Long-term trends of oxygen concentration in the waters in bank and shelves of the Southern Japan Sea

While multiple studies have investigated oxygen decrease in Japan Sea Proper Water (JSPW; > 300 m in depth), oxygen variation in continental slope and shelf waters (< 300 m) must also be investigated in order to assess its socioecological impacts. In this study, historical oxygen data in the waters of three continental shelves and a bank of Japan Sea, off-Awashima area (AW), Wakasa Bay (WB), East of Tsushima Straight (ETS), and Yamato Bank (YB), were collected and analyzed to assess temporal variation of oxygen in each region from 1960 to 2000s. Significant decreasing trends of oxygen were detected in the waters below 150 m depth in WB and YB, and below 300 m in AW, in the summer season. In winter, a decreasing trend of oxygen was detected throughout the water column from 300 m to the sea surface in WB and YB. In ETS, a deoxygenation trend was detected throughout the water column from the bottom to the sea surface in the summer season, while no trend was detected in winter. The results suggested that oxygen decreases in AW, WB, and YB were the consequence of the upward propagation of the deoxygenation signal from JSPW, while that of ETS was caused by horizontal propagation of deoxygenation signal from the East China Sea. Assuming that the observed trend will continue in future, it is predicted that part of the water in Tsushima Strait area will reach the general sublethal threshold of oxygen (134 μmol kg−1) by the end of this century.

CHLOROPHYLL ESTIMATION OF LAKE WATER AND COASTAL WATER USING LANDSAT-8 AND SENTINEL-2A SATELLITE

<p><strong>Abstract.</strong> Chlorophyll-a is an optically active compound (OAC) commonly used as a proxy for phytoplankton biomass in an aquatic environment. Retrieving the concentration of chlorophyll-a remains a challenge due to the presence of several OAC particularly in water bodies which are in proximity to the land-based activities. In this study, an effort has been made to estimate the chlorophyll-a concentration of both the freshwater Lake Biwa and the coastal water of Wakasa Bay in Japan. A spectral decomposition algorithm was used to determine the chlorophyll-a using the satellite images. The algorithm was applied to the satellite images from two different sensors namely Landsat-8/OLI and Sentinel-2A/MSI. The satellite-derived chlorophyll-a concentration for the lake and coastal water from two different sensors were compared to assess the performance of both the sensors. The accuracy of the chlorophyll-a results derived from the images was evaluated with the in-situ measurement data of the chlorophyll-a for the Lake Biwa and the coastal water of Wakasa Bay. Both satellite sensors appear to give the best results for the coastal water (R²&amp;thinsp;&amp;gt;&amp;thinsp;0.80) with an RMSE &amp;lt;&amp;thinsp;0.3&amp;thinsp;&amp;mu;g/L. However, slight underestimation of chlorophyll-a noted for the Landsat-8 image with an increase in chlorophyll-a concentration. For the lake water, Sentinel-2A results were relatively better (R²&amp;thinsp;&amp;gt;&amp;thinsp;0.70) than Landsat-8, with an RMSE of &amp;lt;&amp;thinsp;1.0&amp;thinsp;&amp;mu;g/L. The obtained results will be useful to evaluate the primary productivity of both freshwater and coastal water body.</p>

Geochemical Classification and Determination of Maturity Source Weathering in Beach Sands of Eastern San’ in Coast, Tango Peninsula, and Wakasa Bay, Japan

<span style="font-size: 10.5pt; font-family: 'Times New Roman','serif'; mso-bidi-font-size: 10.0pt; mso-fareast-font-family: 宋体; mso-font-kerning: 1.0pt; mso-ansi-language: EN-US; mso-fareast-language: ZH-CN; mso-bidi-language: AR-SA;" lang="EN-US">Geochemistry of beach sand sediments collected from the Eastern San’in coast (n=17), Tango Peninsula (n=14) and Wakasa Bay (n=7) shorelines were investigated using XRF analyses for major and trace elements to characterize their composition, classification, maturity, provenance, tectonic setting and degree of weathering in source areas. Investigated sands from all sites were very similar showing depletion in all elements except SiO₂, K₂O and As relative to the UCCN and JUCN, suggesting a moderate geochemical maturation. Beach sand sediments from these locations can be classified as arkose, subarkose and litharenite that are chemically immature and formed under arid/semi-arid conditions with a tendency towards increasing chemical maturity suggesting that they are from multiple sources. The relatively low to moderate values of weathering indices of Chemical Index of Alteration (CIA), Plagioclase Index of Alteration (PIA) and Chemical Index of Weathering (CIW), the beach sands from all sites in the source area have undergone low to moderate degree of chemical weathering. A-CN-K and A-CNK-FM plots, which suggest a granitic source composition, also confirm that the sand samples from these sites have undergone low to moderate degree of chemical weathering in consistent with CIA, PIA and CIW values. A plot of the analyzed beach sands data on the provenance discriminating function F1/F2 showed that most of the investigated beach sand sediments in all locations fall within mafic to intermediate ocean island arc source; similar to the tectonic setting discrimination diagrams based on major elements suggesting a passive margin.</span>