Nutrient consumption by diatoms in the dark subsurface layer of Funka Bay, Hokkaido, Japan

We conducted time-series observations in Funka Bay, Hokkaido, Japan, from 15 February to 14 April 2019. The diatom spring bloom peaked on 4 March and started declining on 15 March. Funka Bay winter water remained below 30-m depth, which was below the surface mixed-layer and dark-zone depths on both dates. At depths of 30–50 m, concentrations of NO3–, PO43–, and Si(OH)4 decreased by half between these dates even in darkness. Incubation experiments using the diatom Thalassiosira nordenskioeldii showed that this diatom could consume nutrients in darkness at substantial rates. We conclude that the nutrient reduction in the subsurface layer (30–50 m) could be explained by dark consumption by diatoms that had been growing in the surface waters and then sank to the subsurface layer. We believe that this is the first study to present observational evidence for the consumption of the main nutrients by diatoms in the dark subsurface layer during the spring bloom. Nutrient consumption in this layer might have a substantial influence on the primary production during and after the spring bloom.

Climatic change and its relationship with human society in northern Japan since the mid-Holocene - Quantitative reconstruction of atmospheric and sea surface temperature -

For the prediction for future climate, it is required to enhance our understanding of the Holocene climatic change. By estimating paleo-temperature, we attempted to examine the relationship between climatic changes and human activities in northern Japan. The alkenone SSTs showed a variation of 8.7 °C (14.5 °C to 23.2 °C). Jomon people living around Funka Bay were more dependent on marine products than in Honshu due to cooler climatic conditions. There are two Hypsithermal environments at Funka Bay around 4.6 cal. kyr BP and 1.2 cal. kyr BP. Sea levels for the last 7 kyrs have been controlled mainly by local/regional tectonic vertical movement. Therefore, the highstand never always corresponded to Hypsithermal environments. More than one driving forcing, rather than one, may control climatic/environmental change. The 4.2ka event in northern Japan could be characterized by cooling by a few degrees by the combination of reduced ESAM and El Niño mode. Another notable cooling event occurred in 1.0-0.8 cal. kyr BP due to the La Niña condition and reduced solar activities, which means that the Japanese archipelago never experienced Medieval Warm Event: instead, there was a Medieval Cold Event in Japan. The Okhotsk culture in Hokkaido prospered only under the Hypsithermal condition during the 5 th - 9 th centuries and subsequently it declined and was incorporated into the Satsumon culture under the cooler environmental conditions. For the Yayoi-era (2.9 cal. kyr BP-) and the following periods, our results are consistent with the previous results from Hiroshima Bay, western Japan, that cold periods (6 th - 7 th century BC, 3 rd century BC, 6 th century AD, 10 th -12 h century AD and 16 th century AD) coincided with major shifts in social systems in Japan.

Impact of Climate Extremes on Suitability Dynamics for Japanese Scallop Aquaculture in Shandong, China and Funka Bay, Japan

The assessment of extreme weather events on suitable sites for aquaculture could help in establishing sustainable coastal environmental resource management. Japanese scallop culture is an economically important marine farming activity in the coastal communities of Shandong, China and Funka Bay, Japan. In this study, we improved the suitable aquaculture site-selection model (SASSM) by using Geostationary Ocean Color Imager (GOCI) data instead of Moderate Resolution Imaging Spectroradiometer (MODIS) data, as a complementary source for higher temporal and spatial resolution data that are useful for monitoring fine-scale coastal and oceanic processes. We also applied the newly developed SASSM to the Japanese scallop production site along the Shandong coast. Finally, we analyzed the correlations between environmental factors (chlorophyll a concentration, sea surface temperature (SST), and total suspended sediment), meteorological factors (precipitation, temperature, and wind), and climatic events (winter East Asian monsoon (EAM) and El Niño/La Niña Southern Oscillation), and the impacts of climate events on suitable zones for scallop aquaculture. The new SASSM maps show that GOCI products have the potential for oceanographic investigations in Shandong, China and Funka Bay, Japan. Our results highlighted higher aquaculture site suitability for scallop in Funka Bay than in Shandong coast. During the winter with a strong EAM (2011), the suitable area for Japanese scallop aquaculture increased. Conversely, in the winter during a strong El Niño (2016), we found fewer areas that were highly suitable for scallop aquaculture in Funka Bay. SST was extremely low in Funka Bay during spring and summer 2017, which caused fewer highly suitable areas (scores of 7 and 8) for scallop aquaculture relative to other years. These findings suggest that extreme climatic events significantly impact the availability of suitable sites for marine farming and thus, should be considered in the development and design of coastal aquaculture sites.