Long-term monitoring of bottom environments of the continental slope off Otsuchi Bay, northeastern Japan

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Long-term monitoring of the seafloor environment 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 Earthquake. 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 intensity 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 disturbance and recovery of benthic habitats. The data from these deployments show that long-term monitoring can provide important oceanographic, biological, and sedimentological 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.