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This paper proposes a new method for cruising-type autonomous underwater vehicles (AUVs) to track rough seafloors at low altitudes while also maintaining a high surge velocity. Low altitudes are required for visual observation of the seafloor. The operation of AUVs at low altitudes and high surge velocities permits rapid seafloor imaging over a wide area. This method works without high-grade sensors, such as inertial navigation systems (INS), Doppler velocity logs (DVL), or multi-beam sonars, and it can be implemented in lightweight AUVs. The seafloor position is estimated based on a reflection intensity map defined on a vertical plane, using measurements from scanning sonar and basic sensors of depth, attitude, and surge velocity. Then, based on the potential method, a reference pitch angle is generated that allows the AUV to follow the seafloor at a constant altitude. This method was implemented in the AUV HATTORI, and a series of sea experiments were carried out to evaluate its performance. HATTORI (Highly Agile Terrain Tracker for Ocean Research and Investigation) is a lightweight and low-cost testbed designed for rapid and efficient imaging of rugged seafloors, such as those containing coral reefs. The vehicle succeeded in following a rocky terrain at an altitude of approximately 2 m with a surge velocity of approximately 0.8 m/s. This paper also presents the results of sea trials conducted at Ishigaki Island in 2017, where the vehicle succeeded in surveying the irregular, coral-covered seafloor.

In this study, direct continuous measurements of deep-sea turbulent flows near the seafloor were made in the Izena Cauldron hydrothermal fields, Okinawa Trough, Japan, to understand the dynamics of the bottom mixed layer. The objectives of this study were to reveal time series fluctuations of deep-sea turbulence intensity, that is, the turbulence energy dissipation rate, using an expendable vertical microstructure profiler (VMP-X), and to correlate them with in situ environmental data (flow, turbidity, conductivity, temperature, and depth data) obtained from a monitoring station installed on the seafloor near the VMP-X observation point. The turbulence energy dissipation rate ( $\boldsymbol{\varepsilon}$ ) value near the seafloor changed from 10−10 to 10−7 W kg−1, and $\boldsymbol{\varepsilon}$ maxima were observed from a flood tide between ebb tides (13:00–15:30 and 20:30–23:00) around a depth of 960 m, whereas the $\boldsymbol{\varepsilon}$ values decreased to 10−9 W kg−1 during ebb tide. In the former case, the water temperature increased only slightly, but in the latter case, water temperature increased by 0.1 °C. Furthermore, the turbulent flow was relatively strong from the seafloor to a height of around 50 m from the bottom. Turbulent flow observation points were located to the west of the hydrothermal vent area. The bottom topography of this area is closed. Deep-sea current data were successfully obtained for the layers between the seafloor and 60 m above the bottom, and periodic fluctuations with east–west and north–south components were synchronously observed with water-level variations. According to these environmental data and complex (closed) bottom topography data, the increasing water temperature fluctuation may occur as a result of hydrothermal venting (transport or intrusion of warm water). Therefore, $\boldsymbol{\varepsilon}$ may be affected by hydrothermal venting. However, to better understand the dynamics of vertical mixing processes, more observational data, such as continuous turbulent flow data, are necessary. In future studies, we will collect and analyze more spatiotemporal field data using VMP-X to understand the dynamics of the bottom mixed layer and improve the accuracy of numerical models.

In this study, time series measurements of turbulent flows near the sea floor were taken at hydrothermal fields in the Okinawa Trough. The purpose of this study was to understand the dynamics of the bottom mixed layer (BML), reveal fluctuations in the time series of deep sea turbulent flows (the turbulent energy dissipation rates) using VMP-X, and correlate them with in-situ flow, turbidity, and conductivity, temperature, and depth data obtained from a monitoring station on the sea floor. At the Kumejima west field, time series fluctuations in turbulent energy dissipation rate () between the sea surface and the seafloor were successfully measured. The near the sea bottom changed from 10–10 to 10–7 (W kg-1) and the maximum was observed from a flood tide between ebb tides, and decreased to 10–9 (W kg-1) during the ebb tide on June 24. Moreover, the turbulent flow was relatively strong from the seafloor to a height of around 80 m above it, and water temperature rose 0.1–0.2 degrees Celsius. The measurement points were located approximately 900 m to the northwest of the hydrothermal vent area. Deep-sea current data were successfully recovered for the layers at depths between the seafloor and 30 m above it, and a periodic fluctuation with an amplitude of approximately 10 cm/s for the east-west and north-south components, which was synchronized with variations in the water level, was also observed. According to these environmental and complex bottom topography data, this characteristic water temperature fluctuation may occur as a result of hydrothermal venting (transport or intrusion of warm water). Therefore, the intensity of turbulence may also be affected by hydrothermal venting. However, further continuous in-situ observational data are necessary to understand the dynamics of the BML. In a future study, we will collect and analyze more spatiotemporal turbulent flow data to understand the dynamics of the BML and improve the accuracy of the numerical model.

In deep-sea mining, leakage of lifting water might have an adverse effect on phytoplankton. Therefore, understanding the fluctuation of phytoplankton is important for environmental impact assessment in deep-sea mining. In this study, the maximum fluorescence intensity (F m ) and maximum quantum yield (F v /F m ) of phytoplankton in the Okinawa trough and Izu-Ogasawara waters were determined using fast repetition rate fluorometry (FRRF). The peak F m was noted at a depth of 80–100 m, and the maximum F v /F m was recorded at a depth of 100–130 m. This result suggests the existence of subsurface chlorophyll maximum and distribution of phytoplankton groups with high photosynthetic capacity in the lower layer. Therefore, understanding the dynamics of phytoplankton in not only the surface layer but also the subsurface layer of the open ocean is important. Our observation results could be used as important baseline data. Furthermore, during monitoring surveys, immediately observing the fluctuation in phytoplankton and detecting the influence of lodged lifting water at an early stage are necessary. FRRF can rapidly measure the photosynthetic parameters of phytoplankton in-situ; this could render it suitable for use in monitoring surveys. Thus, observations using FRR fluorometer might be useful for environmental impact assessment in deep-sea mining.

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.

In this study, the current fluctuations were measured at the hydrothermal fields in the Okinawa Trough (Hatoma Knoll, Irabu Knoll, and Iheya North field) to characterize the fluid environment. The purpose of this study was to reveal the fluctuations of the deep-sea currents using an acoustic Doppler current profiler (ADCP), and to correlate them with the development of a bottom mixed layer as predicted by a numerical model. An ADCP (Teledyne RD Instruments Workhorse Sentinel, 300 kHz) was installed on the seabed at the Hatoma Knoll (depth about 1500 m), Irabu Knoll (depth about 2000 m), and Iheya North field (depth about 1000 m). For each observation point, the deep-sea current data were successfully recovered in the layer at depths between the seafloor and 40–60 m above the seafloor. Notably, the measurement results obtained at the Hatoma Knoll and Iheya North field showed a remarkably interesting fluctuation. During a time period of approximately 3 h, the range in the depths at which the currents were successfully recorded increased in a pulse form, up to around the upper layer at 120 m, allowing measurements in almost all of the layers. The time period of this pulse corresponded to the shift from an ebb tide to high tide. With the exception of these pulses, the currents could only be recorded from the seafloor to about a 40 m layer. This result indicated that a suitable environment for ADCP measurements was formed only within this range. To investigate the cause, a numerical model experiment was performed for the bottom turbulence and subsequent occurrence of vertical mixing of the suspended particulate matter in the Okinawa Trough using a large-eddy simulation (LES) model. The simulated results suggested that the semidiurnal tide periodically enhances the bottom turbulence at intervals of approximately 6 h, with the result that the bottom mixed layer thickness increases to a maximum of approximately 40–60 m. It was concluded that a suitable environment for ADCP measurements near the deep seafloor was formed mainly by the bottom mixed layer corresponding to the tide.

We were involved in development of a small lander for low-price and usability. The free fall type hadal zone research lander called “Edokko No.1” consists of 3 glass spheres and wireless connectable rubber bridge, and an observation equipment and levitation transponder device can be installed in the glass sphere. A continuous monitoring operation of periodic variations of surrounding ecosystem at deep-seabed is essential for acquiring baseline data and knowledge towards proposal of an appropriate methodology for underwater environmental impact assessment. In the demonstration test in 2013, the video of the hadal zone at the depth of 7800m has been acquired. By the bate trap survey, to observe the amphipods and a hadal fish (Pseudoliparis amblystomopsis). We will demonstrate the transition of deep-sea ecosystem. Time-lapse observation of the seabed is useful to an understanding of the seabed environment.

For the purpose of environmental impact assessment (EIA) at deep-sea hydrothermal environments towards development and utilization of seafloor massive sulfide (SMS) deposits, we have been developing a cost effective cabled underwater observatory. To realize a flexible and expandable system configuration, the observatory has a simple star-type network and power supply topology by employing an Ethernet interface. Pressure hulls and a framework of the observatory are composed of glass or plastic material to avoid corrosion of metals caused by hydrothermal fluid during long-term operation. The first observatory that has physicochemical sensors, water current profilers, and a video instrument will be deployed at the Oomuro Hole hydrothermal site, the northern Izu-Ogasawara arc for the purpose of system demonstration and practical data acquisition.