Day-night reversion in the horizontal distributions of dissolved oxygen content and pH in aSargassum forest

This paper examines the influence of aSargassum forest on distributions of illuminance, dissolved oxygen content and pH in a small cove facing Wakasa Bay. Spatial distributions of illuminance, dissolved oxygen content, water density and pH were observed for June 1982 during the season of luxuriant seaweed growth, and for August 1982 during the season of little growth. Observations of dissolved oxygen content, water density and pH were made during the day and at night. The values of illuminance at the sea surface were decreased to less than 40% inside theSargassum forest when the sun was highest in the sky during the season of luxuriant growth. Density stratification occurred during every observation. Dissolved oxygen content and pH showed similar patterns of spatial distribution. Their horizontal distributions reversed from day to night, and consisted of two types: (1) higher values inshore and lower values offshore in the upper layer during the day with (2) lower values inshore and higher values offshore at night. Distributions of illuminance about noon, and dissolved oxygen content and pH at night showed patterns corresponding to the vertical distribution of algal density of theSargassum forest. Dissolved oxygen was supersaturated at every observation point during the daytime, but at night it was undersaturated in the lower part of the forest or along the bottom in June and August, respectively. Processes that brought about these spatial distributions are discussed.     

TheKyucho in Uwajima Bay

The phenomenon of rapid increase in water temperature accompanied by a sudden, swift current (Kyucho) in Uwajima Bay is described on the basis of results of long term observations from July 1985 to September 1986. This phenomenon occurs somewhat periodically with an interval of about 15 days in summer, although it does not occur in winter. The increase in water temperature occasionally reaches about 5°C. This phenomenon results from the intrusion of a warm water mass into the bay. NOAA-9 satellite images show that the warm water mass originates from the Pacific Ocean south of the Bungo Channel.     

Spatial and temporal distributions of water temperature in aSargassum forest

The present study examined the influence of aSargassum forest on the spatial and temporal distributions of water temperature in a small cove on a time scale of the order of hours. Water temperature and algal density were measured along a fixed transect at the cove for almost 2 days. The measurements were carried out in May 1977 during the season of luxuriant seaweed growth, and in August during the season of little growth. The average standing crop per unit volume, named the spatial algal density, was employed as a means of expressing the growth condition of the seaweed forest along the transect. Using the spatial algal density and by analyzing water temperature distribution the following facts were revealed. The diurnal fluctuation of water temperature was influenced by theSargassum forest during the season of luxuriant growth, and the seaweed forest delayed the ascent and descent of water temperature, especially under its canopy, on a time scale of the order of hours, i.e., the temperature distribution patterns were influenced by the heights and densities of theSargassum forest during its season of luxuriant growth. It is thought that these phenomena are brought about by two effects: one is the absorption by the seaweed forest of short-wave radiation passing through the sea surface, and the other is the suppression of convection by the seaweed.     

Numerical simulation of tidally induced eddies in the Bungo Channel: A possible role for sporadic Kuroshio-water intrusion (kyucho)

It is well known that the sudden intrusion of Kuroshio warm water into the Bungo Channel (kyucho) is regulated by spring–neap tidal forcing. In order to clarify the physical background behind this regulation, numerical experiments are carried out using a high-resolution non-hydrostatic three-dimensional model. We first reproduce the strong mixing region off the east coast of the Bungo Channel resulting from tidal flow interaction with complicated land configurations during spring tides; behind islands and headlands, small-scale eddies satisfying an approximate cyclostrophic balance are generated. As a result, averaged over the whole model domain, the tidal-mean energy dissipation rate reaches ≈1.6?×?10^?6?W?kg^?1. The model predicted energy dissipation rates at the location and times of direct microstructure measurements in the Bungo Channel are comparable to the observed values. We next examine whether or not strong tidal mixing thus reproduced can inhibit the northward intrusion of Kuroshio warm water in the Bungo Channel. It is shown that the Kuroshio warm water can (or cannot) pass through the tidal mixing regions off the east coast of the Bungo Channel during periods of weakened (or enhanced) tidal mixing at neap (or spring) tides. This indicates that taking into account the realistic spring–neap modulation of tidal mixing intensity is indispensable to further increase the ability of the existing forecast system for kyucho in the Bungo Channel.     

Temporal fluctuations of water temperature in aSargassum forest

This paper examines the influence of aSargassum forest on temporal fluctuations in temperatures of surrounding water in relation to the thermal structure of water in and above theSargassum forest. Water temperature records were obtained at about one-minute intervals for almost two days in May 1977 during the season of luxuriant seaweed growth, and in August 1977 during the season of little growth. The fluctuations were divided into two types. (1) A diurnal fluctuation under the forest with about a three hour lag behind that above the forest during the season of luxuriant growth but with about a 30 min lag during the season of little growth. (2) Sharp spike-like fluctuations with periods shorter than five minutes appearing only in the dense canopy or floating seaweeds in the surface and subsurface layers during the period of luxuriant growth. The luxuriant forest ofSargassum seems to influence the spatial distribution of water temperature and consequently seems to induce the fluctuations mentioned above. The relationship between short period fluctuations and behaviours of larval fishes are discussed.     

秋季湛江港和入海口温盐结构及生态特征

采用2015年10月采集的湛江港海域水体叶绿素a浓度、温度、盐度等参数,分析了秋季湛江港和入海口温盐结构及生态特征。研究结果表明,湛江港海域盐度的水平分布上由湾内往湾外逐渐递增,叶绿素a浓度由湾内往湾外逐渐递减,水深比较浅的区域水温较高,同时在航道入海口底层存在着“高盐低温低叶绿素”的相对稳定的冷水团结构。该水团的形成是由于湛江港出海口独特的地形构造促进区域性水体层化,同时底部水体透明度低,限制航道入海口区域底层的浮游植物的生长等因素所致。     

Effects of tidally induced eddies on sporadic Kuroshio-water intrusion (kyucho)

The sudden intrusion of Kuroshio warm water into the Bungo Channel (kyucho) occurs mainly at neap tides during summer, suggesting that tidal mixing is one of the essential factors regulating kyucho. In order to clarify the physical mechanisms responsible for the regulation of kyucho, we carry out non-hydrostatic three-dimensional numerical experiments allowing Kuroshio warm water to intrude into a strong tidal mixing region. It is shown that the Kuroshio warm water can (or cannot) pass through the tidal mixing regions off the east coast of the Bungo Channel during neap (or spring) tides. The analysis of the dynamic balance off the east coast of the Bungo Channel shows that tidal residual currents generated by tidal flow interaction with complicated land configurations off the east coast of the Bungo Channel can also play an important role in regulating kyucho. In order to assess separately the effects of tidal mixing and tidal residual currents on kyucho, we incorporate the parameterized vertical mixing and tidal stresses into the numerical model instead of tidal currents. It is demonstrated that tidal mixing cannot by itself block the northward intrusion of Kuroshio warm water, and that an additional effect induced by tidal residual eddies equivalent to horizontal mixing is needed to regulate kyucho. This strongly suggests that the basin–ocean water exchange processes in areas with complicated land configurations can only be reproduced by taking into account the effects of tidal residual eddies on a 1-km scale in addition to tidal mixing effects evaluated by microstructure measurements.     

大亚湾及其邻近海域冬季温度、盐度的分布及日变化特征

根据2018年1月冬季航次的水文实测资料,详细分析了大亚湾海水温度(T)、盐度(S)的分布特征。整体而言,观测海区海表相对于海底具有高温低盐的特征;同时,无论是表层还是近底层,大亚湾湾内的海水相对于湾外都呈现高温低盐的特征。观测期间,应是受到大亚湾核电站温排水的影响,湾内西侧存在一个高温中心。盐度的差异在近底层更加明显,低盐中心位于大亚湾的湾顶和大亚湾的中部海域,而高盐中心则主要分布于湾口西侧及惠东以东附近海域。太阳辐射和潮流变化是影响大亚湾温度、盐度变化的两大重要因素。其中,太阳辐射的影响主要局限于表层3~4 m,对近底层海水的影响较小;其加热效应使湾内和湾口附近的表层海水都表现出明显的昼夜变化。由潮汐和温度、盐度的对应关系可知,潮流对湾内温度、盐度的影响较大,而对湾外温度、盐度的影响较小。     

The Kyucho in the Bungo Channel,Japan —Periodic intrusion of oceanic warm water

Kyucho is a sudden and swift current which is usually accompanied by rise of water temperature. Several features of the Kyucho in the Bungo Channel, Japan, are presented through field observations. The Kyucho in the Bungo Channel is an intrusion of warm water from the Pacific Ocean into the eastern half of the Bungo Channel, being driven gravitationally and advancing along the eastern coast of the channel. The Kyucho occurs usually in summer and seldom occurs in winter. It occurs at neap tides showing the prominent spring-neap periodicity. The modulation of the vertical mixing intensity associated with the variations of tidal current, wind and surface heating etc. is supposed to be a main cause of springneap and seasonal periodicities.     

Path and volume transport of the Kuroshio current in Sagami Bay and their relationship to cold water masses near Izu Peninsula

The path of the Kuroshio in Sagami Bay was surveyed through drifter tracking from Oshima-West Channel to Oshima-East Channel. A subsurface drifter with a drogue at 300 m depth flowed around Oshima from Oshima-West Channel to Oshima-East Channel. A difference in flow directions between the upper and lower layers was apparent in the northwest of Oshima. Flow directions there were shown to change from north in the surface layer to east in the bottom layer, and this was confirmed with moored currentmeters.A profile of northward current velocity was estimated from measurements in six layers with currentmeters deployed in the Oshima-West Channel. The profile shows a core of northward flow along the eastern bottom slope and a weak southward flow along the western bottom slope. Volume transport of the Kuroshio into Sagami Bay was estimated to be 1.8×10⁶m³sec^–1 from the profile.Long-term current measurement showed that southward flows were observed in Oshima-West Channel in July 1977, May 1978 and April 1979. Cold or warm water masses appearing south of the Izu Peninsula are suggested to have caused the changes.Displacement of the cold water mass in July 1977 is discussed on the basis of current measurements and offshore oceanographic conditions.     

Numerical Study of a Kyucho and a Bottom Intrusion in the Bungo Channel,Japan: Disturbances Generated by the Kuroshio Small Meanders

The generation and propagation mechanisms of a Kyucho and a bottom intrusion in the Bungo Channel, Japan, have been studied numerically using the hydrostatic primitive equations by assuming density stratification during summer. The experiments are designed to generate a Kuroshio small meander in Hyuga-Nada, which acts as a trigger for these disturbances. After the current speed of the Kuroshio is changed, a small meander is generated. At the head of the small meander, warm Kuroshio water is engulfed, and encounters the southwest coast of Shikoku. However, convergence of heat flux on the bump off Cape Ashizuri suppresses the generation of a warm disturbance, if the current speed is large. As the cold eddy associated with the small meander approaches Cape Ashizuri, the heat flux diverges on the bump. This heat source forces a warm disturbance, which intrudes along the east coast of the Bungo Channel as a baroclinic Kelvin wave (a Kyucho). After the cold eddy passes off Cape Ashizuri, the Kuroshio approaches the bump again. Strong convergence of heat flux then occurs on the bump, which forces a cold disturbance. This disturbance propagates as a topographic Rossby wave along the shelf break at the mouth of the channel. After the topographic wave reaches the west end of the shelf break, it intrudes along the bottom layer of the channel as a density current (a bottom intrusion). These results suggest that a Kyucho and a bottom intrusion are successive events associated with the propagation of the small meander.     

Methane in Izena Cauldron,Okinawa Trough

Methane in the deep water of Izena Cauldron (maximum depth: ca. 1650 m) at the east side of mid-Okinawa Trough was studied by casting a CTD system with 12 Niskin bottles for water sampling at 11 stations inside and outside the cauldron. The water contained much methane up to 706 nmoles/l. The depths of maximum concentration varied widely from station to station, indicating the existence of a considerable number of vents emitting methane and heat. The waters containing less methane formed a straight line in theT-S diagram, while those containing more methane were more largely deviated from the line. The temperature anomaly was virtually proportional to the methane concentration, suggesting that the oxidation rate of methane inside the cauldron is negligibly small and methane can be used as a tracer of the cauldron water. The relation and the estimated vertical diffusivity gave the following fluxes. The emissions of methane and heat out of the bottom below 1450 m turn out to be 1400 moles/day and 7×10¹⁰ cal/day, respectively. The total emission rates inside the cauldron are presumed to be about twice the above values. The turnover time of methane has been estimated to be 240 days, which is also that of heat generated from the bottom and probably that of the bottom water.     

Observation of the Kyucho in the Bungo Channel by HF radar

Observations of sea surface currents by HF radar were carried out in the Bungo Channel in summer 1992. The current ellipses of M₂ constituent obtained by the observational results agree quite well with those obtained by the ADCP observations, showing that the accuracy of the HF radar measurements is of the same level as ADCP. The results revealed the current structures and their change with the Kyucho in detail. The Kyucho is influenced by the complicated coastal geometry and does not propagate straightly into the Bungo Channel. It propagates further inward after charging the coastal bays with warm water. The current directions change largely, since the currents turn around the stagnant region in the bay filled with the warm water. The northward intrusion begins to be weakened in the southern part of the channel, while it still persists in the northern part. The northward current speeds of the observed Kyucho are about 50 cm/s and sometimes attain 60 to 70 cm/s.     

Favorable conditions for cold-water intrusion from the Kuroshio intermediate layer into Osaka Bay

From 1980 to 1995, in August, the bottom layer of Osaka Bay was occupied by cold, nutrient-rich water compared with that observed during both previous and subsequent decades. To investigate the mechanisms for the intrusion of bottom-layer cold water into Osaka Bay, the intrusion into Osaka Bay via the Kii Channel is simulated using a finite-volume coastal ocean model with unstructured triangular cell grids. The initial conditions, boundary conditions, and surface temperature given to the model are obtained from daily reanalysis data provided by the Japan Coastal Ocean Predictability Experiment. The model shows that cold water uplifted on the eastern side of the Kii Peninsula is propagated westward at 1.0 m/s as a coastal boundary current; it reaches the Kii Channel mouth when the Kuroshio axis is located around 74 km south of Cape Shionomisaki. However, the modeled cold water mass at the Kii Channel mouth does not intrude further to the north of the Kii Channel; therefore, another mechanism is required to explain the cold-water intrusion into the bottom layer of Osaka Bay. A plausible explanation is the estuarine circulation established by the freshwater supply at the bay head. When the river runoff is included in the model without forced vertical mixing, the temperature in Osaka Bay decreases 6.6 days later than the temperature decreases at the Kii Channel mouth. Furthermore, the shoreward current speed in the bottom layer of the modeled estuarine circulation is 15 cm/s, which provides the mechanism required for the cold water mass to pass the Kii Channel.     

Upwelling and the distribution of chlorophyll a within the Bay of Concepción,Chile

The influence of upwelling on the distribution of chlorophyll a within the Bay of Concepción, Chile is discussed in light of continuous measurements of surface in vivo chlorophyll fluorescence and temperature taken simultaneously along horizontal transects, and hydrographic stations' data. Results suggest significant temporal variability both in the distribution of in vivo fluorescence, temperature and salinity within the Bay and in the characteristics of the exchange between the Bay and the adjacent shelf waters, induced by variable upwelling. Upwelling is produced by the predominant south-westerly winds during the summer. Significant variations in the wind direction occur with periods from two to seven days. During active upwelling, exchange is characterized by a surface outflow through the mouth of the Bay and an inflow at depth. Low chlorophyll fluorescence is confined to the upwelling areas on the eastern shore either within or outside the Bay; high chlorophyll fluorescence is confined to the central and western Bay. Density data suggest a three-layered circulation pattern at the mouth of the Bay during the upwelling relaxation involving an inflow both at the surface and bottom and outflow at mid-depth. Associated with this exchange is an active high chlorophyll transport from the Bay to the adjacent coastal waters at mid-depth and inflow of low chlorophyll water from the adjacent shelf at the surface and near the bottom.     

Diurnal changes of pH distribution and the cascading of shore water in aSargassum forest

This paper examines the influence of aSargassum forest on the pH distribution in a small cove facing Wakasa Bay. The diurnal changes in spatial distribution of density and pH were observed in May 1977 during the season of luxuriant seaweed growth, and in August 1977 during the season of little growth. The observations were made at three-hour intervals for 24 hr in May, and 9 hr in August. During the period of strong density stratification, the observed pH distributions were divided into two types: (1) lower pH inshore and higher offshore in the upper layer in the evening and early morning, and (2) higher pH inshore and lower offshore in the daytime. During the period of weak stratification, a homogeneous pH distribution was observed. The pH distribution influenced bySargassum forest during the season of luxuriant growth was different from that during the season of little growth. The process that brought these spatial distributions is discussed. From early dawn to forenoon during the season of luxuriant growth, the heavier water, produced inshore by cooling through nocturnal radiation, cascaded as a stable laminar flow under the lighter water remaining under canopies of the forest and detached floating seaweeds. Reaching the offshore lower layer, and co-acting with algal respiration, this cascade influenced pH distribution. Some dynamical parameters for the process of cascading are calculated and examined.     

Changes in the abundance and composition of picophytoplankton in relation to the occurrence of a Kyucho and a bottom intrusion in the Bungo Channel,Japan

The Bungo Channel in southwestern Japan receives both warm, called Kyucho, and cold deep-water intrusions (bottom intrusion) from the Pacific Ocean. Abundances of Prochlorococcus, Synechococcus, and eukaryotic picophytoplankton were monitored from 18 July to 17 August 2001 to clarify whether advected picophytoplankton from the Pacific Ocean can grow in the channel or not. Synechococcus cells were further discriminated into low- and high-PUB types according to their fluorescence property in flow cytometry. From 18 to 25 July, the water temperature decreased by 3 °C at a 5-m depth at all stations, indicating the occurrence of a bottom intrusion. From 25 July to 4 August, a Kyucho occurred and the water temperature rapidly increased. From 4 to 17 August, a bottom intrusion and a Kyucho both occurred twice, although the intensities were smaller than those occurring until 4 August. From 18 to 30 July, the abundance of both Prochlorococcus and a high-PUB type of Synechococcus drastically decreased because of a bottom intrusion; however, the abundances rapidly increased due to the advection by a Kyucho. These advected cells increased from 4 to 17 August in the channel and Kitanada Bay. Changes in the abundance of low-PUB type of Synechococcus and eukaryotic picophytoplankton were less noticeable than those in the abundance of Prochlorococcus and high-PUB type. The present study demonstrated that oceanic picophytoplankton advected by the Kyucho could grow in the channel. However, abundances of low-PUB type and eukaryotic picophytoplankton increased higher than those of Prochlorococcus and high-PUB type did. Thus, these oceanic phytoplankters will be excluded when Kyucho does not occur for a long time. The co-occurrence of various types of picophytoplankton found in the channel is probably achieved by both Kyucho event and their growth capability in the channel.     

Periodic Intrusion of Cold Water from the Pacific Ocean into the Bottom Layer of the Bungo Channel in Japan

A cold-water intrusion, called a “bottom intrusion”, occurs in the lower layer of the Bungo Channel in Japan. It is an intrusion from the shelf slope region of the Pacific Ocean margin in the south of the channel. In order to reveal the fundamental characteristics of the bottom intrusion, we conducted long-term observations of water temperature at the surface and bottom layers of the channel and 15-day current observations at the bottom of the shelf-break region. The long-term water temperature data indicated that the bottom intrusion occurs repeatedly between early summer and late autumn, and its reiteration between early and mid-summer causes a local minimum of water temperature in the lower layer in mid-summer. Moreover, the data revealed that most of the bottom intrusions occurred in neap tidal periods. The current meter recorded a bottom intrusion with a speed of approximately 15 cm⋅s⁻¹. The current meter also revealed that the intruded cold water slowly retreated back to the shelf slope region after the intrusion. This revised version was published online in July 2006 with corrections to the Cover Date.     

三沙湾夏季底边界层动力过程及悬沙输运特征

基于2018年8月福建三沙湾湾内外共两个定点站位的船基和座底三脚架观测数据,研究了三沙湾底边界动力过程及悬沙输运特征。结果表明,三沙湾湾内湾外两个站位均表现出涨落潮历时相近但涨落潮流速明显不对称的现象,即湾内涨潮流速大于落潮流速,湾外则相反。湾内水体受淡水输入影响较大,表现出落潮期间显著的温盐层化,而涨潮期间水体混合良好;湾外水体受淡水影响不明显,表现为水体温度主导的层化。通过对底边界层动力过程的分析表明,湾内(距底0.75m)、湾外(距底0.50m)站位底边界层的平均摩阻流速分别是0.016m/s、0.013m/s,且两个站位拖曳系数基本相等(2.03×10^–3),表明在相同流速下湾内站位的底部切应力更大,近底沉积物再悬浮和搬运相对湾外站位更为显著。因此观测期间悬沙浓度最大值出现在湾内站位,为109mg/L,且悬沙在垂向上的分布可达上层水体;湾外站位悬沙浓度更低,并且底部悬浮泥沙仅能影响至距底5m的水体。悬沙通量机制分解结果表明,三沙湾夏季的潮周期单宽悬沙从湾外向湾内方向净输运,湾内站位向湾内方向净输运74.88 g/(m·s),平流输沙占主导作用,贡献率41....     

Changes of seawater quality in Ofunto Bay,Iwate, after the 2011 off the Pacific coast of Tohoku Earthquake

Ofunato Bay was a semi-closed area because of the breakwater effect at the entrance; however, the breakwater was destroyed by a massive tsunami generated by the 2011 off the Pacific coast of Tohoku Earthquake. Consequently, the physical environment of Ofunato Bay has been changed significantly, i.e., the modification of the stratified structure of seawater inside the bay and the intermittent intrusion of seawater outside the bay. These alterations of physical environment are considered to have an influence on the chemical and biological environment in Ofunato Bay. To elucidate the influence of the tsunami on the aquatic environment, we measured dissolved nutrients, chlorophyll a and dissolved oxygen concentrations, and heterotrophic bacteria abundance inside and outside of Ofunato Bay from 2012 to 2014, and compared these data with those obtained before the earthquake. As compared with before the earthquake, significant changes after the earthquake were (1) decrease of ammonium and phosphate concentrations, (2) increase of chlorophyll a concentration, (3) increase of dissolved oxygen concentration in the bottom, and (4) decrease of heterotrophic bacteria abundance. The collapse of the breakwater and consequential enhanced water exchange were considered to have brought the decrease of nutrient concentration inside the bay. Furthermore, washout of shellfish mariculture rafts by the tsunami decreased the shellfish biodeposits along with the elution of nutrients by heterotrophic bacteria. Decrease of cultivated shellfish further caused a decline in feeding pressure on phytoplankton and, subsequently, increased the phytoplankton biomass that contributed to the decrease of nutrients inside the bay.