Kyushu University [Soichi Yamaguchi (Assistant Professor) Faculty of Engineering Sciences, Department of Advanced Environmental Science and Engineering]

Soichi Yamaguchi

Last modified date：2023.10.06

Assistant Professor / Earth System Science and Technology / Department of Advanced Environmental Science and Engineering / Faculty of Engineering Sciences

Presentations

1.	Japanese society of phycology, [URL].
2.	Soichi Yamaguchi1, Shota Koide1, Takayuki Mine, Takuya Iwanaga, Observation of red tide in an estuary by a fixed-wind type unmanned aircraft, Japan Geoscience Union Meeting 2021, 2021.06, method, enables spatial monitoring, but in the orbit of the satellite, weather condition (clouds), and sea turbidity and seaweed aquaculture in the inner part of the Ariake Sea, which is the target sea area of this research, makes it difficult to obtain data with sufficient spatiotemporal resolution. Observations using unmanned aerial vehicles (UAVs) have been in the limelight in recent years because an aircraft observation has a high degree of freedom in observation such as immediacy and directness, and can perform observations with high spatiotemporal resolution that cannot be performed by satellite or ground observation. In this study, we aimed to establish a spatiotemporal high-resolution sea surface Chl-a concentration remote sensing method using a fixed-wing unmanned aircraft equipped with a spectroradiometer, and attempted to detect red tide in coastal waters. A handheld Spectra PAR Meter (PG200N by UPRtek corp.) was mounted on the UAV to estimate the Chl-a concentration in the sea surface. The PG200N can measure spectral data of light in the visible light region (350 nm to 800 nm) with a wavelength resolution of 1 nm, and can estimate the concentration using the optical characteristics of the Chl-a dye possessed by phytoplankton. It is small and lightweight, suitable for installation on UAV. In the observation, the irradiance incident on the sea surface from the sky and the radiance emitted from the sea surface were measured. In estimating the Chl-a concentration from the obtained spectral data, the reflectance of each wavelength (= upward radiance from the sea surface / downward irradiance from the sky) was calculated, and applied to a three-waveband model (Dall'Olmo, G. and Gitelson, A.A, 2003). Field observations were conducted on December 7 and 18, 2000 and January 5, 2021, and for fixed-wing type unmanned aircraft, OPTiM Hawk V2 (manufactured by OPTiM Corporation) has two PG200N sensors inside the aircraft body, so that it faced directly above and below and measured the spectral data of light from the sky and the sea. The flight speed of OPTiM Hawk V2 was set to 70km / hour, and the spectral data was acquired every 5 seconds, resulting in the spatial resolution of the data to about 100 m. A high correlation was obtained as a whole between the Chl-a concentration estimated by the 3-waveband model and the measured value by the field observation. However, the estimation error tended to increase in the sea area where the Chl-a concentration was high. Generally, when the concentration of phytoplankton is increased, the "packaging effect" that change in the light absorption coefficient per unit Chl-a concentration due to the overlapping of pigments occurs, and the Chl-a concentration estimated by sea color remote sensing is underestimated or overestimated. Therefore, analysis of covariance was performed to estimate the threshold of Chl-a concentration at which the packaging effect appears. Based on this threshold value, we succeeded in developing an algorithm that can estimate Chl-a concentration with high accuracy even in the high Chl-a concentration sea area. In the three observations, a high-concentration local region, which is considered to be the initial occurrence of red tide, was detected. Therefore, this observation using UAV has sufficient performance to grasp the spatiotemporal high resolution of red tide in the coastal area..
3.	In the Ariake Sea, large-scale seaweed cultivation is carried out in the fall and winter, and the production volume is the highest in Japan. However, in recent years, production instability has become a problem, such as the depletion of nutrients due to the occurrence of red tide causing discoloration of seaweed, and elucidation of its formation mechanism has become an urgent issue. Regarding the characteristics of this red tide, Matsubara et al. (2011) reported that red tide is formed mainly in the western sea area of Saga prefecture regardless of species during the aquaculture season. Minamiura and Yamaguchi (2019) pointed out that the high concentration in the western sea area is due to the weakness of seawater exchange, based on the examination using numerical simulation. The dynamics of red tide algae are related not only to the physical environment but also to biochemical reactions such as growth / death and predation of the algae. In addition, seaweed cultivation not only affects the flow, but also uptake water nutrients, so it is in a competitive relationship with red tide algae. Therefore, in order to elucidate the formation mechanism, it is necessary to develop a numerical ecosystem model that incorporates the physiological and ecological characteristics of red tide algae and the effects of seaweed cultivation on the physical, biological, and chemical environments. Therefore, in this study, in order to elucidate the formation mechanism of red tide in the seaweed cultivation environment, we developed a numerical ecosystem model and examined the factors of red tide formation, especially the factors of high concentration in the deep western sea area, using this model..
4.	In the Ariake Sea, which is located in the northwestern part of Kyushu, seaweed is actively cultivated in the fall and winter, and the production volume is the highest in Japan. However, due to the depletion of nutrients due to the occurrence of red tide, the quality of seaweed deteriorates. In order to elucidate the actual condition of red tide, it is important to grasp the process from the early stage of development through the proliferative phase to the decay phase with high resolution in terms of time and space. Although it is possible to spatially monitor the sea surface chlorophyll a (Chla) concentration by satellite, it is affected by the orbit and weather condition, and in the the Ariake Sea, it is affected by turbid water and seaweed on the sea surface. So that, it is difficult to obtain data with sufficient resolution. In recent years, observations using unmanned aerial vehicles (UAVs) have become widespread, and many useful results have been obtained by analyzing various data obtained from the sky. Such UAV observations make it easy to obtain data with high spatiotemporal resolution. In this study, a multispectral spectroradiometer is mounted on the UAV for the construction of a spatiotemporal high-resolution red tide monitoring system using UAV, and a field experiment is conducted to establish a non-contact chlorophyll a concentration estimation method in high turbidity sea areas. Was performed. As a result, it was found that high-precision estimation of sea level Chla concentration is possible..
5.	RELATIONSHIP BETWEEN PHYSICAL ENVIRONMENT AND 3 PHYTOPLANKTON SPECIES, SKELETONEMA SPP., EUCAMPIA ZODIACUS, AND ASTEROPLANUS KARIANUS CAUSING COLOR BLEACHING OF CULTURED NORI IN THE ARIAKE SEA DURING WINTER.
6.	Yuichi Hayami, Soichi Yamaguchi, Naoki Fujii, The mechanism of the progress of hypoxia in Ariake Sea without the increase of terrestrial load, ECSA 57: Changing estuaries, coasts and shelf systems - Diverse threats and opportunities, 2018.09, In the inner area of Ariake Sea, Japan, serious hypoxia occurred every summer and generated massive kill of bivalves. So, it became a big social problem. Since the macrobenthos in this area was richer at the end of 1980s than now, it was supposed that the hypoxia progressed in these 30 years. However, as the terrestrial load of organic matter and nutrients did not increase, it was unknown why the hypoxia progressed. Our recent studies clarified that there were 3 mechanisms that generated the long term progress of hypoxia. The first one was the increase of organic matter from 1970s to early 90s. The box model analysis of COD showed that it was generated by the net ecosystem production. It would be due to the decrease of bivalves and enhancement of the water residence time. The second one was the influence of the sea dike construction in Isahaya Bay which is a sub-basin of Ariake Sea in 1997. The sea dike construction weakened the tidal current between the inner area of Ariake Sea and the bay mouth. Therefore, the vertical mixing in this area decreased and the estuarine circulation was enhanced. The stratification in the inner area became stronger as the enhancement of the intrusion of the dense water in the bottom layer caused by the estuarine circulation. The third one was the long term change in tidal amplitude including 18.6 years lunar nodal cycle. There was an inter-decadal scale decrease of tidal amplitude in outside of the bay. Also, the tidal amplitude of M2 tides decreased from late 1990s to 2000s due to the lunar nodal cycle. So, the tidal amplitude in 2007 was lowest in these 50 years. As the tidal amplitude decreased, the tidal mixing weakened and the stratification was enhanced. It generated further progress of hypoxia..
7.	Soichi Yamaguchi, Naoya. Minamiura, Mechanism of red tide outbreaks in winter in the western area of the inner Ariake Sea, ECSA 57: Changing estuaries, coasts and shelf systems - Diverse threats and opportunities, 2018.09, Aquaculture of fishery resources has been conducted in coastal waters all over the world. In Ariake sea, a region of freshwater influence (ROFI) in Japan, aquaculture of Porphyra yezoensis, namely ”Nori” has been conducted in great magnitude during autumn and winter. Red tide outbreaks by several phytoplankton species frequently occur during the aquaculture season, which causes color bleaching of the nori due to depletion of nutrients in water column and prevents stable production. One of the characteristics of red tide occurrences is for the red tides to be concentrated in western area of the inner Ariake Sea independently of the phytoplankton species. The mechanism for this is poorly understood. The objective of the present study is to clarify factors for the concentration of red tide outbreaks in the western area of the inner Ariake Sea using numerical simulation and the observed data. The results of numerical simulation revealed that the sea water residence time was relatively long in the western area of the inner Ariake Sea compared to those in other areas. There was a high correlative relationship between red tide index (cell number × duration time) and the water residence time. On the other hand, growth rate of a representative diatom : Eucampia zodiacus calculated by the observed nutrient concentration and transparency hardly has a correlation with the red tide index. Finally, the impact of nori aquaculture on the water residence time and transparency was evaluated by the numerical simulation with/without the hydrodynamic resistance of the aquaculture facilities. The results of numerical simulations inferred that the aquaculture increased the water residence time in the western area of the inner Ariake Sea and therefore made the hydrographic environment more suitable for the red tide outbreaks. Sustainable practices of the aquaculture is necessary for the future stable production. .
8.	SEASONAL CHANGE IN TIDAL CURRENT ENERGY DUE TO TIDE-INDUCED EDDY IN GOTO ISLANDS, JAPAN, [URL].
9.	Mechanism of red tide outbreaks in winter in Ariake Sea, [URL].
10.	Long term variation in hypoxia in the inner area of Ariake Sea.
11.	VARIATION OF DO CONCENTRATION IN COLD-DOME WATER IN ARIAKE SEA.
12.	METHOD VALIDATION FOR REDUCING PRESSURE GRADIENT FORCE ERROR IN COASTAL OCEAN MODELING .
13.	山口創一, Tidal Current Energy Potential in Nearshore Waters of Western Japan, International conference on ocean energy 2016, 2016.02, [URL], Much attention has been paid to tidal renewable energy application in Japan since the earthquake in 2011. This study estimates the tidal current energy potential in nearshore waters of western Japan by unstructured-grid finite volume coastal ocean model..
14.	Soichi Yamaguchi, Yusaku kyozuka, TIDAL CURRENT ENERGY MAP AROUND KYUSHU-OKINAWA REGION, JAPAN, Grand renewable energy 2014, 2014.07, Numerical simulation is conducted for obtaining tidal current energy map around Kyushu- Okinawa region, Japan, using finite volume coastal ocean model:FVCOM. Calculation domain covers whole area of Kyushu-Okinawa region and consists of unstructured triangular meshes which can have high resolution near coasts with complicated geometry and low resolution in offshore sea area, so that the present model continuously can calculate the detail distribution of tidal current from offshore area to near coasts with computational efficiency. Simulation results are validated with tide-gauge data observed at coasts over the Kyushu-Okinawa region. The present model shows good agreement with the observed. High tidal current was detected in narrow channels such as those in Goto islands, Kanmon strait and Ariake Sea, exceeding 3.0 m sec-1. Based on the calculated current field, tidal energy map is also obtained. This map can be used for estimating electric power gain and selecting the location best suited to tidal energy plant..
15.	Numerical simulation is conducted for obtaining tidal current energy map around Okinawa-Kyushu region, Japan, using finite volume coastal ocean model:FVCOM. Calculation domain covers whole area of Kyushu- Okinawa region and consists of unstructured triangular meshes which can have high resolution near coasts with complicated geometry and low resolution in offshore sea area, so that the present model continuously can calculate the detail distribution of tidal current from offshore area to near coasts with computational efficiency. Simulation results are validated with tide-gauge data observed at coasts over the Kyushu- Okinawa region. The present model shows good agreement with the observed. High tidal current was detected in narrow channels such as those in Goto islands, Kanmon strait and Ariake Sea, exceeding 3.0 m sec-1. Based on the calculated current field, tidal energy map is also obtained. This map can be used for estimating electric power and selecting the adequate location for tidal energy plant..
16.	Red tides frequently occur in Isahaya Bay, Ariake Sea every summer, especially much attention is paid to the red tide dominated by Chattonella spp. (C. antiqua and C. marina) because it leads to the death of livings in the sea. One of the characteristic phenomena related to the red tide outbreak is the vertical mixing of water column before its occurrence due to meteorological disturbance such as typhoon arrival. In the present study, the author conducted biweekly field measurements on nutrient dynamics in Isahaya Bay before/after the typhoon arrival in summer 2005. Meteorological disturbance by the typhoon caused complete vertical mixing of water column and resuspension of bottom sediment in large quantity. Dissolved inorganic nitrogen (DIN) and phosphorus (DIP) concentration greatly increased after the disturbance, especially by a factor of about 6.4 in vertically averaged DIN concentration. Nutrient budget analysis showed that the nutrient supply from the bottom sediment including pore water and resuspended sediments was dominant for the increase. Estimated nutrient release rate in the present study was more than two times the rate obtained by laboratory experiments that previously conducted using undisturbed bottom sediment. It is important to take into account the effect of bottom sediments dynamics on the nutrient environment in evaluating the flux between water and seabed..
17.	Huge tidal flats with tidal creeks develop around rivers in Ariake Sea. River water that contains anthropogenic pollutants flows into the inner sea area over complicated geometry of the tidal flats. Numerical simulation for the Ariake Sea, up to date, has applied low-resolution numerical model incapable of representing the geometry in detail. The present study applied a finite volume coastal ocean model (FVCOM) with high resolution capable of representing such complex geometry. The model results have been validated against extensive observations and showed that the complex geometry of the tidal flats with tidal creeks had great effects on the dynamics of river water. Also shown is the dependence of the model accuracy on the model resolution around rivers by comparing with coarse resolution models, and its effects on the calculated dynamics of river waters., [URL].
18.	Generation Mechanism of Upwelling current with “Aoshio” occurrence in summer of 2008 in Ariake Sea.
19.	Proposal of open bondary condition for the numerical simulation of Ariake Sea.
20.	Reproduction and its formation mechanism of cold-dome in Ariake sea in summer.
21.	Study on the formation mechanism of density stratification in Isahaya Bay.
22.	Cell division of bloom-forming dinoflagellate Akashiwo sanguinea in the Ariake Sea, Japan.
23.	A NUMERICAL SIMULATION OF PHYSICAL ENVIRONMENTS IN THE ARIAKE SEA INCLUDING THE EFFECTS OF THE TIDAL FLAT.
24.	Variation of Water Quality due to the location of a Mega-Float in Tokyo Bay.
25.	A Numerical Study of Physical Environments in the Ariake Sea.
26.	A NUMERICAL MODEL FOR WATER QUALITY AROUND THE MEGA-FLOAT IINCLUDING THE EFFECTS OF MARINE MUSSEL IN TOKYO BAY.
27.	Variability of Water Quality Due to the Location of a Maga-Float in Tokyo Bay.

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