AD 1928–1930. Furthermore, the peaks of freshwater diatom taxa and chrysophyte cyst showed a freshening event of this lake caused by local extreme events, such as continuous flooding around AD 1900. [ABSTRACT FROM AUTHOR]

Hwajinpo is the largest lagoon in Korea and is located along the east coast of the country. It possesses Holocene sediments that provide an important record of past climate change. We studied the evolution of Hwajinpo Lagoon using grain size data and diatom assemblages in an 11.0-m core (HJ02), which was obtained at the mouth of a small river that drains into the lagoon. Core chronology was established with accelerator mass spectrometry 14C dates and optically stimulated luminescence dates. Diatom assemblages and grain size analysis revealed that estuarine conditions in the inner lagoon area transitioned to an open embayment ca. 8 ka as a result of sea-level rise. Around 7.8 ka, the open bay became a semi-closed bay as a consequence of development of a sand barrier. After the bay was semi-closed, marine water inflow was increasingly restricted as the sand barrier developed, and the semi-closed bay became a completely enclosed, low-salinity, brackish lagoon around 6 ka. There was an erosional hiatus between 5.5 and 1.7 ka (7.0 m depth), likely caused by river flooding and a switch in the location of drainage along the delta. The lagoon became oligohaline around 1.6 ka, likely because of increasing precipitation associated with an intensified Asian summer monsoon. This increase in precipitation resulted in expansion of the sand bar by sediment inflow, driven by agricultural development in the area. About 1000 years ago, the diatom assemblage was similar to the modern assemblage, suggesting the lagoon’s current geomorphic conditions had been established. [ABSTRACT FROM AUTHOR]

Documenting multi-decadal typhoon and storm-rain variability is useful to prevent future typhoon and flood disasters. We present the history of typhoon and storm-rain activity in East Asia inferred from multi-proxy analyses of Lagoon Hwajin-po sediments along the eastern coast of Korea. Anthropogenic effects were enhanced in Lagoon Hwajin-po since ca. AD 1900, by increasing farming in the catchment. To avoid these human-induced effects, we reconstructed the history of typhoon and storm-rain activity only for the interval AD 1400-1900. The record indicates that typhoon frequency throughout the Korean Peninsula varied in response to the state of the El Niño/Southern Oscillation. Typhoon variability was likely modulated further by the state of the East Asia summer monsoon (EASM) pattern, associated with variation in the magnitude of solar irradiance. During periods of minimum solar activity, such as the early Maunder Minimum (AD 1650-1675), typhoons struck the east China coast and Korean Peninsula more frequently because of a strengthened EASM. [ABSTRACT FROM AUTHOR]