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The contribution of dissolved organic carbon (DOC) decomposition to oxygen consumption was estimated in the inner part of the Ariake sea. Oxygen consumption and decomposition of particulate organic carbon (POC) and DOC were estimated using the bottom water samples collected from two stations in the inner part of the Ariake sea from May to August 2018. The contribution of DOC decomposition to total organic carbon decomposition was about 10%. Although the contribution of organic matter decomposition to total oxygen consumption was 47.0-54.2% before the occurrence of hypoxia, the contribution of organic matter decomposition to total oxygen consumption was minor during the hypoxia. These results suggest that organic matter decomposition plays an important role in oxygen consumption before the occurrence of hypoxia, and DOC decomposition is one of the driving forces of hypoxia.

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We surveyed the spatial and temporal distribution of Japanese sea nettle jellyfish Chrysaora pacifica in Omura Bay, Japan, using an environmental DNA (eDNA) method. In 2018, the C. pacifica eDNA concentration increased from March-May at all depths. The seasonal pattern of C. pacifica eDNA was consistent with previous reports based on visual observations along the Japanese coast. Thus, the eDNA method might have advantages to follow the seasonal pattern of C. pacifica while being less time-consuming and less laborious compared with traditional methods. The eDNA concentrations tended to reach a maximum near and/or below the pycnocline throughout this study. Therefore, the vertical distribution of C. pacifica medusae may have been restricted by strong pycnocline formation in July and August 2018. However, even with a weak pycnocline, which C. pacifica should be able to swim across, the apparent distribution of C. pacifica eDNA seems to be restricted by the pycnocline. Therefore, the eDNA method cannot, currently, accurately assess the absolute vertical distribution pattern of C. pacifica, especially when a pycnocline is formed.