Analysing the performance of the NARX model for forecasting the water level in the Chikugo River estuary, Japan.

Estuaries are dynamic environments which are driven by various natural processes like river discharge, tides, waves, influx of saline water and sediments, etc. These ecosystems are the most sensitive to sea level rise and fluctuations in river discharge associated with climate change. A direct response of sea level rise and river discharge can be observed in the water level of estuaries. However, existing models have not considered these parameters for forecasting water level. This paper focuses on developing a water level forecast model for the Chikugo River estuary in Japan using Nonlinear Autoregressive with Exogenous inputs (NARX Model). NARX neural network was used to do the one-step-ahead prediction of water level considering the various parameters that can very well be influenced by climate change: previous water level, river discharge, and salinity. Accordingly, three models were developed: (i) Model I considering previous water level; (ii) Model II additionally considering river discharge; and (iii) Model III additionally considering salinity. All the models showed appreciable performance in forecasting the water level. Model III had the best correlation with the water level with a cross-correlation value of 0.6030, while the river discharge had only a cross-correlation of 0.1113 indicating that the Chikugo River estuary is tide-dominated. The model was trained using different combinations of available data - previous water level, river discharge, and salinity. Cross-correlation results showed a better correlation between water level and salinity than various other combinations trained. Therefore, tidal intrusion influences the water level in the estuary, thereby depicting that sea level rise can affect the water level, and its influence can be well predicted by the developed model. The water level significantly affects the flora and fauna and the predictability of future estuarine floods can help in taking necessary mitigation strategies.

Impacts of sustainable management on the spatial distributions of erosion susceptibility and probable sediment yield in a mixed-forested watershed.

Forest management practices play multifaceted roles in enhancing the geophysical properties that affect raindrop erosion in the watershed, and consequently, sediment deposition in the reservoir. The current work attempts to integrate empirical and physically-based modeling approaches to quantify the impacts of forest conservation on erosion risk and potential sediment accumulation in the mixed-forested Ogouchi Dam watershed in Japan. The reliability of the empirical model for estimating the total erodibility coefficient (TEr), as a function of various forest properties, was evaluated by applying the mathematical expression to multiple forest conditions and comparing the values to field-measured soil erosion rates. The spatial distribution of the empirically derived values showed that about 25.8% of the Government-managed forests and 45.1% of the private forests have higher risks of raindrop splash erosion compared to natural forests. The TEr value in each small Government-divided forest land (less than 5 ha) was then corresponded to the MUSLE management practice factor (MUSLE P) input in each hydrologic response unit (HRU) in the Soil and Water Assessment Tool (SWAT) model to create a sediment yield distribution map and to predict the amounts of sediment accumulation for different management scenarios. The spatial distribution of sediment yield for the base condition showed that 20.9% of the Government-managed forests and 61.6% of the private forests have higher probable amounts of sediment yield relative to the value simulated in the natural forest. A maximum cumulative sediment reduction of about 14.4% is likely attainable upon the complete control of the Government in the entire planted forest area. Overall, this study effectively utilized the field survey datasets to develop a robust empirical model for quantifying erosion risk and was able to couple the results to a GIS-based model that predicts the amounts of sediment yield under varying environmental conditions.

Factors contributing to the minimum water column stability and timing of the winter turnover in the Ogouchi reservoir.

Turnover in lakes and reservoirs causes circulation in the water column from the bottom to the surface when the water column stability becomes low. Previous studies commonly mentioned that turnover occurs when stratification indices become small, but the threshold is rarely discussed. While turnover phenomena have been extensively studied by evaluating changes in bottom dissolved oxygen (DO), the relationship between the disappearance of hypoxia and water temperature indices has not been determined. This study focused on the factors influencing the minimum thermal gradient (TG) and Schmidt Stability Index (SSI), and the timing of turnover events using DO as an indicator of mixing in the Ogouchi reservoir from 1992 to 2001. The results showed that the occurrence of minimum TG and SSI is mainly driven by inflow retention time and average maximum wind speed. Moreover, minimum air temperature and outflow retention time have few contributions to minimum SSI. It was found that 7 out of 10 years exhibited full winter turnover, while the remaining years showed incomplete mixing with persistent hypoxia at the reservoir bottom. This study identifies four cases based on onset thresholds of 0.0035 °C m-1 for TG and 30 J m-2 for SSI to explain turnover event: Case 1: an ideal state with stratification indices below the threshold, resulting in the disappearance of hypoxia; Case 2: indices above the threshold sustain hypoxia; Case 3: an irregular state where the indices exceed the threshold, yet hypoxia disappears; and Case 4: an unexpected persistence of hypoxia despite being below the threshold. The majority of the years (70 percent) were explained by thresholds. The multiple regression analysis indicated the importance of wind speed on the turnover event. Therefore, the effect of wind shear was analyzed for 30 percent of the years that cannot be explained by thresholds (cases 3 and 4). Case 3 shows turnover occurrence due to strong accumulated wind shear, despite exceeding thresholds. Conversely, Case 4 reveals weak wind shear preventing bottom water upwelling, even below thresholds. In conclusion, the precise TG and SSI thresholds for the onset of turnover event were determined using DO data. The thresholds explained the occurrence and non-occurrence of turnover event in most of the years and wind speed clarified unexplained cases by thresholds. The presented method successfully evaluated the timing of turnover and can be applicable elsewhere.

Understanding stratification and turnover dynamics of a tropical lake using extensive field observations and 3D hydrodynamic simulations.

The stratification and turnover dynamics of a tropical lake were evaluated using field observations and 3D hydrodynamic simulations. Located in the Philippines, Sampaloc Lake is a 104-ha and 27-m deep volcanic crater lake with enclosed watershed, which is at risk of the impacts of intensive aquaculture, rapid urbanization and climate change. Temperature, dissolved oxygen (DO) and chlorophyll-a (Chl-a) were measured at seven sampling stations using a multiprobe. Kruskal-Wallis test revealed that the three parameters are not significantly different among stations, indicating that one sampling station can represent the water quality of the whole lake. Schmidt's Stability Index (SSI) and thermocline strength, together with DO and Chl-a gradients decreased from October 2022 (stratified) to January 2023 (turnover). After successfully verifying the 3D numerical model, sensitivity analyses of water temperature to varying weather, together with particle tracking simulations, were implemented to determine the timing of isothermal state, upwelling, partial mixing, and full turnover. Compared to air temperature, variations in wind speed have more pronounced effects on the delay or progression of isothermal conditions in the lake based on SSI, Lake Number and Wedderburn Number. Isothermal conditions do not necessarily coincide with the timing of full turnover, with the latter being delayed by two days than the former, on average. Results revealed that full turnover can occur several weeks earlier with the decrease in AT and increase in WS. This study can advance the understanding of thermal and turnover dynamics of stratified tropical lakes, leading to better management of the water quality of these water bodies.

Phytoplankton habitats and size distribution during a neap-spring transition in the highly turbid macrotidal Chikugo River estuary.

The effect of neap-spring transition on the spatial and temporal changes in the cell size distribution of different phytoplankton species and their size-based habitats distribution were investigated in the highly turbid macrotidal Chikugo River estuary, Japan in 2021. The estuarine mixing changed from stratified to well-mixed over the transition from neap to spring tides and saltwater (1-5) intruded until 17 km with negligible effect from river discharge. The suspended sediment concentration (SSC) was low (<40 mg L-1) during neap tide and highest (1000 mg L-1) during spring tide associated with strong mixing, which led to the formation of an estuarine turbidity maximum (ETM) zone between 8 and 12 km. A total of 159 phytoplankton species were identified in the estuary and are classified into seven groups. The diatoms (freshwater and marine habitats) were found as significant groups (79-86 %). Principal component analysis (PCA) results showed that salinity (p < 0.05) and mixing (p < 0.05) were the major controlling factors in the phytoplankton habitats distribution. The diatoms abundance started to increase from neap tide and reached the maximum abundance (174.714 cells/mL) two days after neap tide, then decreased towards spring tide (69.257 cells/mL). Additionally, the cell sizes of dominant diatoms species (Skeletonema costatum and Nitzschia acicularis) increased from neap to intermediate tides and continued increasing until the spring tide. Spatially, the abundance of diatoms was lower in the middle estuary (67-86 cells/mL) than in other parts because of ETM formation, although larger size distribution occurred. This study concludes that the abundance and size distribution of phytoplankton are influenced by estuarine mixing, and the neap-spring transition phase is more essential for phytoplankton growth than individual neap and spring tides in tide-dominated estuaries. Furthermore, size-increasing patterns play an important role in sustaining their survival strategy under the strong turbulent conditions in addition to saltwater intrusion and ETM.

Erosion and sedimentation pattern of fine sediments and its physical characteristics in a macrotidal estuary.

The seasonal and fortnightly erosion and sedimentation pattern as well as the bonding characteristics of the mud deposit at the estuarine turbidity maximum (ETM) zone of the macrotidal Chikugo River estuarine channel were studied during 2005-2008 using the periodical topographic surveys and mud sampling along with continuous monitoring of water level, turbidity and flow velocity. The results revealed that the estuary was influenced by the tidal forcing during the dry season. This tidal forcing accelerated the net landward sediment transport and sedimentation takes place in the estuarine channel. On the contrary, the river discharge dominated the estuary during the rainy season which eroded those mud deposit in the estuarine channel and exported to the downstream and the sandy base layer appeared at the bottom. During the dry season, a mud deposit of 1.5-1.8 m thick was formed which was characterized by 90% of mud (fine silt and clay) and 10% of fine sand. The high viscosity ratio and loss on ignition of this mud deposit revealed that its consolidation begins at the early stages of deposition due to thixotropy and the presence of organic matter. This consolidation process in the natural environment will be several times higher than that of the disturbed mud, which will help the sediment surface to maintain stability against erosion even at high velocities. Moreover, the consolidation of the mud deposit has a strong impact on the seasonal changes in estuarine morphology other than external forcing such as river discharge, tides, wind and availability of sediments.

Thermal stratification responses of a monomictic reservoir under different seasons and operation schemes.

This study investigates the thermal stratification responses of a monomictic reservoir operated under different facilities. The analysis of 60-year long data showed that the reservoir's thermal regime varies with season and withdrawal scheme and is affected by upstream reach control through the vertical curtain. Isothermal conditions exist during winter (December-March) while stratification onsets in spring (starting April), intensifies in summer (August) and weakens during fall (October-November). Considering summer stratification, deep hypolimnetic withdrawals through the penstock intake promoted thicker epilimnion, with low values of thermal stability (Schmidt Stability Index, SSI) and thermocline strength index (TSI). Meanwhile, shallow withdrawals using selective outflow system resulted in narrower epilimnion, with larger TSI for no curtain scenario and larger SSI for with curtain scenario. Strongest thermoclines do not necessarily translate to largest magnitudes of thermal stability. Longer duration of stratification is associated with shallow withdrawals. Depending on the outflow depth and the occurrence of prolonged hot or cold atmospheric conditions, the onset of stratification could be likely shifted early or late. The 3D numerical simulation determined the individual effects of each operation, which strongly supported the results of the long term analysis. Since thermal stratification directly influences the reservoir's water quality regime, this study can be a helpful reference in optimizing the water quality management of the reservoir.

Environmental DNA preserved in marine sediment for detecting jellyfish blooms after a tsunami.

Environmental DNA (eDNA) can be a powerful tool for detecting the distribution and abundance of target species. This study aimed to test the longevity of eDNA in marine sediment through a tank experiment and to use this information to reconstruct past faunal occurrence. In the tank experiment, juvenile jack mackerel (Trachurus japonicus) were kept in flow-through tanks with marine sediment for two weeks. Water and sediment samples from the tanks were collected after the removal of fish. In the field trial, sediment cores were collected in Moune Bay, northeast Japan, where unusual blooms of jellyfish (Aurelia sp.) occurred after a tsunami. The samples were analyzed by layers to detect the eDNA of jellyfish. The tank experiment revealed that after fish were removed, eDNA was not present in the water the next day, or subsequently, whereas eDNA was detectable in the sediment for 12 months. In the sediment core samples, jellyfish eDNA was detected at high concentrations above the layer with the highest content of polycyclic aromatic hydrocarbons, reflecting tsunami-induced oil spills. Thus, marine sediment eDNA preserves a record of target species for at least one year and can be used to reconstruct past faunal occurrence.

Direct measurement of secondary circulation in a meandering macrotidal estuary.

Secondary circulation, which induces the typical helical flow motion along a curved channel, is responsible for the complex morphodynamic processes in fluvial streams and estuaries with meandering nature. Classical secondary circulation has been extensively documented through flume experiments and numerical simulations but field observations about this phenomenon are scarce and only limited to few channel cross-sections. In this study, intensive measurements of flow velocities were performed using a vessel-mounted ADCP in order to illustrate the spatial distribution of classical secondary circulation in a meandering macrotidal estuary for both flood and ebb phases. Negligible salinity is noted during field measurements and velocity profiles manifesting logarithmic patterns confirm open channel flow conditions. As the flow enters the bend, primary and secondary velocities are shifted towards the outer bank. At the bend apex, circulation is accelerated with near-surface primary and secondary velocities skewed to the outer bank while bottom currents are skewed to the inner bank. As the flow exits the bend, bottom secondary currents gain dominance over surface currents while the bottom velocity vectors are directed towards the inner bank. Both the primary and secondary currents cause the propagation of the three-dimensional helical flow alongside with the asymmetry in the bend cross-section and the formation of point bar. This study successfully pioneers in demonstrating the three-dimensional structure of classical secondary circulation in actual field conditions through intensive ADCP surveys.

Seasonal morphodynamic evolution in a meandering channel of a macrotidal estuary.

The spatio-temporal morphodynamics in relation to the fortnightly tidal variations and seasonal river discharge variations in a meandering estuarine channel were investigated. Intensive topographic surveys (longitudinal and transverse) every 3-4 months using a digital sonar system equipped with a differential global positioning system and continuous monitoring of salinity and turbidity with moored instruments were carried out during 2009-2012 in the upstream section of the highly turbid macrotidal Chikugo River estuary, Japan. The results revealed that the tidal flow dominated in the estuary for most of the year except for the rainy season in which river flow dominates. During the low flow season, tidally induced net upward sediment transport dominates and deposition takes place in the upstream section especially in the inner part of the meander. It contributes towards the strengthening of the existing estuarine turbidity maximum (ETM) in the upstream section of the estuary, and the channel capacity reduced gradually during this period. However, large flood events led to the breakdown of this ETM zone in the upstream during the rainy season and exported the sediments to the downstream tidal flat which resulted in the rapid increase of channel capacity. This further exhibited that the ETM zone was migrated to the downstream part of the estuary during the rainy season. There were significant differences in the seasonal trends of morphological evolution in the estuary which further greatly influenced the channel capacity. The drastic changes in channel morphology due to the sediment export by the high river flow during the flood season was approximately recovered through the sediment import by the fortnightly tidal cycle during the dry season over a seasonal basis. The study concludes that the morphological changes in the tide dominated estuaries are strongly influenced by the fortnightly tidal variations as well as the seasonal river discharge variations.

Effect of neap-spring hydrodynamics on salt and suspended sediment transport in multi-branched urban estuaries.

Unique neap and spring tide hydrodynamic features were studied in the estuarine areas of multi-branched urban rivers in Tokyo, Japan. Intensive measurements of salinity, turbidity, dissolved oxygen, velocity, particulate organic matter (POM), and nutrients were conducted for 13h in five stations simultaneously on July 3 and July 11, 2017. Water sampling, analysis and calculations of salt and sediment transport were performed. Results showed that the Arakawa and Sumida Rivers follow the typical natural estuary hydrodynamics while Shakujii River followed a typical artificial urban estuary. Shakujii River upstream estuary had lower flow velocity during spring tide (-0.05 to 0.04m/s) than neap tide (-0.09 to 0.16m/s) because of the channel slope that does not allow the transit of water to upstream even during high tide. Shakujii River downstream estuary had hypoxic (DO<2mg/L) bottom depths during neap and spring tide. Sumida River and Shakujii River are adjacent yet the nutrient and POM had different nature. Shakujii River is highly influenced by freshwater from combined sewer systems carrying POM, nutrients and sediments. The freshwater was 54% (14,650m3) and 100% (28,671.1m3) by volume during neap and spring tide, respectively. The POM and nutrients in Sumida River is influenced by tidal processes and tributary rivers. SS was transported from Sumida to Shakujii River during flood phase of spring tide. Freshwater from Arakawa River flows to Sumida River during ebb tide. Salt (-0.066kg/m/s) and SS (-0.16 to -2.435kg/m/s) were retained in the estuaries which may lead to river deterioration and occurrence of scum, hypoxia and odor. The results of the study, particularly on Shakujii River, are significant and could serve as basis for water quality management in similar urban estuaries.

A new freshwater shrimp species of the genus Palaemon Weber, 1795 (Decapoda: Caridea: Palaemonidae) from northeastern Japan.

The palaemonid shrimp genus Palaemon Weber 1795 is currently represented by 87 species worldwide, of which 36 species inhabit freshwater environments. In this study, we describe a new species of the genus, P. septemtrionalis, primarily based on material collected from rivers in Miyagi Prefecture, Tohoku District, northeastern Japan. The present new species is morphologically and genetically close to Palaemon paucidens De Haan, 1841, but it is morphologically distinguishable from the latter by the chela of the pereopod 2 being longer than the carpus (versus shorter than the carpus in P. paucidens) and the possession of a low, laminar convexity on the flexor margin of the pereopod 3 dactylus just proximal to the base of the unguis (such a laminar structure is absent in P. paucidens). Comparison of partial sequences of the mitochondrial 16S rRNA gene supports the recognition of the new species. Examination of museum collections and a BLAST search on GenBank revealed that the geographical range of the new species includes the Sea of Japan side ranging from Hokkaido to Hyogo Prefecture and the Pacific side ranging from Aomori to Miyagi Prefecture. An identification key to the 13 Japanese species of the genus is presented.

Germination fluctuation of toxic Alexandrium fundyense and A. pacificum cysts and the relationship with bloom occurrences in Kesennuma Bay, Japan.

While cyst germination may be an important factor for the initiation of harmful/toxic blooms, assessments of the fluctuation in phytoplankton cyst germination, from bottom sediments to water columns, are rare in situ due to lack of technology that can detect germinated cells in natural bottom sediments. This study introduces a simple mesocosm method, modeled after previous in situ methods, to measure the germination of plankton resting stage cells. Using this method, seasonal changes in germination fluxes of toxic dinoflagellates resting cysts, specifically Alexandrium fundyense (A. tamarense species complex Group I) and A. pacificum (A. tamarense species complex Group IV), were investigated at a fixed station in Kesennuma Bay, northeast Japan, from April 2014 to April 2015. This investigation was conducted in addition to the typical samplings of seawater and bottom sediments to detect the dinoflagellates vegetative cells and resting cysts. Bloom occurrences of A. fundyense were observed June 2014 and February 2015 with maximum cell densities reaching 3.6×106 cells m-2 and 1.4×107 cells m-2, respectively. The maximum germination fluxes of A. fundyense cysts occurred in April 2014 and December 2014 and were 9.3×103 cells m-2day-1 and 1.4×104 cells m-2day-1, respectively. For A. pacificum, the highest cell density was 7.3×107 cells m-2 during the month of August, and the maximum germination fluxes occurred in July and August, reaching 5.8×102 cells m-2day-1. Thus, this study revealed the seasonal dynamics of A. fundyense and A. pacificum cyst germination and their bloom occurrences in the water column. Blooms occurred one to two months after peak germination, which strongly suggests that both the formation of the initial population by cyst germination and its continuous growth in the water column most likely contributed to toxic bloom occurrences of A. fundyense and A. pacificum in the bay.

Recovery of Coastal Fauna after the 2011 Tsunami in Japan as Determined by Bimonthly Underwater Visual Censuses Conducted over Five Years.

Massive tsunamis induce catastrophic disturbance in marine ecosystems, yet they can provide unique opportunities to observe the process of regeneration. Here, we report the recovery of fauna after the 2011 tsunami in northeast Japan based on underwater visual censuses performed every two months over five years. Both total fish abundance and species richness increased from the first to the second year after the tsunami followed by stabilization in the following years. Short-lived fish, such as the banded goby Pterogobius elapoides, were relatively abundant in the first two years, whereas long-lived species, such as the black rockfish Sebastes cheni, increased in the latter half of the survey period. Tropical fish species were recorded only in the second and third years after the tsunami. The body size of long-lived fish increased during the survey period resulting in a gradual increase of total fish biomass. The recovery of fish assemblages was slow at one site located in the inner bay, where the impact of the tsunami was the strongest. Apart from fish, blooms of the moon jellyfish Aurelia sp. occurred only in the first two years after the tsunami, whereas the abundances of sea cucumber Apostichopus japonicus and abalone Haliotis discus hannai increased after the second year. Although we lack quantitative data prior to the tsunami, we conclude that it takes approximately three years for coastal reef fish assemblages to recover from a heavy disturbance such as a tsunami and that the recovery is dependent on species-specific life span and habitat.