Ariake Bay Area Research Articles

Use of sea surface discoloration to monitor and discriminate the causative genera of harmful algal blooms (HABs): Practical use of digital repeat photography

Harmful algal blooms (HABs) are associated with water quality degradation, which damages fisheries, ecosystems, and public health. Consequently, techniques for detecting the occurrence of HABs have been developed globally. However, conventional techniques are unable to discriminate the causative species/genera of HABs objectively. Therefore, the aim of the present study was to develop a technique for frequently analyzing the causative genera of HAB events using repeat digital photography with a stationary device named “HABcam.” The HABcam was installed at Okigamisenishi monitoring station in the inner western area of Ariake Bay, Japan, and the digital images obtained were analyzed to quantify the sea surface color and to estimate the probability of a HAB occurrence on a daily basis using Bayes' theorem. The estimated probability of a HAB occurrence accurately discriminated a HAB occurrence or non-occurrence in the research area on 19/21 days (=90.5%) for Chattonella spp. and 7/8 days (=87.5%) for Skeletonema spp. These findings indicate that this technique can be used to objectively determine the causative genera during HAB events and to observe HABs with high accuracy and at a high frequency.

Convolutional Neural Network Considering Physical Processes and its Application to Diatom Detection

Convolutional Neural Network (CNN) considering physical processes with time series of stages for diatom detection with remote sensing satellite derived physical data (Chlorophyll-a, Photosynthesis Available Radiance (PAR), Turbidity, Sea Surface Temperature (SST)) and meteorological data is proposed. Diatom is bloomed under the condition of suitable sea water temperature, nutrition rich water (Chlorophyll-a derived from river water flow), photosynthesis available radiance derived from solar irradiance, transparency of the sea water for photosynthesis (turbidity), and sea water convection between bottom sea water and sea surface water. Almost all the conditions can be monitored by remote sensing satellite-based radiometers. The proposed diatom prediction based on convolutional neural network with remote sensing satellite and meteorological data is validated. Through the experiments at Ariake bay area, Kyushu, Japan with gathered time series of remote sensing data of Moderate resolution of Imaging Spectroradiometer (MODIS) derived turbidity as well as chlorophyll-a data estimated for the winter seasons (from January to March) during from 2010 to 2018 together with measured and acquired meteorological data for the same winter seasons, the proposed method is validated.

Feeding habits, age, growth and reproductive biology of Coilia nasus (Engraulidae) in the Rokkaku River estuary, innermost area of Ariake Bay, Japan

Feeding habits, age, growth and reproductive biology of<i> Coilia nasus</i> (Engraulidae) in the Rokkaku River estuary, innermost area of Ariake Bay, Japan

Duration rather than frequency of hypoxia causes mass mortality in ark shells (Anadara kagoshimensis)

Hypoxia is associated with mass mortality in estuaries, but a direct causal relationship has not been proven to date. This study aimed to demonstrate this relationship and to evaluate how the duration of hypoxia affects the survival of ark shells (Anadara kagoshimensis) using mathematical modeling. The dissolved oxygen concentration was monitored at two stations in the innermost area of Ariake Bay, Japan, to calculate the duration of hypoxia. This was then included in a mathematical model to simulate the population density with sequential computation. The population density decreased with prolonged hypoxia, reaching a value close to the observed population density, indicating that hypoxia is the main cause of mass mortality in ark shells. Furthermore, the ark shell population disappeared in 8days with constant hypoxia but persisted when hypoxia was alternated with normoxia every 6 h. Therefore, mass mortality is caused by the duration rather than the frequency of hypoxia.

Prediction Method for Large Diatom Appearance with Meteorological Data and MODIS Derived Turbidity and Chlorophyll-A in Ariake Bay Area in Japan

Prediction method for large diatom appearance in winter with meteorological data and MODIS derived turbidity and chlorophyll-a in Ariake Bay Area in Japan is proposed. Mechanism for large diatom appearance in winter is discussed with the influencing factors, meteorological condition and in-situ data of turbidity, chlorophyll-a data with the measuring instruments equipped at the Saga University own Tower in the Ariake Bay area. Particularly, the method for estimation of turbidity is still under discussion. Therefore, the algorithm for estimation of turbidity with MODIS data is proposed here. Through experiments, it is found that the proposed prediction method for large diatom appearance is validated with the meteorological data and MODIS derived turbidity as well as chlorophyll-a data estimated for the winter (from January to March) in 2012 and 2015.

Carabid beetles (Carabus blaptoides) from Nii-jima and O-shima isles, Izu-Bonin oceanic islands: Dispersion by Kuroshio current and the origin of the insular populations

Carabus blaptoidesis endemic on the Japanese islands. Two populations of this species were recently found on the oceanic islets of O-shima and Nii-jima, offshore of central Japan. We evaluate the origin of these populations based on morphological and molecular phylogenetic analyses. Molecular analyses show that the O-shima population is closely related to the Boso peninsula population of Honshu, and the Nii-jima population is closely related to the northern Kyushu population. We suggest that the Boso population migrated relatively recently from central Japan, apparently by way of a land bridge formed by an emergent forearc high, followed by further migration to O-shima. In contrast, the Nii-jima population migrated very recently from northern Kyushu, probably by rafting on the Kuroshio current aboard driftwood swept into the sea from floods that occurred in the Ariake bay area in 2012. Similar Kuroshio-driven migration is also expected for the ancient Izu-Bonin oceanic islands.

One of the Possible Causes for Diatom Appearance in Ariake Bay Area in Japan In the Winter from 2010 to 2015 (Clarified with AQUA/MODIS)

One of the possible causes for diatom appearance in Ariake bay area I Japan in the winter seasons from 2010 to 2015 is clarified with AQUA/MODIS of remote sensing satellite. Two months (January and February) AQUA/MODIS derived chlorophyll-a concentration are used for analysis of diatom appearance. Match-up data of AQUA/MODIS with the evidence of the diatom appearance is extracted from the MODIS database. Through experiments, it is found that diatom appears after a long period time of relatively small size of red tide appearance. Also, it depends on the weather conditions and tidal effect as well as water current in the bay area in particular.

Sensitivity of Salt-leached Clay Sediments in the Ariake Bay Area, Japan

This study aims to determine whether the principle that “divalent cation predominance in the pore water precludes quick clay development” applies to the Ariake Bay sediments. The chemical and geotechnical properties of an Ariake clay sediment are presented, and sensitivity is discussed with a focus on pore-water salinity and cation composition. In recent years, reduction of pore-water salinity has occurred due to permeation of river water through the sediments because of overpumping of groundwater. Sodium remains the dominant pore-water cation in an upper zone, whereas divalent cations are dominant in the deeper zone. Divalent cation domination in the deeper zone is ascribed to Ca release from nearby cement-stabilized sediments and to Mg increase in response to a change in river water quality. The upper zone's sensitivity ranged from 15 to 77, and the remolded strength was mostly <0.5 kPa, such that quick clay was present over much of its depth. In contrast, the deeper zone's sensitivity was <40, and its remolded strength exceeded 0.5 kPa; quick clay was not present despite the <2 g/L salinity. The absence of quick clay is ascribed to the high remolded strength caused by the pore-water divalent to monovalent cation ratio being greater than 0.25.

Quick Clay Development and Cation Composition of Pore Water in Marine Sediments from the Ariake Bay Area, Japan

By analyzing the cation composition of pore water in the soil samples of Ariake Bay sediments, the present study assesses the development of quick clay by leaching in both the original and seawater-saturated soil samples. Divalent cations were dominant in the pore water of the original soil sample, whereas Na+ was the major cation in that of the seawater-saturated soil sample. The cation proportion in the pore water for both soil samples remained the same after leaching. The difference in pore water cation composition between the original and seawater-saturated soil samples affected how their geotechnical properties changed through leaching. The undisturbed shear strength of both soil samples remained almost the same, but a large disparity between the soil samples was observed in the remolded shear strength: it remained almost the same in the original soil sample after leaching. Hence, sensitivity was not increased and quick clay was not formed. However, in the seawater-saturated soil sample, the remolded shear strength decreased to a great extent, and quick clay with a sensitivity exceeding 700 developed. The lack of development of quick clay in the original soil sample is attributed to the dominance of divalent cations in the pore water, and the development of quick clay in the seawater-saturated soil sample is ascribed to the dominance of Na+ in the pore water.

Seasonal variations of water column nutrients in the inner area of Ariake Bay, Japan: the role of muddy sediments

To investigate seasonal variations of nutrient distribution in the mudflat-shallow water system, we conducted field surveys once a month from August 2007 to July 2008 in the inner area of Ariake Bay (IAB), Japan. The NH4 (+)-N concentration of the water column increased in autumn because of the high NH4 (+) release from the sediments, ranging from 850 to 3,001μmol m(-2) day(-1). The NO3 (-)-N concentration was maximal in January, which was thought to be caused by NO3 (-) release from the oxic sediments and by NO3 (-) regeneration due to water column nitrification. The PO4 (3-)-P concentration of the water column was high in summer-autumn due to the high PO4 (3-) release from the reduced sediments, ranging from 22 to 164μmol m(-2) day(-1). We estimated the total amounts of DIN and PO4 (3-)-P release (R DIN and [Formula: see text], respectively) from the muddy sediment area of the IAB. In summer-autumn, R DIN and [Formula: see text] corresponded to about 47.7% of DIN input and about 116.6% of PO4 (3-)-P input from the river, respectively. Thus, we concluded that the muddy sediments were an important source of nutrients for the water column of the IAB during summer-autumn. In addition, we found that phosphorus necessary for the growth of Porphyra (Porphyra yezoensis, Rhodophyceae) would be insufficient in the water column when phosphorus during the Porphyra aquaculture period is supplied only from the river. Therefore, the phosphorus release from the muddy sediments was thought to play an important role in the sustainable production of Porphyra in Ariake Bay.

Discovery of an early Middle Pleistocene marker tephra around the Ariake Bay area, Southwest Japan

中部更新世前期のテフラが筑紫平野とその周辺の堆積物コアと露頭から発見された．鉱物の種類組成，屈折率，化学組成，フィッショントラック年代に基づき，このテフラは約0.6 Maに東九州から噴出した由布川火砕流堆積物（Yfg-pfd）に対比される．

有明海湾奥西部海域の底質変動特性

有明海湾奥西部海域の夏季における短期的な底質変動を明らかにするために，2011年夏季に水質および底質調査を週1回行った．貧酸素水塊は，淡水流入，低水温・高塩分水の貫入，気象条件によって形成と消滅を繰り返した．硫化水素は貧酸素水塊が形成することによって生成し，台風の影響による時化によって低濃度化した．また，酸化還元電位（Eh）が-180mV程度の領域と高い硫化水素濃度が見られた領域とは対応していた．高い硫化水素濃度が見られた堆積物のpHは7.2程度であったことから7月下旬のサルボウの斃死要因は，貧酸素水塊の形成と硫化水素の生成であることが示唆された．

Heavy Metal Runoff from a Low Farm Block on the Coastal Creek Area of Ariake Bay in Japan

Heavy Metal Runoff from a Low Farm Block on the Coastal Creek Area of Ariake Bay in Japan

Fractionation of Oxygen Consumption component in the inner western area of Ariake Bay, Japan

Importance of chemical oxygen consumption to the formation of hypoxia was cleared by measuring the DO concentration at the 0.2 m above the bottom, analyzing the hydrogen sulfide profiles in the sediment and some field experiments of the fraction of oxygen consumption (biochemical and chemical oxygen consumption) in the inner western area of Ariake Bay. Contribution rate of the chemical oxygen consumption to the total oxygen consumption at the 0.2 m above the sediment were 99.6% on 1 August, others were low (18 and 31 August). High chemical oxygen consumption before the hypoxic condition implicated that the chemical oxygen consumption has an important role to the generation of hypoxia in the inner western area of Ariake Bay.

Spatio-Temporal Variations of Turbulent Energy Dissipation Rate during Stratification Period in Ariake Bay

In recent years in-situ measurements on microstructures are carried out to investigate turbulence structures in shallow sea areas. However it is difficult to figure out turbulent structures near interfaces, such as sea beds, sea surfaces and pycnocline, because of the measuring principle of a commonly-used shear sensor. In this study, field observations on microstructures were carried out in Ariake Bay by using the microstructure profiler which acquires fine-scale temperature gradients. As a result of these observations, spatio-temporal variations of turbulent energy dissipation rate in the northern area of Ariake Bay were observed and it is clarified that the way to estimate turbulent energy dissipation rate from temperature gradients in shallow sea areas is valid especially under strongly stratified condition.

Characteristics of hypoxia in the inner western area of Ariake Bay

To investigate and verify the mechanism of occurrence of hypoxia in the waters of the inner western part of Ariake Bay, field observations were performed in summer (July to September) 2008. During the observation period, bottom DO decreased at neap tides, especially at the shallow station, and recovered to higher levels during spring tides. The hypoxic waters in the inner western part of Ariake Bay were essentially generated at two points: around the subtidal (between stations T1 and T14) and in the deeper offshore zone (around station P6). In some cases, hypoxic waters were found from stations T1 to P6 along the submarine valley of the Shiota River, suggesting that that the two masses of hypoxia generated individually were unified because of long-term stratification and massive oxygen consumption by SPM and sediments.

Characteristics of nitrogen cycle on a muddy tidal flat and shallow water area of Ariake Bay — Results of field study and laboratory experiments for Shiota River & Kashima River estuary —

Characteristics of nitrogen cycle on a muddy tidal flat and shallow water area of Ariake Bay — Results of field study and laboratory experiments for Shiota River & Kashima River estuary —

On the Relationship Between Nori Production, Water Temperature and Specific Gravity in Fukuoka Prefectural Area of Ariake Bay

On the Relationship Between Nori Production, Water Temperature and Specific Gravity in Fukuoka Prefectural Area of Ariake Bay

Properties of a low-swelling smectitic marine clay of interest in soil engineering

Smectite is generally a high-swelling clay. However, the smectite found in marine quick clays in the Ariake Bay area of Japan is a low-swelling clay like illite and kaolinite. The low swelling properties of an Ariake marine clay are investigated here in terms of consolidation, swelling, and shrinkage characteristics. The void ratios in compression curves of soils containing sodium are lower at 0.01 N than at 1.0 N NaCl concentration, and the slopes of swelling curves are independent of salt concentration in the pore water and cation valency. These tendencies are contrary to those observed for montmorillonite and a paddy soil containing high-swelling smectite. Measurements of swelling pressure suggest that the smectite in the Ariake marine clay exhibits little intracrystalline swelling even after saturation with Na. The volume shrinkage of the Ariake marine clay by air-drying is smaller than that of the paddy soil. Key words: compressibility, marine clays, smectite, swelling.

Relationships of Consistency Limits and Activity to Some Physical and Chemical Properties of Ariake Marine Clays

The physical, chemical, and mineralogical properties of the clay sediment in Ariake Bay area were examined, and the relationships of the consistency limits of the clays to their physical and chemical properties were investigated by statistical methods. The consistency limits were found to be significantly correlated with the clay content, specific surface area, and cation exchange capacity by simple regression analyses. Multiple regression analyses indicated that the clay content and salt concentration in the pore water were the most significant variables explaining the variation of the consistency limits. It was also found that the salt concentration in the pore water was correlated with the activity of the clays, and that the upper clay layers having greater activity contained the higher amount of salt in the pore water.