Spatial and temporal variations of chlorophyll.a in the Seto Inland Sea using AQUA MODIS data Miwa Nakagawa, Xinyu Guo, Naoki Yoshie (Center for Marine Environmental studies, Ehime University, Japan)
Introduction Hiroshima Bay Honshu Aki Nada Bisan strait Harima Nada Osaka Bay Suo Nada Kyushu Iyo Nada Bingo Nada Hiuchi Nada Shikoku Kii Channel The Bungo Seto Channel Inland Sea is divided into several regions (basins, straits and channels). These regions have different features of water temperature and current.
Introduction Terrestrial nutrients are supplied from the river to the basin regions called nada in Japanese. Hiroshima Bay Honshu Aki Nada Bisan strait Harima Nada Osaka Bay Suo Nada Kyushu Iyo Nada Bingo Nada Hiuchi Nada Shikoku Kii Channel Bungo Channel Cold and nutrient-rich water intrudes from offshore area of the Pacific Ocean in the bottom layer.
Objective Most previous studies focused on only individual region. The relation of phytoplankton between all the regions in the Seto Inland Sea is still not well known. In this study... We focus on all the regions in the Seto Inland Sea. We compare the spatial and temporal variations of phytoplankton between regions.
Satellite Chl.a data Data: daily chlorophyll.a concentration of AQUA MODIS provided by JAXA (horizontal resolution of 500m) Period: April 2006 ~ March 2015 We calculated... (1)daily data monthly mean (2) monthly mean in different year monthly climatology
Representative regions We selected 12 representative regions. (6 basins, 4 straits and 2 channels) In this study, we calculated Chl.a average in the frame of representative regions. Kurushima Str. Hiroshima Bay Harima Nada Akashi str. Osaka Bay Naruto str. Suo Nada Iyo Nada Hiuchi Nada Kii Channel Hoyo Str. Bungo Channel 6
Accuracy of satellite Chl.a Hiroshima Bay Honshu Aki Nada Bisan strait Harima Nada Osaka Bay Kyushu Suo Nada Iyo Nada Bungo Channel Bingo Nada Hiuchi Nada Shikoku Kii Channel We compared the satellite Chl.a data with the in situ data (Yoshie et al 2011) from spring to autumn (Apr. ~ Oct.) in the Iyo Nada and the Bungo Channel.
Observation [mg/m 3 ] Observation [mg/m 3 ] Accuracy of satellite Chl.a The satellite Chl.a data was relatively accurate from spring to autumn in the Iyo Nada and the Bungo Channel. Iyo Nada Bungo channel n=35 r=0.45 RMSE=0.38 p<0.01 n=11 r=0.15 RMSE=0.19 p>0.05 AQUA/MODIS [mg/m 3 ] AQUA/MODIS [mg/m 3 ] significant correlation insignificant correlation RMSE is enough low
[mg/m 3 ] Spatial The satellite variation Chl.a is lower in the western regions than in the eastern regions and central regions. spatial distribution of climatology Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
[mg/m 3 ] Seasonal variation 10 5 1 0.1 10 5 1 0.1 10 5 1 basin channel strait in the representative regions The satellite Chl.a has a clear seasonal variations: high from autumn to winter and low in summer. The variations are large in basins and small in channels. The variations are also different among basins. large: Hiuchi Nada small: Hiroshima Bay 0.1 Apr Jun Aug Oct Dec Feb 10
Interannual The interannual variations of the satellite for 9 Chl.a years showed a significant in downward the representative trend in entire Seto Inland regions Sea. Suo Nada 2006 2015 monthly average Iyo Nada Hiroshima Bay climatology monthly average - climatology Hiuchi Nada Harima Nada Osaka Bay Bungo channel Kii channel Hoyo Str. Kurushima Str. Naruto str. Akashi str. 11
[mg/m 3 ] [mg/m 3 ] Comparison of the seasonal variation with the interannual variation seasonal variation standard deviation of climatology interannual variation Especially, the interannual variations in autumn has largest variability. standard deviation of monthly average - climatology May(spring) Aug(summer) Nov(autumn) Feb(winter)
20062008 2010 2012 2014 20062008 2010 2012 2014 Interannual variation in autumn The Chl.a has high concentration in 2008 and low in 2013. in the representative regions The variations in the Hiuchi Nada and the Osaka Bay are large among basins. The Kurushima and Akashi Straits have large variability due to the large variability of Hiuchi Nada and Osaka Bay. Suo Nada Iyo Nada Hiroshima Harima BayNada Hiroshima Bay Harima Nada Osaka Bay Suo Nada Iyo Nada Kurushima Str. Bungo channel Naruto str. Hiuchi Nada Hoyo Str. Hoyo Str. Kurushima Str. Naruto str. Hiuchi Nada Akashi str. Osaka Bay Kii channel Kii Channel Akashi str. Bungo Channel
Accuracy of satellite Chl.a in winter We compared the satellite Chl.a data with the continuous Chl.a data by the ferry from Kobe to Busan (Tsukamoto et al., 1997). Kyushu
1991 1992 1993 Accuracy of satellite Suo Nada Chl.a in winter Kurushima Str. high turbidity water Hiuchiassociated Nada with the Bisan Str. consistent between Harima two Nada studies (ours and Akashi Str. climatology of this study The satellite Chl.a is overestimated Iyo Nada during winter in all the regions of the Seto Inland Sea. The overestimate might be caused by the winter mixing in the shallow water. However, the minimum Chl.a concentration is Tsukamoto et al., 1997).
DIN conc.(μmol/l) DIP conc.(μmol/l) Causes for decreasing trend of Chl.a The decreasing trend of satellite Chl.a might be caused by the long term reduction of nutrients associated with the severe limitation of nutrient emissions. Osaka Bay Harima Nada (North area) DIN Harima Nada (South area) Kii Channel DIP From 水産庁委託 沿岸海域の栄養塩管理技術の開発 事業成果ダイジェスト, H27 19
Summary In the Seto Inland Sea... <temporal variation> Chl.a increase from summer to winter. Satellite Chl.a is overestimated during winter. Chl.a shows decreasing trend in recent years. The long term reduction of nutrient contributes to it. The magnitude of interannual variation of Chl.a is largest in autumn. <spatial variation> Chl.a is higher in the eastern than in the other regions. The magnitude of variations; basins > straits > channels.
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