Manabu Shimizu

Spring-neap tidal variations of residual flow in Tokyo Bay, Japan

Sea surface current observations with the use of two high-frequency radars were carried out near the mouth of Tokyo Bay, Japan from 1 December to 23 December 1998. A peculiar fortnightly variability of residual flow was found near the mouth of Tokyo Bay, that is, sea surface residual flow was weak and southward in spring tide but it was strong and formed a clockwise circulation in neap tide. Strong sea surface residual flow resulted from large vertical shears of estuarine circulation and small vertical viscosity by weak tidal currents in neap tide. The clockwise circulation was produced by an expansion of horizontal circulation from the river mouth to the bay mouth.

Variability of factors driving spatial and temporal dispersion in river plume and Chattonella antiqua bloom in the Yatsushiro Sea, Japan.

The dynamics of river plume in relation to harmful blooms of the raphidophycean flagellate, Chattonella antiqua in summer 2008-2010 in the Yatsushiro Sea, Japan were studied using a hydrodynamic model and monitoring data. In the southern area, the bloom formed in the waters stratified by a halocline caused by the southward expansion of riverine water from the Kuma River after the bloom initially forming in the northern area. The timing of the southward riverine water advection can be explained by the balance between the wind stress term and the pressure gradient term calculated from the horizontal density difference between the northern and southern areas. The wind stress and pressure gradient terms were evaluated using the sea surface temperature, salinity, wind speed and direction at two stations. Real time monitoring or continuous observations in these areas will enable nowcasts of bloom expansion when a bloom develops in the northern area.

A Late Winter Hydrography in Hidaka Bay, South of Hokkaido, Japan

Hydrographic observations in Hidaka Bay, south of Hokkaido, Japan were carried out in late winter 1996 and 1997 to examine the spatial distributions and circulation features of two different water masses, i.e., Coastal Oyashio Water (COW) and Tsugaru Warm Water (TWW), and their modifications. It is known that COW is mostly composed of cold and low-salinity water of the melted drift ice coming from the Okhotsk Sea and flows into Hidaka Bay from winter to spring and TWW with high-salinity continuously supplies from the Tsugaru Strait to the North Pacific. Cold surface mixed layers (<26.2σθ, 0–100 m depth) were found mainly over the shelf slope, confirming that anti-clockwise flow of COW was formed. TWW was relatively high in salinity and low in potential vorticity, and had some patch-like water masses with a temperature and salinity maximum in the limited area in the further offshore at the deeper density levels of 26.6–26.8σθ. The fine structure of vertical temperature and salinity profiles appeared between TWW and COW is an indication of enhanced vertical mixing (double-diffusive mixing), as inferred from the estimated Turner angles. At a mouth of the Tsugaru Strait in late winter 1997, a significant thermohaline front between TWW and the modified COW was formed and a main path of TWW spreaded south along the Sanriku coast, probably as the bottom controlled flow. Hence, the patch-like TWW observed in late winter is isolated from the Tsugaru Warm Current and then rapidly modified due to a diapycnal mixing.

Interregional differences in mortality of aquacultured yellowtail Seriola quinqueradiata in relation to a Chattonella bloom in the Yatsushiro Sea, Japan, in 2010

Mass mortality of Seriola quinqueradiata was caused by a Chattonella bloom in summer 2010 in the southern area of the Yatsushiro Sea. We analyzed the diurnal records of the mortality occurrence and cell density of C. marina to examine the short-term mortality dynamics. In addition, a hydrodynamic model and Lagrangian particle tracking method were used to examine interregional differences in the mortalities. Mortalities were concentrated along the southern coast of Shishi Island and western coast of Nagashima Island in early July. In late July, mortalities occurred along the southern coast of Shishi Island and eastern coast of Nagashima Island. Severe mortalities occurred where the bloom (>100 cells ml−1 of Chattonella marina) appeared in both early and late July. Model results show that interregional difference of the bloom was controlled by the advection system in the Yatsushiro Sea. The consistent spatio-temporal pattern between fish mortalities and bloom dynamics indicates the potential benefits of real-time monitoring of harmful algae and assessing factors controlling hydrographic conditions to mitigate mortalities.

Chattonella (Raphidophyceae) bloom spatio-temporal variations in Tachibana Bay and the southern area of Ariake Sea, Japan: Interregional displacement patterns with Skeletonema (Bacillariophyceae)

In 2010, a massive bloom of the raphidophycean flagellate Chattonella occurred in the Ariake Sea and Tachibana Bay. Bloom dynamics and hydrographical conditions were examined by field survey. The development and decline of the bloom occurred three times in Tachibana Bay. First and third bloom developments synchronized with precipitation, and the second bloom developed in synchronization with a salinity decrease which occurred in relation to an increase of river discharge from the Chikugo River which takes several days to flow from the Ariake Sea. These results imply that the bloom was transported with the low salinity water from the Ariake Sea to Tachibana Bay. During blooms along the northern coast of Shimabara Peninsula, the predominant phytoplankton species changed from Chattonella to Skeletonema. Low salinity water intrusion induced an interregional difference of the Chattonella and Skeletonema bloom spatially-differentiated by the salinity in the Ariake Sea and Tachibana Bay.

Meteorological conditions preceding Chattonella bloom events in the Yatsushiro Sea, Japan, and possible links with the East Asian monsoon

The relationship between harmful blooms of raphidophycean flagellate Chattonella spp. and preceding meteorological conditions was investigated in the Yatsushiro Sea, Japan, using data over a 25-year period from 1988 to 2012. The occurrence date of Chattonella blooms was significantly correlated with the average air temperature from February through April and with the onset date of Baiu, the Japanese name for the rainy season in early summer. These two meteorological factors showed significant differences between occurrence and non-occurrence years of Chattonella blooms in the Yatsushiro Sea. The Chattonella blooms tended to occur in years that were relatively warm from winter to spring and had a relatively late onset of Baiu. The duration of Chattonella blooms was significantly correlated with the interval between the closing date of Baiu and the occurrence date of the Chattonella blooms, suggesting an important role of Baiu as nutrient input affecting the magnitude of the bloom event. The East Asian monsoon, which controls local meteorological conditions, is likely involved in the population dynamics of Chattonella through its life cycle and physiological characteristics.

Circulation and material transport in Suo-Nada during spring and summer

An attempt was made to reproduce the circulation pattern in Suo-Nada, Japan during spring and summer season in order to elucidate the water exchange mechanism in the basin. Two hydrographic surveys at the end of each season were conducted covering the entire Suo-Nada area. A three-dimensional hydrodynamic Princeton Ocean Model (POM) was used to compute the current resulting from the observed density and wind field. During spring, a very pronounced counter clockwise gyre is situated near the opening of the basin. This is replaced by a clockwise circulation which seemed to occupy the whole domain during summer. Within each season, however, the vertical distribution of current does not show any remarkable differences, indicating the dominance of horizontal current and a very weak estuarine flow. These observational and numerical results were used to estimate the remnant function and the corresponding average residence time of permanently dissolved matter (PDM) and transformable matter (TM). The results revealed a small difference in the average residence times of materials within each season but a large seasonal variability between spring and summer. Furthermore, calculations based on climatological density fields have indicated a similar trend of variation between the seasonal values of average residence times.

Numerical study for specifying the major origin of low salinity water associated with Chattonella (Raphidophyceae) blooms in Tachibana Bay, Japan

In both 2009 and 2010, massive Chattonella blooms occurred in Tachibana Bay. Observation results show that high cell densities of Chattonella were distributed in the central area of Tachibana Bay with low salinity water. Model results indicate that the low salinity water originated from the Ariake Sea and intruded into Tachibana Bay during the northerly or weak winds. It is suggested that low salinity water was mainly discharged from the northern area of the Ariake Sea. Northerly wind enhanced the horizontal advection of the low salinity water intruding into Tachibana Bay originating from the northern area of the Ariake Sea.

Seasonal Variability in Circulation Pattern and Residence Time of Suo-Nada

The development and evolution of the persistent counterclockwise circulation in Suo-Nada have been studied in detail using a three-dimensional numerical model. The realistic circulation has been reproduced from the monthly climatological fields of salinity, temperature along the open boundary, wind, heat flux and buoyancy input from twenty-two major rivers surrounding the basin. The seasonal variation of fresh-water discharge from the rivers proved to be the most important forcing mechanism driving the circulation. The expansion and contraction of coastal low salinity water well corresponded with the development of counterclockwise circulation from spring to summer and dissipation from autumn to winter. The circulation was found to be vertically homogenous and quasi-barotropic. However, confined along the coastal zone is an estuarine flow (i.e., oppositely directed currents in the upper and lower layers) whose horizontal dimension conforms to the period of maximum buoyancy flux from the rivers. This indicates that the flow pattern in Suo-Nada can be separated into two distinct regions where the relative dominance of estuarine circulation plays a significant role. Furthermore, the monthly water exchange capacity for autonomous purification of the basin has been evaluated from the average residence time of conservative material based on the calculated current field. It is demonstrated that the kinetic energy of the basin is directly responsible in promoting water exchange in Suo-Nada.