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To investigate the mass mortality of the macrobenthos community, primarily asari clams, triggered by upwelling-driven hypoxia, we conducted continuous observations of temperature, salinity, and DO, and monthly macrobenthos monitoring on the Rokujo tidal flat in Mikawa Bay, central Japan, from 2014 to 2016. Additionally, laboratory experiments were conducted using sediments on a tidal flat containing macrobenthos to examine the possibility of hydrogen sulfide formation in tidal flats. The bottom layer at the offshore station was intermittently hypoxic, and the station of the tidal flat was occasionally hypoxic in August and September for three years. Hypoxia was mostly observed on the tidal flat when constant easterly winds were recorded offshore. The biomass of asari clams decreased considerably from September to October 2016 when hypoxia was intermittent. Hypoxia persisted for approximately one week from 20 September, which was associated with the calm weather and stagnation of tidal currents owing to the neap tide. Conversely, the hydrogen sulfide concentration in the water directly above the sediment exceeded 30 mg L−1 after 3 days of incubation in the laboratory experiment. Therefore, the possibility of oxygen consumption on tidal flats due to hydrogen sulfide formed by biological die-offs was considered in the long-term persistence of hypoxia. [ABSTRACT FROM AUTHOR]

In eutrophic waters, such as Mikawa Bay, Japan, anoxic bottom water develops in summer. This causes H2S release into seawater due to sulfate reduction in the sediment, leading to environmental problems. The addition of Fe to sediments is a method used to improve the sediment environment. However, this method, which was devised from a natural mechanism, has not yet been quantitatively evaluated. In this study, we aim to quantitatively evaluate the suppressive effect of Fe on H2S release. First, we developed a sediment model that focuses on S and Fe. We then attempted to reproduce the observations and experiments on H2S dynamics using the model. Consequently, the proposed model was able to reproduce field S and Fe dynamics in sediments even under anoxic conditions. Additionally, the model could describe the difference in H2S release depending on the amount, type, and time of iron-based additives. Finally, we conducted predictive calculations for one year assuming 637 g/m2 of iron materials, and quantitatively evaluated the effects of adding iron materials to the sediments in terms of S and Fe cycles. We found iron-based additives suppressed H2S release by 14 mmol/m2/d compared to the case without additives. [ABSTRACT FROM AUTHOR]

• This study examines lake Shinji, a mesohaline lake with intermittent saline water intrusion. • It employs a 3-D hydrodynamic model, EcoPARI, in hindcast mode for 2012 (Jan-Sep). • The focus was on optimizing the vertical turbulence diffusion model and identifying ideal parameters. • Differential model constants for temperature and salinity are explored, enhancing simulation results. • The tuned model successfully replicates a historical upwelling event in September, driven by strong westerly winds from a nearby typhoon. Lake Shinji is a mesohaline lake that has intermittent saline water intrusion from its east coast connection of Ohashi River. This intrusion is often in the shape of a very thin layer that moves along the Lake bottom and stayed at the center due to bowl shape bathymetry of the Lake having maximum depth at the center. To elucidate the intrusion mechanism a 3-D hydrodynamic model (EcoPARI) was employed in a hindcast mode covering the year 2012 from the beginning of January to the end of September. Initially, the core emphasis was given to the vertical turbulence diffusion model and optimum model parameters were determined to accurately simulate the hydrodynamics. It was highlighted by the numerical simulations that alteration of one model constant, which is the multiplying coefficient of Richardson number, "α", is influential instead of altering all the constants to tune the turbulence model. The possibility of using differential model constants for temperature and salinity was also tested and it helped in the improvement of simulations. Finally, the performance of the model was also assessed under extreme weather conditions and a historical upwelling event was simulated. The model was able to reproduce the intrusion in the summer months as well as the upwelling event in September due to strong westerly winds caused by a nearby passing typhoon. [ABSTRACT FROM AUTHOR]

The Ohashi River is a narrow water stream that connects two brackish lakes in Japan. Intermittent saline water intrusion often occurs in Lake Shinji from Lake Nakaumi through Ohashi River. In this study, two approaches were discussed to reproduce the hydrodynamic conditions of a morphologically complex river. In the first approach, the river sinuosity was straightened. The straightening of the river resulted in a higher flow velocity and water flux coefficient due to the reduction in the flow path and the resistance, and this approach was found to be appropriate for the reproduction of the flow velocity. However, the river shape was visually quite different from the actual river morphology. In the second approach, the prime focus was given to the shape and bathymetry to quantitively reproduce the flowrate of the saline water intrusion. This approach resulted in an underestimation of the flow velocity, which was compensated by increasing the cross-sectional area of the river. A slower flow velocity causes up to a 3-h time lag for the water mass to pass through the Ohashi River, which in principle should affect the temporal variations of the water temperature and salinity. Fortunately, as the typical time scale for water temperature and salinity fluctuations in the Ohashi River is a few days, a 3-h time lag did not cause any problems. [ABSTRACT FROM AUTHOR]

In a recent review of the seagrass genus Halophila in Japan, eight taxa including four new species were listed: (i) H. decipiens, (ii) H. minor, (iii) H. major, (iv) H. ovalis, (v) H. mikii, (vi) H. nipponica, (vii) H. okinawensis, and (viii) H. gaudichaudii. In this study, the specific concept of Halophila in Japan as well as some other geographic areas was re-assessed by a combination of nuclear ribosomal ITS1, 5.8S rDNA, and ITS2 sequence analyses and morphological examinations of both recently collected materials and available type collections. The results confirm the distinctness of the Australian endemic species H. australis and the putative pan-tropical taxon H. decipiens; but also demonstrate conspecificity of: (i) H. minor and H. ovalis; (ii) H. major and H. mikii; and (iii) H. nipponica, H. okinawensis and H. gaudichaudii. As an outcome of this study, we recommend restricting the list of Halophila representatives in Japan to the following four species: (i) H. decipiens, (ii) H. major, (iii) H. ovalis, and (iv) H. nipponica. In addition, we predict the possibility of H. major occurring further south than currently recorded. [ABSTRACT FROM AUTHOR]

A new species of the green algal genus Pseudocodium is described on the basis of specimens collected from Okinawa, southern Japan. It differs from the generitype species P. de-vriesii Weber-van Bosse from South Africa, P. australasicum Womersley from Australia and P. floridanum C. J. Dawes & A. C. Mathieson from Florida in its smaller size (1-2 cm in height, axes 0.5-1.0 mm in width) and infrequent branching. Unlike P. de-vriesii and P. australasicum also is its total lack of constricted or articulated axes. Molecular-phylogenetic analyses based on plastid encoded rbcL gene sequences show the Okinawan plants to group with Atlantic P. floridanum but to be specifically distinct owing to substantial base pair differences (88 bp, 11.876%). Remarkable differences in morphometric data, ecological preferences and geographic distribution were also found between them. We thus describe the Okinawan entity as Pseudocodium okinawense E. J. Faye, M. Uchimura et S. Shimada sp. nov. In addition, on the basis of laboratory culture experiments, a diminutive, entirely tubular growth form showing great morphological dissimilarity with field materials is newly observed in the genus, thus adding further details to our understanding of the plants vegetative phenology at early developmental stages. [ABSTRACT FROM AUTHOR]

Although most fatalities in tsunami-related disasters are conjectured to be a result of drowning, injury risk owing to collision with other floating debris or fixed buildings has not been studied sufficiently. In this study, the impact force corresponding to the collision of a concrete block and drifting test body in a tsunami wave was experimentally investigated, and the injury risk was evaluated in terms of different biomechanical indexes; specifically, maximum acceleration, head injury criterion, and impact force. The injury risk indicated by the considered indexes was reasonably low. It was noted that if a healthy adult collided with a concrete wall under a velocity of 2.5 m s-1 and wave height of 0.59 m, the adult would likely not be critically injured. However, a similar collision impact poses considerable risk to infants and children, as well as the more sensitive regions of the adult body. Moreover, in the case of large tsunamis, such as that in the 2011 Great East Japan Earthquake, a drifting person may be at considerable risk for injuries. The collision impact occurring on the tip of a surge flow is notably significantly larger than that on a bore flow. This is because a surge flow, which arrives at the concrete block earlier than a bore flow, forms a certain water layer along the concrete wall and that layer acts as a cushion for any body drifting on the bore flow, indicating the importance of such a buffering effect. These findings can provide practical guidance regarding the formulation of effective tsunami-protection measures., Competing Interests: The authors have declared that no competing interests exist.