Impact assessment of the Japanese tsunami on ocean-surface chlorophyll concentration using MODIS-Aqua data

Abstract

National Aeronautics and Space Administration (NASA) moderate resolution imaging spectroradiometer (MODIS) Aqua-retrieved chlorophyll concentration data sets have been analyzed and interpreted, as have changes in chlorophyll concentrations associated with the tsunami that occurred on March 11, 2011, in Sendai, Tohoku (latitude 38.3°N, longitude 142.4°E). The eight-day and three-day composite data sets were archived from the NASA MODIS website to generate chlorophyll images during March 2011, before, during, and after the tsunami. The chlorophyll concentration variability was studied with reference to satellite images, a transect line, eight points, and mean plots over two subsets in the coastal and offshore water. An increase in chlorophyll concentration of up to 5.0 mg/m3 along the transect line and in the range of 1.0-4.3 mg/m3 in the coastal water points and subset (10×10) was observed. The chlorophyll mean ranged from 0.289 to 1.237 mg/m3 and 0.123 to 0.271 mg/m3 for subsets 1 and 2, respectively. The offshore water subset was about 300 km from the tsunami site, and the chlorophyll range has not shown more gradients in post-tsunami images, as was evident from subset-2 mean data. Data from March 2010 and 2009 was processed for the same locations to comply with any seasonal trend. But the recent 2011 tsunami had a sudden episodic impact on the coastal water of Japan, enhancing chlorophyll concentration without being tallied with seasonal trends. So, the episodic effect seemed to be significant instead of being a seasonal trend.As observed from field photographs and previous tsunami events, water receded from the land, along with a tsunami return wave that probably carried nutrients and minerals, and even upwelling would have enriched the phytoplankton growth and sudden chlorophyll concentration increase in the post-tsunami period found convincing in satellite images. The anomaly of chlorophyll observed was significant, with p<0.02 during 2011.