Regional characteristics of diurnal variation of localized heavy rainfall frequency in Tokyo and its surroundings

The present study aimed to clarify the regional characteristics of the diurnal variation of localized heavy rainfall frequency in the Tokyo metropolitan area and Saitama Prefecture based on the dense hourly rainfall data acquired from 290 stations across summers of 15 years between 1994–2010 (June to September). The obtained results can be summarized as follows. The central to western parts of Saitama Prefecture (Cluster 2) displayed a single peak with a large maximum from the evening till the early night hours. In comparison, the western and southern parts of the Tama region (Cluster 4) and the western Tokyo area (Cluster 5) indicated a bimodal pattern with two peaks during the day and night. Moreover, central Tokyo, the northern Tama region, and southeastern Saitama Prefecture (Cluster 3) are located between these areas and can be considered a transition zone between the single-peak and bimodal areas. Despite the moderate increase in the frequency of heavy rainfall from evening to night in the eastern part of the Tokyo metropolitan area as well as the Saitama Prefecture (Cluster 1), it did not exhibit a clear maximum

Significant nutrient consumption in the dark subsurface layer during adiatom bloom : a case study on Funka Bay, Hokkaido, Japan

We conducted repetitive observations in Funka Bay, Hokkaido, Japan, on 15 February, 4 and 15 March, and 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-layer depth (0.1 % of the surface photosynthetically active radiation, PAR, depth) on 4 and 15 March. In the subsurface layer at depths of 30-50 m, concentrations of NO3-, PO43-, and Si(OH)(4) decreased by half between these dates, even in the dark. Incubation experiments using the diatom Thalassiosira nordenskioeldii showed that this diatom could consume added nutrients in the dark at substantial rates after pre-culturing to deplete nutrients. Incubation experiments using natural seawater collected in the growing phase of the bloom on 8 March 2022 also showed that nutrient-depleted phytoplankton could consume added nutrients in the dark. We excluded three physical process - water mixing, diffusive transport, and subduction - as possible main reasons for the decrease in nutrients in the subsurface layer. We conclude that the nutrient reduction in the subsurface layer (30-50 m) between 4 and 15 March 2019 could be explained by nutrient consumption by diatoms in the dark in that layer

Isoprene production in the water column and sediment in Funka Bay, Hokkaido, Japan

We conducted shipboard observations in Funka Bay, Hokkaido, Japan, in 2015, 2016 and 2019 to observe temporal variations of isoprene (C5H8) concentration in water and sediment. We found increases in C5H8 concentrations below and within the euphotic zone, which were associated with chl-a peaks and changes in nutrient and dissolved oxygen (DO). We found a C5H8 peak in the subsurface layer within the euphotic zone at 20-30 m depth in May-June 2019 that coincided with a decrease in DO, suggesting that there was net oxygen consumption. We hypothesize that the rapid cycle of regenerated production in the subsurface layer in early summer resulted in C5H8 production (1.00-1.75 pmol (mu g chl-a) d(-1) at 30 m), decreased DO, relatively high chl-a levels, and nutrient depletion. We propose that C5H8 can be used as a new chemical parameter reflecting primary production, in addition to the common parameters of chlorophyll-a, nutrients, and oxygen. We attributed a C5H8 peak at the surface in May-June 2019 to photosynthetic production of C5H8 associated with new production, likely stimulated by the sporadic input of nutrients to the sea surface. In addition, this is the first study to report the presence of C5H8 in seafloor sediments. We found very high C5H8 concentrations in the sediment from the surface to 5 cm depth. We propose that C5H8 production and degradation by the bacterial community in coastal marine sediments are approximately in balance