Ariake Bay Research Articles

Copepod community structure in the highly turbid innermost part of Ariake Bay in the 2000s with special reference to high abundances of <i>Oithona davisae</i>

Ariake Bay has Japan’s biggest nori seaweed farming field in the innermost part and has suffered from environmental degradation since the late 1990s. To elucidate the seasonal patterns in the copepod community structure in the inner bay, we examined monthly plankton-net samples collected in the top 2-m layer at 11 stations for 5 years from April 2002 to March 2007. Of the 11 stations, 7 were at the high-turbidity site (HTS) representing the nori farming field and 4 in the offshore low-turbidity site (LTS) for comparison. The copepod community in the HTS was dominated almost year round by Oithona davisae, whose 5-year mean abundance of 203 ind. L−1 comprised 84% of copepods and was more than double that in the LTS. O. davisae became extremely abundant in summer with the maximum monthly mean abundance in the HTS of 2756 ind. L−1 and the maximum sample-specific abundance of 10116 ind. L−1. The following predominant copepods in the HTS were Parvocalanus crassirostris in summer–autumn, Microsetella norvegica in summer, Acartia omorii in spring, and Paracalanus parvus s.l. irregularly. Possible causes for high abundance of O. davisae in the HTS are assumed to be sufficient food resources, water temperature over 21°C, and a wide salinity tolerance range of the species.

July 2020 heavy rainfall in Japan: effect of real-time river discharge on ocean circulation based on a coupled river-ocean model

In July 2020, a stationary atmospheric front over Japan caused persistent, nearly continuous rain for most of the month that resulted in new historical highest rainfall records in several areas and caused serious river floods, landslides, and debris-flow events. An existing hindcast and forecast ocean circulation model that includes climatological discharge information of major rivers failed to represent the extreme river discharge under heavy rainfall. New experiments were conducted using real-time river discharge information based on Today's Earth CaMa-Flood simulation that includes 368 rivers in Japan. The inclusion of real-time river discharge improved the salinity bias in the near-surface waters. The differences were significant compared to the observations in the heavy rain region (i.e., Ariake Bay and Tosa Bay), and insignificant at offshore stations. The ensemble experiments of real-time river discharge cases suggested the difference between the climatological and the real-time river discharge experiments was not random, but was robust. The freshening water changed the shelf circulation, and its far-reaching effect appeared hundreds of kilometers away from the shore. Passive particle tracking was conducted for examining the cross-shelf exchange. More particles released from Bungo Channel went offshore in near-surface water when the real-time river discharge was used compared to using the climatology discharge. Particles released in Tosa Bay, Seto Inland Sea, and Kii Channel showed the opposite tendency. The real-time river discharge not only changed the modeled coastal salinity distribution, but also the coastal and offshore currents. The role of the real-time river discharge on modeling normal flow periods or drought events, and its influence on a longer time scale model simulation remain to be explored.

Occurrence of planktonic larvae of manila clam Ruditapes philippinarum in Ariake Bay: Characteristic of spatial and temporal variation among the populations

Occurrence of planktonic larvae of manila clam <i>Ruditapes philippinarum</i> in Ariake Bay: Characteristic of spatial and temporal variation among the populations

Weekly Observations of Estuarine Microbial Assemblages during Summer in the Inner Part of Ariake Bay, Japan; Microbial Water-sediment Coupling in Turbid Shallow Waters.

Estuarine microbial assemblages are altered by a number of environmental factors, and knowledge of these changes is essential for understanding the functions of microbes in estuarine ecosystems. The aims of the present study were to examine the relationship between microbial assemblages in the water column and sediment surface, and to identify the environmental factors that influence the short-term dynamics of microbial assemblages in these two zones in summer in the inner part of Ariake Bay. The microbial assemblage of each sample consisted of a mean of 71.1% operational taxonomic units (OTUs), which commonly occurred in the water column and sediment surface, although their relative composition markedly differed between the two zones. In the water column, spatiotemporal changes in microbial assemblages correlated with several environmental factors, such as the nitrogen content in suspended particles, turbidity, and salinity. On the other hand, temporal changes in the sediment’s microbial assemblages were governed by a single environmental factor, namely, the oxygen reduction potential. These results suggest that the composition of microbial assemblages in the water column and sediment surface differed even in highly turbid brackish waters with high sediment resuspension, and the environmental factors contributing to the change in the assemblage composition also differed between the water column and sediment.

Mechanisms of ecosystem crisis originated in frequent occurrence of red tides in Ariake Bay

Mechanisms of ecosystem crisis originated in frequent occurrence of red tides in Ariake Bay

Population dynamics of a hypoxia-tolerant bivalve: A comparison of two sites in the inner part of Ariake Bay, Japan

Population dynamics of a hypoxia-tolerant bivalve: A comparison of two sites in the inner part of Ariake Bay, Japan

Genetic diversity and population structure of razor clam Sinonovacula constricta in Ariake Bay, Japan, revealed using RAD-Seq SNP markers

The razor clam Sinonovacula constricta is a commercially important bivalve in Japan. The current distribution of this species in Japan is limited to Ariake Bay, where the fishery stock is declining. It is necessary to understand the genetic population structure of this species in order to restore the fishery stock while preserving the genetic diversity of the clam. Here, we report for the first time the genetic population structure of S. constricta in Ariake Bay, Japan. Paired-end restriction site-associated DNA sequencing (RAD-Seq) analyzed samples of S. constricta collected from seven mudflats located along Ariake Bay. Two different genetic populations exist in Ariake Bay, one inhabiting wild habitats and the other inhabiting the transplanted area of artificial seedlings. Our results suggest that genetic differentiation occurred between these two populations (Fst value = 0.052), and a high level of genetic differentiation is maintained between the two groups. In the future, monitoring the interbreeding status of the two genetically distinct populations and the genetic differentiation within each population is important for conserving the genetic diversity of S. constricta in Japan.

Effects of the Physicochemical Environment on the Development and Maintenance of Winter Diatom Blooms in Turbid Water in the Inner Western Part of Ariake Bay

Effects of the Physicochemical Environment on the Development and Maintenance of Winter Diatom Blooms in Turbid Water in the Inner Western Part of Ariake Bay

Analyses of faunal changes of macrobenthos in Ariake Bay after the construction of dike for the Isahaya Reclamation Project ―Research Group for Conservation Ecology on the Ariake Bay―

Analyses of faunal changes of macrobenthos in Ariake Bay after the construction of dike for the Isahaya Reclamation Project ―Research Group for Conservation Ecology on the Ariake Bay―

Introduction and evaluation on the US West Coast of a new strain (Midori) of Pacific oyster (Crassostrea gigas) collected from the Ariake Sea, southern Japan

We describe the introduction, taxonomic identification and genetic parameters for farm traits of the Midori strain of the Pacific oyster Crassostrea gigas - a possible alternative to the widely farmed Miyagi strain on the US West Coast. Broodstock (G0) oysters collected from the mouth of the Midori river and near Iwajima Island, Ariake Bay, Japan were identified as C. gigas based on the nuclear internal transcribed spacer (ITS) region and 28S gene as well as a mitochondrial (COI) marker. G0 and G1 generations and G2 larvae and spat were maintained in quarantine conditions and subjected to numerous histological exams and PCR tests for detection of pathogens of concern. After the G1 generation and G2 early life stages were found to be free of detectable diseases, G2 spat were planted at farm test sites and adults were used to produce a G3 generation. This G3 generation was cultured under non-quarantine conditions and families were planted at sub-tidal and inter-tidal test sites, in Oregon and California, respectively. Field performance traits were measured to determine the potential for improvement through selection. Overall, Midori oysters showed significantly lower values for harvest yield, survival, individual average weight and dry meat weight compared with oysters from a selected stock of the Miyagi strain of Pacific oyster; however, shell shape index (a measure of shell depth) was significantly greater in Midori oysters. Heritability estimates for group traits at harvest (survival, yield and individual average harvest weight) were high, whereas heritability estimates for individual traits (individual weight, wet meat weight and dry meat weight) and for shell dimension measurements (shell length, shell width and shell depth) were moderate-to-high. Family-by-site (GxE) correlations were low-to-moderate for all measured traits. Genetic correlations were positive and moderate-to-high among group traits and shell dimensions. In contrast, correlations between shell shape and other traits had small-to-moderate negative values but these were not statistically significant. Estimated potential genetic gain for dry meat weight was high (46%) whereas estimated gains for yield, individual average weight, individual weight were high-to-moderate (approximately 10%). In contrast, shell shape showed very little potential for genetic gain through selection. Overall, the Midori strain of the Pacific oyster shows promise as a deep-cupped oyster for the half-shell trade. Statement of relevanceWe describe the introduction of a new strain of Pacific oyster (Midori) collected from southern Japan to the US West Coast. Rigorous quarantine protocols and disease tests were applied to ensure that the risks of introducing diseases were minimized. The Midori strain was found to have a deeper cup than the commonly farmed Miyagi strain but the potential estimated gain for improving this trait through selection was small.

Survival and behavior patterns associated with hypoxia at different life stages of the pen shell Atrina cf. japonica

Pen shell (Atrina cf. japonica) resources have been devastated in Ariake Bay, Japan, and to facilitate the recovery of this species, there is an urgent need to fully understand the factors contributing to its high levels of mortality. Pen shells living in natural waters grow through successive life stages, and environmental factors may affect these bivalves differently at different stages. Accordingly, to elucidate the causes of mortality in natural waters, it is necessary to gain an understanding of the quantitative effects of environmental factors on bivalves at each life stage. In this study, we sought to determine the differential effects of hypoxic conditions on 1-year-old (average shell length: 93.9 mm) and 2-year-old (146.5 mm) pen shells bred under artificial conditions. We exposed shells of each age group to six different dissolved oxygen (DO) concentrations for 96 h and monitored their behavior and survival rate. Based on the survival status, we estimated the lethal DO concentrations that induced 50%, 95%, and 5% mortality (LC50, LC95, and LC5, respectively) at each age. We found that for 1-year-old shells, the LC50 values at 48, 72, and 96 h were 0.51, 0.74, and 0.84 mg/L, respectively, whereas the corresponding values for 2-year-old shells were 0.74, 1.27, and 1.80 mg/L. Furthermore, we found that for 1- and 2-year-old shells, the estimated ranges from LC95 to LC5 at 48, 72, and 96 h were 0.39–0.68, 0.62–0.88, and 0.64–1.12 mg/L, and 0.31–1.75, 0.77–2.09, and 1.29–2.53 mg/L, respectively. Under low DO concentrations (0.47 to 1.93 mg/L and 0.49 to 3.30 mg/L for 1- and 2-year-old shells, respectively), we observed pen shells with more than half of their shell length protruding above the substrate. In addition to age and body size, the 1- and 2-year-old pen shells used in the present study also differed with respect to reproductive status, with 7.6% of 1-year-old and 96.7% of 2-year-old shells considered to be fully ripe. Collectively, our observations indicate that 2-year-old pen shells are less tolerant to hypoxic conditions than are 1-year-old pen shells, and we suspect that the differences in hypoxic tolerance could be attributable to differences in the physiological status of the pen shells during gonadal development. We believe the findings of this study will make an important contribution to enhancing our understanding of the effects of hypoxia on the viability of A. cf. japonica in natural waters.

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.

Primary Productivity Dynamics in the Summer Arctic Ocean Confirms Broad Regulation of the Electron Requirement for Carbon Fixation by Light-Phytoplankton Community Interaction

Predicting conversion of photosynthetic electron transport to inorganic carbon uptake rates (the so-called electron requirement for carbon fixation, KC) is central to the broad scale deployment of Fast Repetition Rate fluorometry (FRRf) for primary productivity studies. However, reconciling variability of KC over space and time to produce robust algorithms remains challenging given the large number of factors that influence KC. We have previously shown that light appears a proximal driver of Kc in several ocean regions and therefore examined whether and how light similarly regulated KC variability in the Arctic Ocean during a summer cruise 2016. Sampling transited ice-free and ice-covered waters, with temperature, salinity and Chl-a concentrations all higher for the ice-free than ice covered surface waters. Micro- and pico-phytoplankton generally dominated the ice-free and ice-covered waters respectively. Values of KC, determined from parallel measures of daily integrated electron transport rates and 14C-uptake, were overall lower for the ice-covered versus ice-free stations. As in our previous studies, KC was strongly linearly correlated to daily PAR (r=0.68, n=46, p<0.001) and this relationship could be further improved (r=0.84, n=46, p<0.001) by separating samples into ice-free (micro-phytoplankton dominated) versus ice-covered (Nano- and Pico-phytoplankton dominated water. We subsequently contrasted the PAR-KC relationship form the Arctic waters with the previous relationships from the Ariake Bay and East China Sea, and revealed that these various PAR-KC relationships can be systematically explained across regions by phytoplankton community size structure. Specifically, the value of the linear slope describing PAR-KC decreases as water bodies have an increasing fraction of larger phytoplankton. We propose that this synoptic trend reflects how phytoplankton community structure integrates past and immediate environmental histories and hence may be better broad-scale predictor of KC than specific environmental factors such as temperature and nutrients. We provide a novel algorithm that may enable broad-scale retrieval of CO2 uptake from FRRf with knowledge of light and phytoplankton community size information.

Growth environment of diatoms in turbid water in the inner western part of Ariake Bay during winter

To evaluate nutrient and light conditions as environmental factors that affect diatom growth in coastal areas of the inner western part of Ariake Bay, observations were conducted in the Kashima River and Shiota River estuary system. During high tide, nutrient concentrations increased between the upper and intermediate areas of the estuary due to river discharge. In these areas, cell densities of genus Skeletonema and Asteroplanus karianus tended to be high, reaching 22,350 and 6850 cells mL−1, respectively, whereas the euphotic depth (zeu)-to-water depth (z) ratios were low (0.1–0.3). In the lower estuary, where high salinities (> 28) were observed, zeu/z ratios were high (0.5–0.9) due to decreased turbidity, but nutrients were depleted. These results show that the diatom growth environment at high tide was characterized by low zeu/z ratios in the upper and intermediate areas of the estuary and low-nutrient conditions in the lower estuary. Time series observations in the lower estuary, performed to investigate the movement of water mass and variability in the growth environment with the tidal cycle, show a decrease in salinity during ebb tides and increases in nutrient concentrations and diatom cell densities, indicating movement of the water mass from the upper and intermediate areas to the lower estuary as the tide ebbs. Furthermore, the reduced water depth resulted in high zeu/z ratios. These results imply that nutrient and light conditions were improved in the lower estuary at low tide, and those better conditions made this area suitable for diatom growth.

A relationship between the mechanisms of the environmental crisis in the inner part of Ariake Bay and the reclamation project of the tidal flats conducted in Isahaya Bay

A relationship between the mechanisms of the environmental crisis in the inner part of Ariake Bay and the reclamation project of the tidal flats conducted in Isahaya Bay

Reproduction and embryonic development of the red stingray Hemitrygon akajei from Ariake Bay, Japan

The reproductive system of the red stingray Hemitrygon akajei was described from 1,418 specimens, 682 males and 736 females, sampled year-round (2003–2014) from Ariake Bay to provide reproductive life history information for conservation and management of the species. Females reach sexual maturity at a larger size than males with the size at 50% sexual maturity 522.2 mm disc width (DW) and 321.5 mm DW, respectively. Male stingrays had semen in the seminal vesicles year-round. Dental sexual dimorphism was aseasonal. The mating period is protracted, spanning 7 consecutive months October–April but ovulation occurred during May. This suggests female sperm storage, which has not been described for dasyatid stingrays. Females have a single functional ovary and uterus (left) and reproduce via aplacental viviparity with lipid histotrophy. Gestation required 3 months with parturition during late July and early August. Uterine eggs without macroscopic embryos were observed during the first half of gestation suggesting a short period of arrested development or diapause. Developmental cohorts based on morphological features were described for embryos and can be used to characterize embryo growth and development for other stingray species. Observations of foetal mortality (1.25%) and morphologically abnormal embryos (0.72%) were uncommon. Pregnancy rate was 90% and reproduction was annual and synchronous. Hemitrygon akajei fecundity ranged from 7 to 25 and increased with female size. Although H. akajei is a medium-sized dasyatid ray, it has the highest fecundity reported for any batoid species. Elasmobranchs life histories usually are associated with a low degree of productivity that results in rapid population decline with increased fishing pressure. Producing high numbers of small young is a successful reproductive strategy for H. akajei that may be partly responsible for their resilience to fishing pressure and continued abundance in Ariake Bay. This study is the first to describe reproduction of H. akajei, an economically important top predator inhabiting coastal ecosystems throughout Asia.

Influence of water discharged from a reservoir on reclaimed land into Isahaya Bay (Kyushu, Japan) on the regeneration of NH4+ in the water column

In 1997, the inner part of Isahaya Bay on the west coast of Ariake Bay, western Japan, was separated from the sea by dikes, leading to the formation of a freshwater reservoir. Since the Isahaya Reclamation Project began in the 1990s, red tides and hypoxia have become more common in the bay. Eutrophic high-turbidity water from the reservoir is frequently discharged into the bay, so NH4+ regenerated from organic matter in the discharge may be a source of nutrients for the red tides. In this work, we mixed reservoir water and seawater from the bay in experiments aimed at quantifying the NH4+ regeneration caused by the direct decomposition of labile organic matter in the high-turbidity drainage water. By comparing the results of field monitoring and mixing experiments, we estimated that the NH4+ regeneration rate under the influence of the drainage water was 192–501 μmol m−2 h−1, which was 31.5–46.8% higher than the natural NH4+ regeneration rate. This suggests that the additional NH4+ regeneration induced by the drainage water is non-negligible and that the organic matter in the drainage water may be an important source of NH4+ for red tides in the bay. This is the first study to provide data on the non-negligible amount of NH4+ regenerated upon the direct decomposition of labile organic matter in high-turbidity water drained from a reservoir on reclaimed land into the bay.

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

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.

Improved MODIS-Aqua Chlorophyll-a Retrievals in the Turbid Semi-Enclosed Ariake Bay, Japan

The accurate retrieval of chlorophyll-a concentration (Chl-a) from ocean color satellite data is extremely challenging in turbid, optically complex coastal waters. Ariake Bay in Japan is a turbid semi-enclosed bay of great socio-economic significance, but it suffers from serious water quality problems, particularly due to red tide events. Chl-a derived from the MODerate resolution Imaging Spectroradiometer (MODIS) sensor on satellite Aqua in Ariake Bay was investigated, and it was determined that the causes of the errors were from inaccurate atmospheric correction and inappropriate in-water algorithms. To improve the accuracy of MODIS remote sensing reflectance (Rrs) in the blue and green bands, a simple method was adopted using in situ Rrs data. This method assumes that the error in MODIS Rrs(547) is small, and MODIS Rrs(412) can be estimated from MODIS Rrs(547) using a linear relation between in situ Rrs(412) and Rrs(547). We also showed that the standard MODIS Chl-a algorithm, OC3M, underestimated Chl-a, which was mostly due to water column turbidity. A new empirical switching algorithm was generated based on the relationship between in situ Chl-a and the blue-to-green band ratio, max(Rrs(443), Rrs(448)/Rrs(547), which was the same as the OC3M algorithm. The criterion of Rrs(667) of 0.005 sr−1 was used to evaluate the extent of turbidity for the switching algorithm. The results showed that the switching algorithm performed better than OC3M, and the root mean square error (RMSE) of estimated Chl-a decreased from 0.414 to 0.326. The RMSE for MODIS Chl-a using the recalculated Rrs and the switching algorithm was 0.287, which was a significant improvement from the RMSE of 0.610, which was obtained using standard MODIS Chl-a. Finally, the accuracy of our method was tested with an independent dataset collected by the local Fisheries Research Institute, and the results revealed that the switching algorithm with the recalculated Rrs reduced the RMSE of MODIS Chl-a from 0.412 of the standard to 0.335.