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Kagami M.,Toho University | Hirose Y.,Toho University | Ogura H.,Chiba Prefectural Environmental Research Center

Lake Inba is one of the most eutrophic lakes in Japan. In this study, field sampling and nutrient enrichment bioassays were conducted to determine the seasonal patterns of nutrient limitation for phytoplankton growth in this lake. Phytoplankton biomass increased significantly with the additions of phosphorus (P) on almost all sampling dates, indicating P limitation of phytoplankton growth from spring to autumn. However, nitrogen (N) limitation was also observed during summer (i. e., 19 August). On 10 August, a typhoon struck Lake Inba. After this event, dissolved inorganic nitrogen (DIN) and phosphorus concentrations increased, probably because of increased river discharge. At the same time, phytoplankton growth in the control treatment became relatively high, with the addition of neither P nor N stimulating the growth. However, 10 days after the typhoon, the phytoplankton growth rate in the control treatment decreased, with only the addition of N having a significant positive effect on phytoplankton growth. N limitation during summer is caused by the low concentrations of DIN, as well as changes in the N:P ratio due to allochthonous nutrient loads. These results indicate that a reduction of both P and N input is necessary to control phytoplankton blooms in Lake Inba. © 2012 The Japanese Society of Limnology. Source

Ishizuka O.,Geological Survey of Japan | Ishizuka O.,Japan Agency for Marine - Earth Science and Technology | Taylor R.N.,University of Southampton | Geshi N.,Geological Survey of Japan | And 4 more authors.
Earth and Planetary Science Letters

To discover how magmas move and interact beneath an arc we have examined the temporal and spatial evolution of the largest Izu-Bonin frontal arc volcano Izu-Oshima and the adjacent Izu-Tobu field of backarc volcanoes. Extensive 14C ages and geochemical analysis of subaerial satellite cones as well as other effusives has enabled us to construct a well-constrained ~14 ka record of Izu-Oshima volcanism.The geochemistry of Izu-Oshima is found to change systematically through the last 14000 yr. Ba/La, Pb/Ce, 87Sr/86Sr, 143Nd/144Nd and 206Pb/204Pb all decrease between 10 ka and 5 ka before increasing between 5 ka and the present, while La/Yb and Nb/Zr show the reverse. These changes in composition match the addition of Izu-Tobu (backarc) magma to the Izu-Oshima plumbing system with a maximum of a 40% Izu-Tobu at around 5 ka. Progressive but asymptotically declining changes in composition through the 10-5 ka period are found to fit a model where pre-mixed magma is episodically added to, and mixed with, a chamber beneath Izu-Oshima. The 5-0 ka period reverses this trend, but is again progressive and declining, suggesting a switch to a progressive influx of pure Izu-Oshima frontal arc magma. Combining flux and eruption volume estimates with the observed geochemical mixing rates indicates that the accessible melt volume of the Izu-Oshima magma system is ~16 km3. Interaction and pre-mixing between the fluid-dominated frontal arc melt and the sediment-bearing backarc magmas must occur at deeper levels within the arc crust. This deep reservoir receives a continuous feed from the frontal arc mantle, but may periodically intercept rising magmas from the backarc source to produce episodes of magma mixing on timescales of ~5000 yr. This study demonstrates that interaction between frontal arc and backarc magma needs to be considered to achieve better understanding of material transfers and elemental budgets at subduction zones. © 2015 Elsevier B.V. Source

Ichikawa Y.,Chiba Prefectural Environmental Research Center | Naito S.,Chiba Prefectural Environmental Research Center | Ishii K.,Chiba Prefectural Environmental Research Center | Oohashi H.,1 Ichiba cho
Asian Journal of Atmospheric Environment

In order to survey the seasonal variation of the chemical composition of particulate matter of 2.5 μm or less (PM2.5), PM2.5 was sampled from 8 February 2013 to 31 March 2014 in an industrial area of Chiba Prefecture, Japan. Chemical measurements of the sample included: ionic components (Na+, NH4 +, Ca2+, Mg2+, K+, Cl-, NO3 - and SO4 2-), carbonaceous components - organic carbon (OC) and elemental carbon (EC), and water-soluble organic carbon (WSOC). Also, secondary organic carbon (SOC) was measured based using the EC tracer method, and char-EC and soot-EC were calculated from the analytical results. The data obtained were interpreted in terms of temporal variation. Of the overall mean value of PM2.5 mass concentration obtained during the study period, ionic components, OC and EC accounted for 45.3%, 19.7%, and 8.0%, respectively. NO3 - showed a unique seasonal distribution pattern due to a dependence on temperature and absolute humidity. It was estimated that an approximate temperature of 14°C, and absolute humidity of 7 g/m3 were critical for the reversible reaction of NH4NO3 (p) ⇋ NH3 (g)+HNO3 (g). The amount of OC and EC contributing to the monthly PM2.5 mass concentration was higher in autumn and winter compared to spring and summer. This result could be attributed to the impact of burning biomass, since WSOC and the ratio of char-EC/soot-EC showed a similar pattern during the corresponding period. From the comparison of monthly WSOC/OC values, a maximum ratio of 83% was obtained in August (summer). The WSOC and estimated SOC levels derived from the EC tracer method correlated (R=0.77) in summer. The high occurrence of WSOC during summer was mainly due to the formation of SOC by photochemical reactions. Through long-term observation of PM2.5 chemical components, we established that the degree to which the abovementioned factors influence PM2.5 composition, fluctuates with seasonal changes. Source

Hanno K.,Chiba Prefectural Environmental Research Center | Hanno K.,University of Tokyo | Oda S.,University of Tokyo | Mitani H.,University of Tokyo

The aryl hydrocarbon receptor (AhR) binds to polyaromatic compounds, including dioxins, and enhances the expression of several target genes, including drug-metabolizing cytochrome P450s (CYP1As). Four AhR genes (AhR1b-1, AhR1b-2, AhR2a, and AhR2b) were identified in the medaka genome. The molecular machinery involved in the dioxin response has been clarified chiefly in mammals, although fish models, such as zebrafish (Danio rerio), medaka (Oryzias latipes), and Fundulus, are excellent candidates for examining the mechanisms of developmental dioxin toxicity. Using these fish models, several experimental studies investigating the induced expression of CYP1A1 and AhRs, including functional evaluations by 2378T4CDD exposure, have been performed. However, few studies have examined the exposure to other dioxin isomers and it is not certain whether similar induced expressions patterns and toxicity-mediating functions of CYP1A1, AhRs, and AhR repressor (AhRR) compare with 2378T4CDD exposure. In this study, we investigated the toxicity of 13 dioxin isomers, including 2378T4CDD, and the induced expression of AhRs, AhRR, and CYP1A1 (CYP1A1_ORYLA) in the early life stages of medaka embryos. After exposure to dioxin isomers for 24-48 h, the expression of AhR2a and CYP1A1_ORYLA correlated to the dioxin toxicity, and AhRR mRNA was widely expressed indicating it modulates AhR activity during the early stages of medaka embryos. © 2009 Elsevier Ltd. All rights reserved. Source

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