1 3 DOI 10.1007/s10872-015-0330-4 J Oceanogr SPECIAL SECTION: ORIGINAL ARTICLE Long-term monitoring of bottom environments of the continental slope off Otsuchi Bay, northeastern Japan Kazumasa Oguri 1 · Yasuo Furushima 1 · Takashi Toyofuku 1 · Takafumi Kasaya 1 · Masahide Wakita 1 · Shuichi Watanabe 1 · Katsunori Fujikura 1 · Hiroshi Kitazato 1 Received: 31 March 2015 / Revised: 21 September 2015 / Accepted: 27 September 2015 © The Oceanographic Society of Japan and Springer Japan 2015 Keywords Off Otsuchi Bay · The Tohoku Earthquake of 2011 · Long-term monitoring · Water mass · Marine snow · Benthic habitats · Sediment 1 Introduction On 11 March 2011, the Tohoku Earthquake off the Pacific coast of Japan caused strong shaking and a huge tsunami along the entire Pacific coast of the Tohoku region of Japan. After the earthquake and many aftershocks, dense nephe- loid layers were widely observed in the sea off northeastern Japan (Kawagucci et al. 2012; Noguchi et al. 2012; Oguri et al. 2013). Fine sediment with a high water content, pre- sumably related to turbidity currents, widely covered the seafloor, in both shallow and hadal regions (Ikehara 2012; Ikehara et al. 2012; Oguri et al. 2013; Nomaki et al. 2015). Surface sediments (Arai et al. 2013) and meiobenthic habi- tats (Toyofuku et al. 2014) were disturbed not only by earthquake shaking but also by unusually intensive hydro- dynamic currents triggered by the tsunami. Physical distur- bances to the seafloor caused by the series of earthquakes and the tsunami were investigated during many cruises carried out just after the disaster. However, changes to the seafloor environments have not been fully explored because access to the seafloor has been limited. The northeastern Pacific coast and continental slope of Japan are character- ized by high primary productivity, owing to the mixing of warm and cold water systems (Saino et al. 1998), and fisher- ies are quite active. In 2010, for example, 97,857 tonnes of fish were caught by the fisheries of Iwate Prefecture (Iwate Prefecture 2011). When the area of coseismic slip is taken into account (Ozawa et al. 2011; Sato et al. 2011), physi- cal disturbances to the seafloor resulting from the earth- quake are likely to have influenced fisheries throughout Abstract Long-term monitoring of the seafloor environ- ment off Otsuchi Bay, northeastern Japan, was carried out to investigate environmental changes of the deep-sea floor after the 2011 off the Pacific coast of the Tohoku Earth- quake. We deployed two deep-sea stations, one on an upper continental slope site (around 300 m water depth) and the other on a bathyal (998 m) site, to measure current inten- sity and direction, water temperature, salinity, dissolved oxygen, and turbidity, and to obtain seafloor images. The monitoring period on the upper slope was 9 months (March–September 2013), and 14 months (August 2012– October 2013) at the bathyal site. The oceanographic data from the upper slope site recorded the seasonal exchange of water masses and dense marine snowfall from April to May 2013. On 7 December 2012, a large aftershock of the 2011 earthquake caused increased turbidity at the bathyal site, and seafloor photographs and videos recorded the dis- turbance and recovery of benthic habitats. The data from these deployments show that long-term monitoring can provide important oceanographic, biological, and sedimen- tological data from deep-sea sites. Together with shipboard and deep-tow observations and data collected by remotely operated vehicles, long-term monitoring can be a valuable tool for understanding deep-sea environments and their variability. Oceanographic observations after the 2011 earthquake off the Pacific coast of Tohoku Electronic supplementary material The online version of this article (doi:10.1007/s10872-015-0330-4) contains supplementary material, which is available to authorized users. * Kazumasa Oguri ogurik@jamstec.go.jp 1 Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, Kanagawa, 237-0061, Japan