Hydrodynamic and water column properties at six stations associated with mussel farming in Pelorus sound, 1984–85

Mussel farming places a benthic organism in a pelagic environment; it is therefore important to understand the driving force that transports the food to the mussels. The hydrodynamic regimes in the sidearms and embayments in Pelorus Sound are dominated by the lunar tide, and a net estuarine circulation in the main channel flowing inwards along the bottom and outwards along the top. Salinity gradients extend throughout the sound from the river inflows, with strongest density stratification in the sidearms and embayments: nearest the head of the sound. There, the water column is separated at the pycnocline into upper and lower layers which tend to move in different directions or at different velocities. Local circulation patterns modify tidal flushing patterns, producing extended residence times in some embayments, whereas other embayments off the side of the main channel tend to be flushed more rapidly by through‐flow water and have shorter residence times than would otherwise be expected. The changing inflow of fresh water modifies the local hydraulic regimes in the inner sounds, especially during flood conditions.     

Residence times of contaminants in a lake-canal system: a delta case study in the SW Netherlands

As final part of the Delta Works in the SW Netherlands, the interior part of the tidal Eastern Scheldt will be enclosed by dams and flushed with contaminated Rhine-Meuse water. The new name of this area will be Lake Zoom (Zoommeer).Since the flushing of Lake Zoom will be a regular process, this system offers an excellent opportunity to study the residence times of various contaminants over a lake-canal system of 35 km. The in situ measurements will be linked to mathematical model studies.A number of models describing transport with water, settling, sedimentation and migration in the bottom is investigated. Explicit definitions of residence and adaptation times are given and applied to the studied models. As an example, a hypothetical application is made to the Lake Zoom case.     

Biogeochemical factors influencing the residence time of microconstituents in a large tidal estuary, Delaware Bay

Generally the large tidal estuaries of the eastern United States, such as Delaware Bay, are characterized by rather high suspended particle concentrations at the landward end and high biological activity at the seaward end. As such, these estuaries can be conceptualized as geochemical and biochemical “reactors” for those processes controlling the transmission of trace elements from fresh to the coastal shelf waters. The efficiency of these reaction processes relative to estuarine flushing will control the residence times of microconstituents in such estuaries.Evidence is drawn from the Delaware estuary to illustrate biogeochemical estuarine reaction processes using salinity distribution data and mass balance calculations. The Delaware retains some of its estuarine trace elements as sedimented estuarine particles, while others are more conservative and largely exported. Those retained by sedimenting processes include trace elements in primarily geochemical (particle reactive) chemistries, while those exported appear recycled by biochemical (nutrient reactive) chemistries. Often, the behavior of trace elements (e.g., Fe, Cd) and nutrients (e.g., PO₄) appear biogeochemically linked. Other examples are drawn from mixing studies to illustrate particle interaction, and benthic flux measurements to illustrate limited diagenetic reflux.The residence time of estuarine microconstituents should depend seasonally on the relative turbidity, flushing rate, and primary production of tidally dominated estuaries such as the Delaware. Thus, residence times of the more biogeochemically reactive microconstituents must be substantially shorter (days to weeks) than the average flushing times of these larger estuaries, while the residence times of the less reactive ones should approach such flushing times (weeks to months). True estuarine residence times of microconstituents can only be modeled after using large data sets averaged over time (season, tides) and space (salinity).     

Estuary/ocean exchange and tidal mixing in a Gulf of Maine Estuary: A Lagrangian modeling study

A Lagrangian particle method embedded within a 2-D finite element code, is used to study the transport and ocean–estuary exchange processes in the well-mixed Great Bay Estuarine System in New Hampshire, USA. The 2-D finite element model, driven by residual, semi-diurnal and diurnal tidal constituents, includes the effects of wetting and drying of estuarine mud flats through the use of a porous medium transport module. The particle method includes tidal advection, plus a random walk model in the horizontal that simulates sub-grid scale turbulent transport processes. Our approach involves instantaneous, massive [O(500,000)] particle releases that enable the quantification of ocean–estuary and inter-bay exchanges in a Markovian framework. The effects of the release time, spring–neap cycle, riverine discharge and diffusion strength on the intra-estuary and estuary–ocean exchange are also investigated.The results show a rather dynamic interaction between the ocean and the estuary with a fraction of the exiting particles being caught up in the Gulf of Maine Coastal Current and swept away. Three somewhat different estimates of estuarine residence time are calculated to provide complementary views of estuary flushing. Maps of residence time versus release location uncover a strong spatial dependency of residence time within the estuary that has very important ramifications for local water quality. Simulations with and without the turbulent random walk show that the combined effect of advective shear and turbulent diffusion is very effective at spreading particles throughout the estuary relatively quickly, even at low (1 m²/s) diffusivity. The results presented here show that a first-order Markov Chain approach has applicability and a high potential for improving our understanding of the mixing processes in estuaries.     

The impact of human activities on the flushing properties of a semi-enclosed lagoon: Xiaohai, Hainan, China

In this study, a Lagrangian particle tracking model driven by hydrodynamic fields was used to investigate the changes of flushing properties in Xiaohai Lagoon resulting from natural evolution and influences of human activities. Comparisons of residence times between 1936 and 2001 indicate that the flushing efficiency of the lagoon has deteriorated greatly during the past seven decades. Over this period, the average residence time of the lagoon has increased from 45 to 71 days, an increase of 59%. The Lagrangian residual velocity has decreased from 1936 to 2001 in the large portions of the lagoon, and the spatial distribution pattern of the residual current has changed significantly. The well-developed horizontal circulations in 1936 disappeared. Instead, the horizontal Lagrangian residual velocity showed uniformly seaward motion with reduced velocity in 2001. Human activities have incurred great impacts on the deterioration of flushing efficiency. The reduction of river inflow imposed the most significant impact, causing an increase of average residence time by 33%. The land reclamation in the lagoon came as the second most significant factor, causing an increase of 15%. Closure of the North Opening had posed minor impact. The model results suggest that restoring the river inflow, as well as the dredging of the tidal channel and the inner lagoon, should be the top proprieties for future water quality management.     

Control of domoic acid toxin expression in Pseudo-nitzschia multiseries by copper and silica: Relevance to mussel aquaculture in New England (USA)

The production of the toxin Domoic Acid (DA) by the diatoms Pseudo-nitzschia spp. is affected by several environmental factors, among them copper and silica. The effects of these nutrients upon DA production have been studied individually, but not in combination. There is evidence, however, that in diatoms copper can enter the cell via the silicic-acid transport site. The goal of this study was to analyze the effect of the interaction between copper and silicic-acid supply upon DA production in Pseudo-nitzschia multiseries. The study was motivated by concerns about the risk of toxigenic Pseudo-nitzschia spp. impacting mussel aquaculture in New England (USA). The results of the present study do not indicate that copper uses the silicic acid transport site to enter the cell; nevertheless, there is an interaction between these two nutrients that produces a synergistic affect upon toxin production. A small increase in copper, without a simultaneous increase in silicate, as well as an increase in both copper and silicate, leads to DA up-regulation. Furthermore, the field component of this study reports the presence of species of Pseudo-nitzschia on the New England coast that are capable of producing DA. Together these findings indicate that risk of DA impacting mussel aquaculture along the coast of New England would be increased by an unusual enrichment of copper in the vicinity of mussel farms.     

影响坦帕湾水交换的三种因素

本文利用高分辨率数值模型,以2001年秋季为例,详细分析了影响坦帕湾水交换的三种因素:潮汐、河流和风。论文共设置了三组实验,驱动力分别为潮汐,潮汐和河流,潮汐、河流和风。模拟结果显示:只有潮汐作用时,由于坦帕湾潮汐较弱,潮程较短,坦帕湾与其临近海域的水交换主要发生在湾口附近;当潮汐和河流共同作用时,由于河流和湾口海水盐度的不同形成了水平密度梯度,在其产生的水平密度梯度力的作用下,坦帕湾形成了表层流向湾外、底层流向湾内的重力环流,从而加强了坦帕湾跟其临近海域的水交换;由湾内指向湾外方向(2001年秋季平均)的风应力加强了流向湾外的表层流,同时水位梯度力发生了反转,变成了由湾口指向湾顶,这加强了流向湾内的底层流,表层流和底层流的加强最终促进了坦帕湾跟其临近海域的水交换;在航道处,水深较深瑞利数较大,该处的重力环流较强,这使得相对于两侧的浅水区,航道处的水交换能力较强。此外,文章还分析了坦帕湾水交换的空间差异,在Old Tampa Bay的西侧和北侧,滞留时间最长,水交换能力最弱。为减少海洋生态灾害发生,今后应重点加强对该地区的生态环境保护。     

Influence of Wind on Dynamics and Flushing of Shallow Estuaries

Observations of two small estuaries in Cape Cod, U.S.A. indicate large variations in salinity structure that are forced by variations in along-estuary wind stress. During onshore winds, the estuarine circulation is reduced, and the along-estuary salinity gradient increases as freshwater accumulates. During offshore winds, the surface outflow is enhanced, freshwater is flushed out of the estuary, and the along-estuary salinity gradient becomes weak. Constrictions block the wind-induced flushing, resulting in strong salinity fronts across the constrictions. The residence time of one of the estuaries varies by more than a factor of three in response to variations in wind-induced flushing. The other estuary has little variation of flushing associated with winds, due to a constriction at the mouth that inhibits the wind-induced exchange. The strong influence of winds on the flushing of these estuaries is due in part to their shallow depths, which accentuates the influence of wind stress relative to the effects of the horizontal density gradient. In addition, the residence times of the estuaries are comparable to the time scale of wind forcing, allowing large changes in water properties during wind events.     

Potential alteration of fjordal circulation due to a large floating structure—Numerical investigation with application to Hood Canal basin in Puget Sound

Circulation in typical fjords is characterized by a shallow brackish layer at the surface over a deep long and narrow saltwater column. This surface layer is responsible for the outflow of water from the fjord and is important for flushing of the basin and water quality maintenance. The vertical structure of circulation and transport is known to be easily disrupted, and we postulate that the stability of fjordal circulation may also be vulnerable to impacts from anthropogenic alterations, such as floating structures, which could constrict the mixing and transport in the upper layers of the water column. First, using simplified fjord geometry and a three-dimensional finite volume coastal ocean model (FVCOM), a baseline examination of tidally averaged current profiles in basins with and without sills is presented. The response, varying from a partially mixed estuary regime to classical fjord conditions matches many fjord-like basins such as those in Puget Sound, Washington. The effect of surface obstruction on tidally averaged currents and residence times was then examined by incorporation of a narrow block in the surface layer of the model across the width of the simplified fjord channel such that normal velocity in the horizontal direction was forced to zero. This block approximated the presence of a floating bridge and was further tested using the geometry of Hood Canal, a fjordal sub-basin with a sill in Puget Sound. The results show that tidally averaged mean outflow under the influence of such a constraint at the water surface could be reduced significantly. In the case of Hood Canal, preliminary results indicate that the presence of the floating bridge might have increased the residence times in the basin by 8–13%, which could be an important factor affecting water quality.     

Correspondence between the distribution of hydrodynamic time parameters and the distribution of biological and chemical variables in a semi-enclosed coral reef lagoon

Hydrodynamic modeling can be used to spatially characterize water renewal rates in coastal ecosystems. Using a hydrodynamic model implemented over the semi-enclosed Southwest coral lagoon of New Caledonia, a recent study computed the flushing lag as the minimum time required for a particle coming from outside the lagoon (open ocean) to reach a specific station [Jouon, A., Douillet, P., Ouillon, S., Fraunié, P., 2006. Calculations of hydrodynamic time parameters in a semi-opened coastal zone using a 3D hydrodynamic model. Continental Shelf Research 26, 1395–1415]. Local e-flushing time was calculated as the time requested to reach a local grid mesh concentration of 1/e from the precedent step. Here we present an attempt to connect physical forcing to biogeochemical functioning of this coastal ecosystem. An array of stations, located in the lagoonal channel as well as in several bays under anthropogenic influence, was sampled during three cruises. We then tested the statistical relationships between the distribution of flushing indices and those of biological and chemical variables. Among the variables tested, silicate, chlorophyll a and bacterial biomass production present the highest correlations with flushing indices. Correlations are higher with local e-flushing times than with flushing lags or the sum of these two indices. In the bays, these variables often deviate from the relationships determined in the main lagoon channel. In the three bays receiving significant riverine inputs, silicate is well above the regression line, whereas data from the bay receiving almost insignificant freshwater inputs generally fit the lagoon channel regressions. Moreover, in the three bays receiving important urban and industrial effluents, chlorophyll a and bacterial production of biomass generally display values exceeding the lagoon channel regression trends whereas in the bay under moderate anthropogenic influence values follow the regressions obtained in the lagoon channel. The South West lagoon of New Caledonia can hence be viewed as a coastal mesotrophic ecosystem that is flushed by oligotrophic oceanic waters which subsequently replace the lagoonal waters with water considerably impoverished in resources for microbial growth. This flushing was high enough during the periods of study to influence the distribution of phytoplankton biomass, bacterial production of biomass and silicate concentrations in the lagoon channel as well as in some of the bay areas.     

The Spring Phytoplankton Bloom in the Coastal Temperate Ocean: Growth Criteria and Seeding from Shallow Embayments

A method based on time-series of conductivity, temperature and depth (CTD) profiles which successfully determines favourable phytoplankton growth conditions for the spring bloom in nearshore temperate coastal waters was developed. The potential for shallow embayments to influence phytoplankton species composition in larger adjacent waters was also investigated. At temperate latitudes, such embayments should have favourable phytoplankton growth conditions earlier in the spring than open waters as bathymetry limits vertical mixing and thus increases light availability. The study area was Nanoose Bay, which is connected to the Strait of Georgia, British Columbia. Data were collected 2–3 times per week during the winter-spring of 1992 and 1993. A mooring with 5 current meters was placed at the mouth of the bay in 1992. The conservation equation for a scalar was used to estimate the balance between advective transport and biological source and sink terms. Variability in physical conditions and biological response between years was tremendous. Results indicate that seeding from the bay was not possible in 1992 but could have been in 1993. However, to conclusively determine the importance of Nanoose Bay on the spring bloom species composition in the Strait of Georgia, more extensive work is required. This revised version was published online in August 2006 with corrections to the Cover Date.     

Transport timescales for identifying seasonal variation in Bass Strait, south-eastern Australia

A three-dimensional σ-coordinate ocean model with realistic forcing is used to derive and compare flushing time, residence time and water age distributions in Bass Strait, a broad shallow shelf sea in south-eastern Australia which exhibits seasonal variation in water mass properties and circulation. Results illustrate flushing out of most old water occurs each winter–spring period and the circulation in summer increases age of remnant older water in the Strait. A correlation between water age and salinity is found in winter which is the result of advective “erosion” of the interior water mass. The presence of an area in the south-eastern part of the interior with low flushing is suggested to result from a number of physical factors including the presence of quasi-stationary eddies which entrain age, leading to relatively older water residing there.     

A nested numerical tidal model of the southern New England Bight

A nested, barotropic numerical tidal model has been applied to the southern New England Bight to include Long Island Sound (LIS), Block Island Sound (BIS), Rhode Island Sound (RIS), Buzzards Bay (BB) and a portion of the southern New England shelf. An explicit finite difference representation of the two-dimensional vertically averaged equations of motion and continuity, employing forward time and centered spatial differences with the bottom friction term being evaluated at both time levels, is applied to the study area to predict tidal circulation dynamics. Horizontal friction and advective terms have been neglected. Tidal height data along the open boundaries is specified from available data. Model predictions of corange and high and low cophase lines agree with available data and show the strongly co-oscillatory behavior of LIS. Tidal current ellipses agree reasonably well with available data showing rectilinear motion in LIS, BIS and BB and rotary motion in RIS.     

Numerical study on tidal currents and seawater exchange in the Benoa Bay, Bali, Indonesia

A three-dimensional （3-D） finite volume coastal ocean model （FVCOM） was used for the study of water cir culation and seawater exchange in the Benoa Bay, Bali Island. The M2 tidal component was forced in open boundary and discharge from six rivers was included in the numerical calculation. The M2 tidal elevation produced by the FVCOM has a good agreement with the observation data. The M2 tidal current is also suc cessfully calculated under the ebb tide and flood tide conditions. The non-linear M2 tidal residual current was produced by the coastline geometry, especially surrounding the narrow strait between the Serangan Is- land and the Benoa Peninsula. The tidal residual current also generated two small eddies within the bay and one small eddy in the bay mouth. The salinity distribution influenced by river discharge could be success- fully calculated, where the numerical calculation and the observation results have a good correlation （r2） of 0.75. Finally in order to examine the seawater exchange in the Benoa Bay, the Lagrangian particle tracking method and calculation of residence time are applied. The mechanism of particle transport to the flushing of seawater is depicted clearly by both methods.     

Mussel reefs on soft sediments: a severely reduced but important habitat for macroinvertebrates and fishes in New Zealand

Green-lipped mussels (Perna canaliculus) formed extensive reefs on soft sediments in sheltered embayments around northern New Zealand until overfishing and/or increased sediment input caused their virtual disappearance by 1980. To determine the role of mussel reefs as habitat for other animals, we located remnant soft-sediment reefs in five locations and compared the density, biomass, productivity and composition of mobile macroinvertebrate communities, and the density of small fishes associated with mussels, with fauna in the surrounding soft sediments. The mussel reefs had a distinct assemblage of macroinvertebrates, which had 3.5 times the density, 3.4 times the biomass and 3.5 times the productivity of surrounding areas. The density of small fishes was 13.7 times higher than in surrounding areas. These results show that soft-sediment mussel reefs support an abundant and productive fauna, highlighting the probable large loss of productivity associated with the historical decline in mussel habitat and the consequent desirability of restoration efforts.     

Organic geochemistry of sediments from the continental margin off southern New England,U.S.A. — part I. Amino acids,carbohydrates and lignin

Total organic carbon (TOC), lignin, amino acids, sugars and amino sugars were measured in recent sediments from the continental margin off southern New England. The various organic carbon fractions decreased in concentration with increasing distance from shore. The fraction of the TOC that was accounted for by these major components also decreased with increasing distance from shore. The concentration of lignin indicated that only about 3–5% of the organic carbon in the nearshore sediment was of terrestrial origin. The various fractions were highly correlated, which was consistent with a simple linear mixing model of shelf organic matter with material from the slope and rise and indicated a significant transport of sediment from the continental shelf to the continental slope and rise.     

潜式与离岸式人工岬头护滩作用和水交换特征比较

潜式人工岬头和离岸式人工岬头是两种改良式人工岬头,与传统人工岬头相比,它们对岸滩的保护效果稍弱,但能改善岬湾内的水交换。以北戴河西海滩养滩工程为研究对象,采用数值模拟的方法,对潜式岬头和离岸式岬头的护滩效果和改善水交换效果进行研究比较。首先建立岸线演变模型和物质输运模型,对工程后的岸线演变及水交换情况进行模拟。岸线演变的模拟结果表明,潜式岬头和离岸式岬头的护滩效果均较好,但潜式岬头后的岸线形状相比离岸式岬头后更为平滑。同时讨论海滩的侵蚀和淤积特性对潜式岬头和离岸式岬头的影响以及潮汐条件对冲淡时间的影响。与传统岬头相比,潜式岬头和离岸式岬头可以很好地减少岬后冲淡时间以及改善岬湾内水交换,而护滩效果只是稍有减弱。因此,潜式岬头和离岸式岬头都是改善岬湾水交换的有效工程措施。     

Tidal flushing time of marine fish culture zones in Hong Kong

Accurate determination of flushing time is crucial for maintaining sustainable production in fish culture zones(FCZs),as it represents the physical self-purification capability via tidal exchange with clean water in the outer sea.However,owing to the temporal and spatial complexity of the coastal flushing process,existing methods for determining flushing time may not be generally applicable.In this paper,a systematic method for determining the flushing time in FCZs is presented,in which bathymetry,runoff,tidal range and stratification are properly accounted for.We determine the flushing time via numerical tracer experiments,using robust 3D hydrodynamic and mass transport models.For FCZs located in sheltered and land-locked tidal inlets,the system boundary can be naturally defined at the connection with the open sea.For FCZs located in open waters,hydrodynamic tracking is first used to assess the extent of tidal excursion and thus delimit the initial boundary between clean water and polluted water.This general method is applied to all designated marine FCZs in Hong Kong for both the dry and wet seasons,including 20 sheltered FCZs(in semi-enclosed waters of Tolo Harbour,Mirs Bay,and Port Shelter) and 6 FCZs in open waters.Our results show that flushing time is the longest in inner Port Shelter(about 40 days in dry season),and the shortest for the FCZs in open waters(less than one week in dry season).In addition,the flushing time in dry season is commonly longer than that in wet season:20%~40% for most well-sheltered FCZs;2.6~4 times for the others.Our results indicate a positive correlation between the flushing time and distance to open boundary,supporting the view that the flushing time of a FCZ is closely related to its location.This study provides a solid basis for mariculture management such as the determination of carrying capacity of FCZs.     

Modelling and observations of tidal wave propagation, circulation and residence times in Puttalam Lagoon, Sri Lanka

Tidal measurements and a depth-averaged 2D model are used to examine wave progression and circulation in a long, shallow, micro-tidal lagoon in Sri Lanka. Ranges and phase lags for different tidal constituents are used to calibrate the model. A single drag coefficient, C_d = 0.0032, gives almost perfect agreement with data. Current measurements are used for validation of the model. The lagoon tide consists of a combination of progressive and standing waves, where progressive waves dominate in the outer part and standing waves in the inner. A Lagrangian based particle-tracking method is developed to study tidally and wind induced residence times. If tides were the only factor affecting the residual circulation, the residence time inside the narrowest section would be approximately 100 days. Steady winds (of typical monsoon average) decrease the residence times to 60–90 days. Estuarine forcing due to net freshwater supply is not modelled (due to lack of reliable runoff data), but independent, long-term salinity observations and calculations based on volume and salt conservation during periods of negligible freshwater supply (the lagoon is seasonally hypersaline) indicate residence times ranging from 40 to 80 days. Model derived residence times based on tides alone represent a minimum exchange. Even weak forcing, through winds, excess evaporation or freshwater supply efficiently reduces residence times.     

Net Sand Transport Direction in a Tidal Inlet, using Foraminiferal Tests as Natural Tracers

The relationship between the flux of exotic benthic foraminiferal tests (i.e. tests which are supplied from open-sea sources alone) in a tidal inlet and that of bulk sediment was analysed, which can be expressed as two first-order linear equations. According to this relationship, in order to determine net sediment transport directions in the entrance, the test concentration in surficial sediments of the tidal basin can be compared against a ‘ critical level ’. The critical level is determined for the conditions that no net transport of bulk sediment is present within the entrance. If the observed concentration (averaged over the tidal basin) is higher than the simulated critical level, then the net sediment transport is directed to landward. This method is applied to the analysis of net sand transport at Christchurch Harbour, a tidal inlet system located in southern England. In this investigation, concentrations of exotic foraminiferal tests in the surficial sediments of the tidal basin and ebb tidal delta area were obtained from the analysis of sea-bed sediment samples. A series of probable critical levels were calculated based upon the data sets with regard to: (1) sediment discharge from the rivers; (2) magnitude of sediment discharge within the entrance during the ebb; (3) the test concentration outside the harbour; (4) the thickness of the moving layer; and (5) two parameters associated with dispersive processes. The results show that the concentration in the tidal basin sediment is higher than a number of simulated critical concentrations for representative cases. Consequently, the high level of the concentration of exotic benthic foraminiferal tests within the harbour should be explained as a result of landward net transport of sands within the entrance.