Coastal currents adjacent to the Caeté Estuary,Pará Region,North Brazil

We examined and compared tidal currents and water column structure between a near-shore station (12 km from the coast) and an offshore station (32 km from the coast) adjacent to the Caeté River, Pará Region, Brazil. Although the coastal system of Pará is largely influenced by local tides and wind, we found substantial differences in the dominant forcing agents between stations. Water column dynamics at the near-shore station were largely affected by local tidal processes, while differences between surface and bottom layer flows also indicated the importance of gravitational circulation at this station and a substantial influence of the adjacent Caeté River discharge. In comparison, at the offshore station, water column structure was largely influenced by a semi-diurnal tidal flow, an along-coastal current flow (mainly associated with the North Brazil Current) and the dynamics of local wind flow. The near-shore station at low tide showed a high level of stratification; at high tide such stratification was reduced. In comparison, stratification was only apparent within the upper 6 m at the offshore station, the rest of the water column was relatively well-mixed. The stratification within Station 1 at low tide was a result of the bi-directional movement of water discharged from the Caeté River, with lower salinity surface water and high salinity bottom water resulting in an estuarine-like circulation environment. The spatial variability and lack of correlation in current flow and water column structure between the near-shore and offshore stations suggest that a flow field resulting from differences in local circulation, tidal variability and wind persistence separate areas. We argue that this separation may indicate that the offshore station is located in a transition region between the Caeté River waters and the local coastal area.     

Current and Turbidity Variations in the Western Part of Suo-Nada, the Seto Inland Sea, Japan: A Hypothesis on the Oxygen-Deficient Water Mass Formation

A hydrographic survey and a 25-hour stationary observation were carried out in the western part of Suo-Nada in the summer of 1998 to elucidate the formation mechanism of the oxygen-deficient water mass. A steep thermocline and halocline separated the upper layer water from the bottom water over the observational area except for near the Kanmon Strait. The bottom water, in comparison with the upper layer water, indicated lower temperature, higher salinity, lower dissolved oxygen, higher turbidity, and higher chlorophyll a. Turbidity in the upper layer water changed with semi-diurnal period while the bottom water turbidity showed a quarter-diurnal variation, though the M₂ tidal current prevailed in both waters. From the turbidity distribution and the current variation, it is revealed that the turbidity in the upper layer water is controlled by the advection due to the M₂ tidal current. On the other hand, the quarter-diurnal variation in the bottom water turbidity is caused by the resuspension of bottom sediments due to the M₂ tidal current. The steep thermocline and halocline were maintained throughout the observation period in spite of the rather strong tidal currents. This implies an active intrusion of the low temperature and high salinity water from the east to the bottom of Suo-Nada. Based on the observational results, a hypothesis on the oxygen-deficient water mass formation was proposed; the periodical turbidity variation in the bottom water quickly modifies the oxygen-rich water in the east to the oxygen-deficient bottom water in Suo-Nada in a course of circulation.     

Sensitivity studies with the three-dimensional multi-level model for tidal motion

A three-dimensional multi-level hydrodynamic model has recently been developed and applied to tidal motion in Singapore’s coastal waters. This paper describes a series of numerical experiments to evaluate the sensitivity of the tidal currents and elevations to model parameters. The results show that the predicted tidal elevations are insensitive to three model parameters: horizontal eddy viscosity coefficient (Smagorinsky constant, c_h), bottom friction coefficient (c_b) and internal friction coefficient (c_v), whereas the effects of these parameters are quite different for tidal current velocities. The velocities are slightly reduced with an increase in c_h and c_b. The bottom friction effects on velocity profiles increase with water depth. The effect of c_v might be significant for the tidal velocities at all levels. The velocities at upper layers of the water column decrease with the increase in c_v, whereas the velocities at the bottom layer show the reverse trend. The effects of three model parameters on the magnitude and phase of the simulated currents are in the order (from strong to weak) of c_v, c_b and c_h.     

Dispersal and recruitment of blue crab larvae in Delaware Bay,U.S.A.

Results of a three-year survey of the occurrence of Callinectes sapidus larvae in the mouth of Delaware Bay indicated that stage I zoea larvae were most abundant insurface water as compared to mid-depths and near bottom. The major peak in abundance of stage I zoea larvae occurred in early August with a secondary peak in early September. Peaks in abundance of megalopae occurred five weeks after the respective peaks in zoeal abundance. Zoea stages II–VIII were not collected in the bay mouth. Results of sampling every 3 h over consecutive tidal cycles showed that stage I zoea larvae were most common in the water column on ebbing tidal currents. Megalopae were most common in the water column on flooding tidal currents, suggesting a tidally related, vertical migration. It was concluded that stage I zoea larvae are flushed from the estuary and undergo development on the continental shelf. Megalopae are then transported back to inshore waters by a combination of winds and currents and invade the estuary by means of migration into the water column on flooding tidal currents and migration to the bottom on ebbing tidal currents.     

Semidiurnal Dynamics of Salinity, Nutrients and Suspended Particulate Matter in an Estuary in the Seto Inland Sea, Japan, during a Spring Tide Cycle

The physical and chemical variability of the water column at subtidal station of an estuary in the Seto Inland Sea, Japan, was studied over a 24-hour period during a spring tide (tidal range ca. 2 m) in May 1995. Surface water and several depths through the water column were monitored every one and two hours, respectively. At each occasion, water temperature, salinity and dissolved oxygen concentration were measured and water samples were collected for the determination of nutrients and suspended particulate matter (SPM). Disruptive changes in the physical and chemical characteristics of the water was produced by the tidal cycle and the mixing of water masses of different origin. These changes were highly significant both spatially and temporally, yet with varying effects on physical parameters, nutrients and the different components of SPM. Significant differences in nutrient concentrations were also observed when the data-set was divided into ebb and flood components, irrespective of the depth. Nitrate and nitrite rose to 1.8 times higher during the flood. Spatial differences of SPM were less marked than those of nutrients, only particulate organic carbon (POC) being significantly higher at the surface than in the intermediate and the lower layer. Both POC and pheopigment concentrations increased markedly through the water column, being highest shortly before the lower low tide. In contrast, suspended solid (SS) content increased sharply after the lower low tide (>40 mg l⁻¹) and this coincided with a marked decrease of the C/SS content (<20 mg g⁻¹). The lagtime between POC and SS tidal transport was caused by particle resuspension from the exposed intertidal sediments as the tidal level rose, and particle transport selection in relation to the tidal state. This revised version was published online in August 2006 with corrections to the Cover Date.     

Hydrodynamic controls on cold-water coral growth and carbonate-mound development at the SW and SE Rockall Trough Margin,NE Atlantic Ocean

Long-term (⩽1-year) records obtained by seabed observatories (BOBO) and repeated (24-h) CTD casts show the presence of a highly energetic environment in and around two cold-water carbonate-mound provinces, on the Southwest and Southeast Rockall Trough (SW and SE RT) margin. Carbonate mounds, covered with a thriving coral cover, are embedded mainly in the Eastern North Atlantic Water (ENAW) and are observed in a confined bathymetric zone between 600 and 1000 m water depth. Cold-water corals seem to be restricted in their growth by temperature and food availability. The presence of living corals on top of the carbonate mounds appears linked to the presence of internal waves and tidal currents in the water column, and consequently carbonate mound structures are shaped by the local hydrodynamic regime. Mound clusters have an elongated shape perpendicular to the regional contours and corresponding to the direction of the highest current speeds. On the SW RT margin temperature, salinity and current speed reflect a diurnal tidal pattern, causing maximum temperature variations at 900 m depth of more than 3 °C. Current speeds up to 45 cm s⁻¹ occur, and a residual current of 10 cm s⁻¹ is directed along the slope to the southwest. At the SE RT margin the temperature of the bottom water fluctuates more than 1 °C with a semi-diurnal tidal cyclicity. Amplitudes of average and peak current speeds here are comparable with those measured on the southwest margin, but the residual current in this area is directed to the northeast. Tidal currents and internal waves at both margins force the formation of intermediate and bottom nepheloid layers and bring fresh food particles with increased velocity to the mounds. The distribution of corals in both mound areas is considered directly related to the presence of enhanced turbidity. An increase in temperature can be directly related to an increase in the amount of particles in the water column. Current velocity increases when a transition occurs from cold to warm waters. High current velocities prevent local sedimentation but provide sufficient food particles to the corals, so that the corals thrive at the mound summits.     

江苏近海潮流湍流切应力研究

潮流的流速分布和湍流切应力的分布密切相关。为了研究潮流湍流切应力的特征和变化规律,本文从潮流运动方程推导出随潮流呈周期性变化的湍流切应力随深度的分布表达式。从江苏近海现场实测流速剖面中计算得到一个潮周期内的湍流切应力的变化过程。通过将实测值与本文理论解的比较发现,计算值能很好地反映出实测值的变化特征,尤其是能正确地反映潮流在加速和减速过程中,湍流切应力偏离线性分布呈现上凹和下凹的现象,以及只有在加减速转换的时候切应力才呈现线性分布的特点。最后,通过分析湍流切应力振幅及相位沿水深的变化情况,认为反映潮流周期、涡粘性和水深之间关系的参数Ri是决定湍流切应力弯曲程度及分布形态的重要参数。对于近海潮流,水深是影响湍流切应力偏离线性程度的主要因素。     

Methane in the southern North Sea: Sources, spatial distribution and budgets

Measurements of methane (CH₄) so far have always shown supersaturation in the entire North Sea relative to the atmospheric partial pressure and the distribution of surface CH₄ reveals a distinct increase towards the shore. Since North Sea sediments presumably are an insignificant source for CH₄ the coastal contribution via rivers and tidal flats gains in importance.In this work, CH₄ data from the River Weser, the back barrier tidal flats of Spiekeroog Island (NW Germany), and the German Bight are presented. Results from the River Weser are compared to other rivers draining into the German Bight. Measurements in the tidal flat area of Spiekeroog Island highlight this ecosystem as an additional contributor to the overall CH₄ budget of the southern North Sea. A tidally driven CH₄ pattern is observed for the water column with maximum values during low tide. Tidal flat sediments turn out to be the dominating source because pore waters discharged during low tide are highly enriched in CH₄. In contrast, the freshwater contribution to the tidal flats by small coastal tributaries has almost no impact on water column CH₄ concentrations. The CH₄ level seems to be disturbed irregularly by wind forcing due to elevated degassing and prevention of advective flow when tidal flats remain covered by water.Based on our data, two model calculations were used to estimate the impact of tidal flats on the CH₄ budget in the German Bight. Our results demonstrate that the back barrier tidal flats of the east Frisian Wadden Sea contribute CH₄ in an order of magnitude between the Wash estuary and River Elbe and thus have to be considered in budget calculations.     

Processes influencing iron distribution in the coastal waters of the Tsugaru Strait,Japan

We report measurements of iron, nutrients, dissolved oxygen, humic-type fluorescence intensity and chlorophyll a concentrations in the coastal waters at the inflow (western) and outflow (eastern) ends of Tsugaru Strait (Japan) in June 2003 and 2004. Two different water masses (intensive eastward flow “subtropical Tsugaru Warm Current Water (TWCw)” and weak westward flow “subarctic Oyashio Water (OW)”) were observed at the eastern end of the strait. TWCw at the southern part of the eastern strait was vertically homogeneous with a uniform concentrations of iron (0.7–1.1 nM for labile dissolved Fe and 14–20 nM for total dissolvable Fe in 2003) as well as other chemical, biological and physical components throughout the water column of 200 m due to strong vertical mixing in the strait. The degree of mixing in the Tsugaru Warm Current (TWC) is predominantly affected by diurnal tidal current, which is strong during the period of tropical tides and weak during the period of equinoctial ones. The especially strong vertical water mixing in 2003 is caused by large dissipation energy input due to the bottom friction of passage-flow through the strait and tidal current. At the northern part of the eastern strait, the fresh surface layer overlying the OW and the deep-bottom waters in 2003 contained large concentrations of dissolved iron, resulting from iron supplied from river runoff and shelf sediments, respectively. These results suggest that the most important mechanism for transporting iron in the strait is the strong vertical water mixing due to the tidal current, and that the iron sources in the coastal waters are the organic-associated, iron-rich freshwater input into the surface water.     

Tidal effect on nutrient exchange in Xiangshan Bay, China

Xiangshan Bay is an estuary in China which borders on the East China Sea. The circulation in the estuary is driven by tidal movement, residual current, the internal density distribution, and synoptic wind forcing; however, the last three are not the main dynamic factors affecting nutrient transport. Because the estuary tends to be eutrophic, a synoptic study was carried out to assess the influence of tidal movement on the nutrient distribution patterns within the estuary and to estimate the fluxes of nutrient transport between the estuary and the sea. Nitrate and ammonium are found to be exported from the estuarine water to the coastal water under usual tidal conditions, except for storm tides which result in large amounts being imported because of the extremely high concentrations in the coastal bottom water. Exports of phosphate and silicate are shown to be consistent during spring tides and neap tides in all seasons. However, the usual tidal regimes resulted in only minor nutrient exchange except during abnormal events.     

Vertical structure of larval fish assemblages during diel cycles in summer and winter in the southern part of Bahía de La Paz, México

The effect of environmental variables on the vertical structure of larval fish assemblages in a tropical coastal lagoon was analyzed. Ichthyoplankton samples were collected from the near-bottom and surface strata near the mouth of a subtropical lagoon during contrasting seasonal conditions of temperature, photoperiod, light intensity, and tidal heights. During summer, larval fish assemblages had high species richness (R) and were dominated by tropical species. During winter, assemblages had lower R values and were dominated by subtropical and temperate species. Vertical distribution patterns of the taxa were determined by the interaction of environmental variables and behavior of each species to maintain their position in a stratum in the water column, or to achieve vertical migrations induced by environmental stimuli that, in this case, were thermal gradient, column water stratification, and intensity of light. Depth position and vertical migration of fish larvae, coupled with the flood and ebb tide conditions, played an important role in their retention and displacement toward the lagoon. Fish larvae with distribution restricted to the inner part of the inlet, such as Achirus mazatlanus, Etropus sp., and several gobies, were more abundant in the near-bottom stratum during the ebb tide, allowing them to avoid exportation, whereas those that could spawn outside, but depended on the inlet as a nursery area, were more abundant near the surface during flood tide, such as Abudefduf troschelii and Stegastes rectifraenum.     

胶州湾水交换及湾口潮余流特征的数值研究

利用基于普林斯顿海洋模式建立的胶州湾及临近海域潮汐潮流数值模型,结合胶州湾口走航式声学多普勒海流剖面仪(ADCP)测流资料,研究了胶州湾口的潮(余)流特征,并在潮流模型的基础上耦合建立了水质模块,模拟了胶州湾的水交换过程。考虑M2,S2,K1,O1,M4和MS4六个主要分潮,胶州湾口潮流场的模拟与ADCP观测数据吻合较好。外湾口水道上的潮流非常强,大潮期间观测到201 cm/s的峰值流速。团岛岬角的两侧分别存在一个流向相反的余流涡旋,两涡旋在团岛附近辐合,形成了57 cm/s的离岸强余流。整个胶州湾平均水体存留时间为71 d,平均半交换时间为25 d。胶州湾水体交换能力在空间分布上有很大差异:湾口海域最强,向湾顶逐渐减弱。湾内存在两个弱交换区,分别位于湾的西-西南部和东北端,水体存留时间多超过80 d,湾西局部水域最长达120 d,而半交换时间也大多超过40 d。潮流场的结构、强度,以及与湾口距离的远近是造成湾内水交换能力空间差异的主要原因。     

台湾海峡M_2分潮的三维数值模拟

本文利用普林斯顿海洋模式对台湾海峡 Ｍ ２ 分潮作了分辨率较高的三维数值模拟。在较准确地模拟了潮汐分布的基础上,研究了 Ｍ ２ 分潮流椭圆分析、最大流同时线分布、潮流场的水平及垂直结构,最后还给出了 Ｍ ２ 分潮余流、余水位的分布。结果表明, Ｍ ２ 分潮最大流同时线在海峡中部同时形成密集区和一个圆流点;构成海峡潮波系统的两股潮波中,南支潮波的影响似超出了以往所认为的范围;最大潮流仅在近海底处急剧减小,最大流方向随深度增加右转,到近底层又向左转;潮汐余流和余水位均较弱,仅在澎湖水道、台湾浅滩附近余流较大。     

舟山绿色石化三维潮流特性数值研究

本文基于MIKE3平台建立宁波、舟山及其附近海域三维水动力模型,通过与实测数据进行潮流验证的基础上,对比了三维模型与二维模型的区别,并主要对舟山绿色石化三期围垦工程实施前后周边潮流场的变化影响进行了预测分析。结果表明:（1）同样的初始条件下,二维模型和三维模型的潮位和潮流验证结果都是良好的,而在某些情况下二维数值结果更接近于实测值;（2）三维垂向上的流速分布呈现出随水深增加而减小的趋势,总体上潮流拟合效果较好,其中0.6H水深以下的流速相对误差结果在10%～15%之间,考虑模型可能受未考虑风场和波浪影响的缘故,局部测站表层计算流速较实测值稍大;而垂向上的流向分布近乎一致;（3）围垦工程前后并未对工程周边潮流场有明显影响,仅在大鱼山岛东北侧、东南侧和西北角受“截弯取直”和小岛屿间水道围堵的影响,有局部潮流形态的改变。整体上看,以顺应岸线方向上的围垦工程布置方案会对局部近岸潮流产生影响,对外海大范围潮流场的影响几乎不计。     

A three-dimentional model of the northeast East China Sea, with application to the M2 current field

A simple three-dimensional tidal model is used to examine the M2 tidal current distribution in a northeastern part of the East China Sea, especially the vertical variation of the current in the region. Computed M2 current is compared with observations available and found to be in good agreement.Main features of the calculating method in this study are: (1) Vertical variation of the tidal current is taken as a funetion of the depth-mean velocity: (2) the method is applicable to a variety of the vertical eddy viscosities; (3) it has a fine vertical resolution, especially near the sea bootom. So, this method not only enables us to get a steady state solution easily but also depicts effects of the friction on the vertical variation of the current much better.     

Vertical structure of the M<Subscript>2</Subscript> tidal current in the Yellow Sea

The vertical structure of the M₂ tidal current in the Yellow Sea is analyzed from data acquired using an acoustic Doppler current profiler. The observed vertical profiles of the M₂ tidal current are decomposed into two rotating components of counter-clockwise and clockwise, and restructured using a simple one-point model with a constant vertical eddy viscosity. The analyzed results show that the internal fictional effect dominates the vertical structure of the tidal current in the bottom boundary layer. In the Yellow Sea, the effect of the bottom friction reduces the current speed by about 20–40% and induces the bottom phase advance by about 15–50 minutes. In the shallower coastal regions, the effects of bottom topography are more prominent on the vertical structure of tidal currents. The vertical profile of the tidal current in summer, when the water column is strongly stratified, is disturbed near the pycnocline layer. The stratification significantly influences the vertical shear and distinct seasonal variation of the tidal current.     

杭州湾北部潮流深槽区细颗粒物质输运与再悬浮过程

于2005年5月大潮期间在杭州湾北部潮流深槽区的4个站位进行潮周期观测,获得了流速、悬沙等数据,并对其进行了分析,计算了水沙通量和再悬浮通量。分析结果表明,该深槽区涨潮流速大于落潮流速,涨潮历时小于落潮历时,潮差自湾口向湾内方向增大;悬沙的组分以粉砂为主,分选较差,偏态以负偏为主,这些特征与底质一致;深槽中部和东部的悬沙沿岸线向湾内方向输运;深槽西部和东部外侧的悬沙输运方向与余流方向一致,分别向湾内和南部输运;除转流和流速加速初期外,垂线流速分布符合Kûrmûn-Prandtl模型,摩阻流速与垂线平均流速变化趋势一致。计算得到的表观粗糙长度在涨落潮时段的水流加速或减速阶段都呈增大趋势,且数值较大,这难以把它简单地归结为床面形态的作用,表观粗糙长度的变化趋势可能是高悬沙浓度和浓度成层性共同作用的结果,但对这一假说的验证还有待于进一步的现场观测和机制分析。计算所得的再悬浮发生的周期性与实测悬沙浓度的周期性相符,而且最大悬沙浓度的出现滞后于最大再悬浮通量,说明再悬浮作用对水层中悬沙浓度的变化具有重要影响。     

Coupled penetrometer, MBES and ADCP assessments of tidal variations in surface sediment layer characteristics along active subaqueous dunes, Danish Wadden Sea

In-situ geotechnical measurements of surface sediments were carried out along large subaqueous dunes in the Knudedyb tidal inlet channel in the Danish Wadden Sea using a small free-falling penetrometer. Vertical profiles showed a typical stratification pattern with a resolution of ∼1 cm depicting a thin surface layer of low sediment strength and a stiffer substratum below (quasi-static bearing capacity equivalent: 1–3 kPa in the top layer, 20–140 kPa in the underlying sediment; thickness of the top layer ca. 5–8 cm). Observed variations in the thickness and strength of the surface layer during a tidal cycle were compared to mean current velocities (measured using an acoustic Doppler current profiler, ADCP), high-resolution bathymetry (based on multibeam echo sounding, MBES) and qualitative estimates of suspended sediment distributions in the water column (estimated from ADCP backscatter intensity). The results revealed an ebb dominance in sediment remobilization, and a general accretion of the bed towards low water. A loose top layer occurred throughout the tidal cycle, likely influenced by bedload transport and small events of suspended sediment resettlement (thickness: 6 ± 2 cm). Furthermore, this layer showed a significant increase in thickness (e.g. from 8 cm to 16 cm) related to periods of overall deposition. These findings imply that dynamic penetrometers can conveniently serve to (1) quantify potentially mobile sediments by determining the thickness of a loose sediment surface layer, (2) unravel sediment strength development in potentially mobile sediments and (3) identify sediment accumulation. Such data are an important complement and add a new geotechnical perspective during investigations of sediment remobilization processes in highly dynamic coastal environments.     

渤、黄、东海三维潮波运动数值模拟

采用POM三维海流数值模式模拟渤、黄、东海潮波运动。所得潮汐调和常数与77个测站的观测值的平均绝对误差：m1振幅1．92cm,迟角4．39°;M2振幅4．70cm,迟角5．08°。模拟结果显示M2分潮流共存在8个圆流点,m1岁潮流共存在9个圆流点。其中,M2分潮流在舟山群岛附近与m1分潮流在黄海北部和台湾海峡谷存在一个圆流点,这三个圆流点是本文数值模拟所首次显示的,需进一步的观测验证它们实际存在与否。模式还得出湍流涡动系数分布。