Tidally phased emergence events in a strongly tidal estuary

Acoustic backscatter from an active sonar system over a range of six frequencies between 265 kHz and 3 MHz in the tidally dominated Damariscotta River estuary, Maine, United States, revealed that the major emergence event of the night commenced on the first tidal deceleration after dark (3.5–4 h after local slack), irrespective of flow direction. Emergence traps identified the mysid shrimp,Neomysis americana, as the dominant migrator. Water-column-integrated, acoustically estimated biovolume at our 10-m deep study location increased by a factor of about 6 during these large events, entirely dominating the holoplanktonic contribution and likely being a major component in benthic-pelagic coupling. Application of the same algorithm used to locate this nighttime emergence revealed a parallel but considerably smaller daytime emergence event near the same phase of the tide. Daytime trap samples failed to recover the organisms responsible, but transmissometry rejected the alternative hypothesis that we observed resuspension events. We suspect, but have yet only weak evidence, that animals emerging in daylight are copepods rather than mysids.     

Spring-neap tidal contrasts and nutrient dynamics in a marsh-dominated estuary

This contribution presents a new perspective on water chemistry and its relation to tidal hydrology in marsh-dominated estuaries. Results are derived from both field and modeling experiments. A heuristic model based on a tidally-averaged advection-dispersion equation is used in conjunction with source-sink terms (for benthic, marsh surface, and open-water exchanges) to make predictions of nutrient concentrations in the water column. Spring-neap tidal contrasts are associated with significant changes in water-column chemistry for a variety of nutrients sampled during the growing season in the Parker River estuary (Massachusetts). For ammonium, phosphate, nitrate plus nitrite, total dissolved N, and total dissolved P, concentrations are significantly lower during spring tides (marshes flooded) than during neap tides (marshes unflooded). Model results indicate that physical changes and open-water processing are insufficient to produce the observed effect, and that explicit biogeochemical processing on marsh surfaces is required. Field observations of changes in nutrient to nutrient ratios with the onset of marsh inundation also support this conclusion. As tides progress from the neap to spring condition, a “spectrum” of trajectories emerges in salinity-nutrient plots developed from both observational datasets and model output. Care must therefore be exercised in designing sampling programs for water chemistry in marsh-dominated ecosystems and in interpreting the resulting mixing diagrams.     

Seasonal variation in food web composition and structure in a temperate tidal estuary

Seasonal variation in aquatic food web structure at Mad Island Marsh, Matagorda Bay, Texas, was examined using dietary information obtained from the analysis of gut contents from large samples of fish and crustacean specimens. Unique aspects of this study include the use of large samples of consumer gut contents (n=6,452), long-term sampling (bimonthly surveys over 18 mo), and standard methods of data collection and analysis facilitating comparisons with other aquatic food webs. Dietary data were partitioned for analysis into warm (summer) and cold (winter) seasons. Most consumers fed low in the food web, with trophic levels ranging from about 2 to 3.5 during both summer and winter. Vegetative detritus was more important in macroconsumer diets than live algae and macrophytes. Low trophic levels of consumers reflected the important role of abundant detritivores (e.g., striped mulletMugil cephalus, Gulf menhadenBrevortia patronum, and macroinvertebrates) in linking detritus to top predators via short food chains, a finding consistent with many other estuarine food web studies. Despite changes in community composition and population size structure of certain species, most food web properties revealed comparatively little seasonal variation. The summer food web had more nodes (86), more links (562), a higher density of links as indicated by connectance (0.08), and a slightly higher predator: prey ratio (0.51) compared to the winter food web (75 nodes, 394 links, connectance = 0.07, predator: prey ratio = 0.47). Proportions of top (0.06–0.07), intermediate (0.75–0.76), and basal (0.19) species did not vary significantly between seasons, but mean trophic level was higher during summer. Addition of feeding links based on information from the literature increased connectance to 0.13 during both seasons; other web parameters had values similar to those obtained for our directly estimated food webs. Seasonal variation in food web structure was influenced by changes in community composition (e.g., influxes of postlarval estuarine-dependent marine fishes during winter), availability of resources (e.g., more submerged macrophytes and amphipods during summer), and size structure and ontogenetic diet shifts of dominant consumer taxa. Our findings suggest that some basic properties of estuarine food web are resilient to seasonal changes in population and community structures and food web architecture.     

Sedimentation cycle of a freshwater tidal flat in the St. Lawrence estuary

Cap-Tourmente tidal flat is located on the north shore of the St. Lawrence middle estuary, 50 km downstream from Quebec city, Canada. Seasonal variations in sedient accretion were studied during the period without ice from May to December, using horizontal plates in conjunction with vertical stakes. The sedimentological regime which is characterized by very rapid changes, can be divided into four periods. There are two periods of intense erosion in May and October–November; during these months, erosion is initiated by walking and beak probing of very large flocks of snow geese. During the summer, very rapid accretion occurs for approximately 100 days; during this period, the sedimentation rate is 2 mm per day in the lower section of the tidal marsh which is covered with vegetation. Among the numerous factors influencing the sedimentation cycle, the very gradual slope of the upper part of the flat (0.25%) seems to be one of the most important because it results in a very large area (550 m wide) which is protected from strong tidal currents. This latter area is covered with dense and high vegetation which provides an effective protection against erosion unitl the snow geese arrive in October and raze it in a few days. The presence of vegetation reduces the velocities of tidal currents, particularly during ebb and modifies their direction for both flood and ebb, orienting them perpendicularly to the main tidal flow.     

Variation in surface turbulence and the gas transfer velocity over a tidal cycle in a macro-tidal estuary

The gradient flux technique, which measures the gas transfer velocity (k), and new observational techniques that probe turbulence in the aqueous surface boundary layers were conducted over a tidal cycle in the Plum Island Sound, Massachusetts. Efforts were aimed at testing new methods in an estuarine system and to determine if turbulence created by tidal velocity can be responsible for the short-term variability ink. Measurements were made during a low wind day, at a site with tidal excursions of 2.7 m and a range in tidal velocity of nearly 1 m s⁻¹. Estimates ofk using the gradient flux technique were made simultaneously with the Controlled Flux Technique (CFT), infrared imagery, and high-resolution turbulence measurements, which measure the surface renewal rate, turbulent scales, and the turbulent dissipation rate, respectively. All measurements were conducted from a small mobile catamaran that minimizes air- and water-side flow distortions. Infrared imagery showed considerable variability in the turbulent scales that affect air-water gas exchange. These measurements were consistent with variation in the surface renewal rate (range 0.02 to 2 s⁻¹), the turbulent dissipation rate (range 10⁻⁷ to 10⁻⁵ W kg⁻¹), andk (range 2.2 to 12.0 cm hr⁻¹). During this low wind day, all variables were shown to correlate with tidal speed. Taken collectively our results indicate the promise of these methods for determining short-term variability in gas transfer and near surface turbulence in estuaries and demonstrate that turbulent transport associated with tidal velocity is a potentially important factor with respect to gas exchange in coastal systems.     

Modeling the transport and distribution of lead in tidal Keelung River estuary

A vertical, two-dimensional heavy metal (lead) transport model incorporated into the hydrodynamic, salt, and sediment transport modules was developed to simulate the lead concentration in the tidal Keelung River estuary of northern Taiwan. We validated the developed model with measured data, including longitudinal velocity, salinity, suspended sediment, and heavy metal (lead) concentration, obtained in 1998. An exponential relationship relating the salinity and suspended-sediment concentrations was established to calculate the partition coefficient of lead in the estuary. The simulated results of dissolved, particulate, and total lead concentrations agreed well with the measured data. A model sensitivity analysis indicated that the partition coefficient plays an important role in the distribution of dissolved and particulate lead concentrations along the tidal Keelung River estuary.     

Mercury sources and transformations in a man-perturbed tidal estuary: The Sinnamary Estuary, French Guiana

The distribution, partition and speciation of mercury (Hg) were studied along the redox gradient of an anthropogenically perturbed tropical estuary, the Sinnamary Estuary in French Guiana. This system is a partially mixed estuary characterized by an anoxic freshwater end-member, while the marine end-member consists of the Amazon Plume.The set up of an artificial oxygenation system in the anoxic freshwater end-member generates sharp gradients of major chemical species (iron, sulfides, etc.) coupled with intense organic matter (OM) turnover. The coexistence of oxygenated waters and dissolved sulfides in an organic rich environment depicts the Upper Sinnamary Estuary (USE: part of Sinnamary Estuary under the tidal influence but upstream of the salt intrusion) as a potential site for Hg methylation. The concentrations of all mercury compounds (HgT) in the unfiltered samples (HgT_UNF), in the dissolved (HgT_D) and particulate (HgT_P) phases of the USE average 11 ± 3, 6 ± 2 and 5 ± 3 (i.e. 600 ± 200 pmol g⁻¹) pmol L⁻¹, respectively. Average concentrations of monomethylmercury (MMHg) in the unfiltered (MMHg_UNF), dissolved (MMHg_D) and particulate (MMHg_P) phases were 3.7 ± 1.0, 2.0 ± 0.9 and 1.8 ± 1.2 (i.e. 220 ± 130 pmol g⁻¹) pmol L⁻¹, respectively. Water oxygenation and sulfides concentrations emerged to play a critical role in controlling MMHg levels. Additionally, iron cycling, acid-base mechanisms, and redox-dependent processes were involved in the MMHg partitioning between phases.Overall, the USE constitutes a biogeochemical reactor that gathers partitioning and methylation processes. The permanent MMHg inputs from the anoxic freshwater end-member combined with the intense endogenous Hg methylation ensures high-MMHg levels in both dissolved and particulate phases. To illustrate, the USE exports 60 ± 20% more MMHg_UNF than it imports: 5.5 ± 0.7 vs. 3.5 ± 1.2 kg year⁻¹.     

Light attenuation and submersed macrophyte distribution in the tidal Potomac River and estuary

Changing light availability may be responsible for the discontinuous distribution of submersed aquatic macrophytes in the freshwater tidal Potomac River. During the 1985–1986 growing seasons, light attenuation and chlorophylla and suspended particulate material concentrations were measured in an unvegetated reach (B) and in two adjacent vegetated reaches (A and C). Light attenuation in reach B (the lower, fresh to oligohaline tidal river) was greater than that in reach A (the recently revegetated, upper, freshwater tidal river) in both years. Reach B light attenuation was greater than that in reach C (the vegetated, oligohaline to mesohaline transition zone of the Potomac Estuary) in 1985 and similar to that in reach C in 1986. In reach B, 5% of total below-surface light penetrated only an average of 1.3 m in 1985 and 1.0m in 1986, compared with 1.9 m and 1.4 m in reach A in 1985 and 1986, respectively. Water column chlorophylla concentration controlled light availability in reaches A and B in 1985, whereas both chlorophylla and suspended particulate material concentrations were highly correlated with attenuation in both reaches in 1986. Reach C light attenuation was correlated with suspended particulate material in 1986. The relationship between attenuation coefficient and Secchi depth was K_PAR=1.38/Secchi depth. The spectral distribution of light at 1 m was shifted toward the red portion of the visible spectrum compared to surface light. Blue light was virtually absent at 1.0 m in reach B during July and August 1986. Tidal range is probably an important factor in determining light availability for submersed macrophyte propagule survival at the sediment-water interface in this shallow turbid system.     

Metals in tidal flats colonized by microbial mats within a South-American estuary (Argentina)

In this study, we measured the concentrations of metals (Cd, Cu, Pb, Ni, Cr, Zn, Hg, Mn, and Fe) and assessed the characteristics of tidal flats (grain size and organic matter content) in sediments and their overlying microbial mats fractions to evaluate the anthropogenic impact within the Bahía Blanca Estuary (BBE). Puerto Rosales (PR) and Almirante Brown (AB), located in the middle and inner zone of the estuary, respectively, were used as sampling sites. Sediments were composed mainly of silt–clay in AB, whereas first fine-grained particles were coarser in depth in PR. Regarding the concentration of metals in both fractions, we found differences between sites: There were higher concentrations of overall metals in AB relative to PR. In addition, higher concentrations of Cu were recorded in the first centimeters of AB tidal flats, whereas higher concentration of Cd were recorded in microbial mats of PR. Considering that the grain size was similar between sites, these results are consistent with the high concentration of organic matter found in AB, probably because this site is close to a former municipal dump and sewage discharges. Also, the higher Cd content found in PR site would highlight both the influence of untreated urban discharges and port anthropogenic activities. In conclusion, this study allowed identifying high values of some metals in the presence of microbial mats in the BBE, thus suggesting a possible interaction between both, at least for metals like Cu or Cd.     

Water column light attenuation estimation to simulate phytoplankton population in tidal estuary

The penetration of sunlight into the water column plays a critical role in the aquatic ecosystem. The irradiance available for primary production in a water body depends on the incident light at the water surface, light extinction in the water column, and depth. In this study, the light attenuation through the water column of the Danshuei River–Keelung River estuary was estimated. The measurement of photosynthetically active radiation (PAR) indicates that the conventional exponential attenuation of light with depth is a very good model. A light attenuation coefficient may be derived from the PAR measurements at each location. The regression with salinity yields a good correlation, indicating that the fraction of seawater should be a good parameter for estimating the water column light attenuation coefficient (K _d ). A laterally averaged two-dimensional finite difference model for hydrodynamic and water quality model was performed and applied to simulate the phytoplankton population at the lower reach of the Danshuei River estuary. In the process of phytoplankton population simulation, the regression model of K _d and salinity was incorporated in the water quality model. The simulated results show that the modeled concentration of chlorophyll a matched the measured values at the lower reach of the Danshuei River estuary.     

Seasonal and tidal monthly patterns of particulate matter dynamics in the Columbia River estuary

We investigated seasonal and tidal-monthly, suspended particulate matter (SPM) dynamics in the Columbia River estuary from May to December 1997 using acoustic backscatter (ABS) and velocity data from four long-term Acoustic Doppler Profiler (ADP) moorings in or near the estuarine turbidity maximum (ETM). ABS profiles were calibrated and converted to total SPM profiles using pumped SPM samples and optical backscatter (OBS) data obtained during three seasonal cruises. Four characteristic settling velocity (W _s) classes were defined from Owen Tube samples collected during the cruises. An inverse analysis, in the form of a non-negative least squares minimization, was used to determine the contribution of the four,W _s-classes to each, total SPM profile. The outputs from the inverse analyses were 6–8 mo time-series ofW _s-specific SPM concentration and transport profiles at each mooring. The profiles extended from the free surface to 1.8–2.7 m from the bed, with 0.25–0.50 m resolution. These time series, along with Owen Tube results and disaggregated size data, were used to investigate SPM dynamics. Three non-dimensional parameters were defined to investigate how river flow and tidal forcing affect particle trapping: Rouse numberP (balance between vertical mixing and settling) trapping efficiencyE (ratio of maximum SPM concentration in the estuary to fluvial source concentration), and advection numberA (ratio of height of maximum SPM concentration to friction velocity). The most effective particle trapping (maximum values ofE) occurs on low-flow neap tides. The location of the ETM and the maximal trapping migrated seasonally in a manner consistent with the increase in salinity intrusion length after the spring freshet. Maximum advection (high values ofA) occurred during highly stratified neap tides.     

钱塘江河口大规模治江围涂后对涌潮高度的影响

根据钱塘江河口治江围涂前和大规模治江围涂后现场实测的涌潮高度数据,对盐官以下河段大潮期涌潮高度的变化进行分析,结果表明,大规模治江围涂后的涌潮最高河段已由治江围涂前的新湾下移至大缺口段,并且大规模治江围涂后的涌潮最高值比治江围涂前增高了0.4 m,盐官河段增高0.1 m.盐官以上河段治江围涂后的涌潮高度变化,主要与径流对沙坎顶的冲淤密切相关,在其它条件相同时,治江围涂后的涌潮高度有所增大.这一结果与某些年份因涌潮增高,标准堤大量溢流等实例基本相符.     

Transport in the Hudson estuary: A modeling study of estuarine circulation and tidal trapping

The effects of estuarine circulation and tidal trapping on transport in the Hudson estuary were investigated by a large-scale, high-resolution numerical model simulation of a tracer release. The modeled and measured longitudinal profiles of surface tracer concentrations (plumes) differ from the ideal Gaussian shape in two ways: on a large scale the plume is asymmetric with the downstream end stretching out farther, and small-scale (1–2 km) peaks are present at the upstream and downstream ends of the plume. A number of diagnostic model simulations (e.g., remove freshwater flow) were performed to understand the processes responsible for these features. These simulations show that the large-scale asymmetry is related to salinity. The salt causes an estuarine circulation that decreases vertical mixing (vertical density gradient), increases longitudinal dispersion (increased vertical and lateral gradients in longitudinal velocities), and increases net downstream velocities in the surface layer. Since salinity intrusion is confined to the downstream end of the tracer plume, only that part of the plume is effected by those processes, which leads to the largescale asymmetry. The small-scale peaks are due to tidal trapping. Small embayments along the estuary trap water and tracer as the plume passes by in the main channel. When the plume in the main channel has passed, the tracer is released back to the main channel, causing a secondary peak in the longitudinal profile.     

Abandoned channel fill sequences in the tidal estuary of a small mountainous,dry‐summer river

This study proposes a modification of the current model for abandoned channel fill stratigraphy produced in unidirectional flow river reaches to incorporate seasonal tidal deposition. Evidence supporting this concept came from a study of two consecutive channel abandonment sequences in Ropers Slough of the lower Eel River Estuary in northern California. Aerial photographs showed that Ropers Slough was abandoned around 1943, reoccupied after the 1964 flood, and abandoned again in 1974 with fill continuing to the present. Planform geomorphic characteristics derived from these images were used in conjunction with sub‐centimetre resolution stratigraphic analyses to describe depositional processes and their resultant sedimentary deposits. Both abandonment sequences recorded quasi‐annual scale fluvial/tidal deposition couplets. In both cases, tidal deposits contained very little sand, were higher in organic and inorganic carbon content than the sandier, fluvially dominated deposits, and possessed millimetre‐scale horizontal laminations. The two abandonment fills differed significantly in terms of the temporal progression of channel narrowing and fluvial sediment deposition characteristics. Aerial photographic analysis showed that the first abandonment sequence led to a more rapid narrowing of Ropers Slough and produced deposits with a positive relationship between grain size/deposit thickness and discharge. The second abandonment resulted in a much slower narrowing of Ropers Slough and generally thinner fluvial deposits with no clear relationship between grain size/deposit thickness and discharge. The δ¹³C values and organic nitrogen to organic carbon ratios of deposits from the first phase overlapped with Eel River suspended sediment characteristics found for low flows (one to five times mean discharge), while those of the second phase were consistent with suspended sediment from higher flows (seven to ten times mean discharge). When considered together, the results indicate that the early fill sequence recorded a reach experiencing regular fluvial deposition through flow conditions during the wet season, while the latter fill sequence records a reach more disconnected from the main stem in terms of flow and sediment. The major factor affecting the difference in sedimentation between the two fill periods appears to have been the morphology of the upstream river bend in relation to the position of the bifurcation node. During the first fill period, the upstream entrance to Ropers Slough seems to have remained open, in part due to the placement of its entrance on the outside of the mainstem river bend, and despite stronger tidal effects caused by a larger tidal prism and closer proximity to the tidal inlet. By the second fill sequence, the upstream bend morphology had altered, placing the entrance to Ropers Slough on the inner bank of the mainstem bend, which resulted in more rapid plug bar formation. The role of tidal effects in the geomorphic trajectory of the two abandonment sequences is unclear, but appears to have been less important than local bifurcation geometry.     

Flux of particulate matter in the tidal marshes and subtidal shallows of the Rhode River estuary

There was a net influx of suspended particulate matter to the uppermost part of the Rhode River estuary during the several years of this study. Most of the influx was due to episodic discharges of suspended sediment from the watershed during heavy rains. In contrast, tidal exchange of particulate matter was not related to rainstorms. Sediment composition data and historical records indicate that marsh accretion accounts for only 13% of the sediment trapping although marshes occupy 60% of the study area. Influx of particulate matter to the marshes is directly related to the amount of time they are submerged during tidal cycles.     

The paleoecological development of a late holocene,tidal freshwater marsh of the Upper Delaware River estuary

Changes in groundwater tables brought about by sea level increases in the Delaware River Basin (near Philadelphia) about 2,500 years B.P., initiated wetland development at the Princeton-Jefferson Branch of the Woodbury Creek marshes. Continual increases in sea level pushed groundwater tables further upward, and by approximately 800 years B.P., groundwater tables had risen to the upper limits for woody vegetation at the site. By the time European settlers arrived in the late 1600s nontidal sedge marshes dominated the site. Upon arriving colonists began manipulating the hydrology of the Delaware River Basin by constructing dams and dikes for flood control. Soon many areas were cut off from direct contact with the river. During the next one and one-half centuries sea level continued to rise, and because of channelization of the Delaware River the tidal range doubled. During the early 1900s flood control structures began to fail allowing tidal waters to periodically inundate these protected sites. At that time the site was dominated by a Quercus-Castanea swamp forest with hummocks of Cyperaceae interspersed throughout. In 1940 the dike surrounding the Princeton-Jefferson marsh collapsed and the site was immediately inundated with tidal waters on a regular basis. Within a short period of time tidal freshwater marsh developed and has continued to the present day. It is clear from this investigation that changes in hydrology brought about by cultural modifications have been directly responsible for the ontogeny of this tidal marsh. The influence cultural impacts have had on wetland development at the Princeton-Jefferson marsh suggest that it may be necessary to reevaluate the extent humans have modified the development and structure of the present day upper Delaware River estuary. Although the ability to discern historic vegetation zonation patterns is limited, these marshes can record individual events that have shaped these wetlands through time. Due to differences in the structure of the plant community, rates of decomposition, and processes of accretion, Redfield’s model (1972) of tidal salt marsh development does not apply to the Princeton-Jefferson marsh. Along a submerging coast, the development of tidal freshwater marsh in many estuaries may be necessary for the establishment of brackish and salt marshes by creating and maintaining a suitable habitat for the eventual colonization of more salt-tolerant plant species. The roles these wetlands have played in the development of the estuaries has been underestimated in the past.     

Evaluation of the influence of urbanization processes using mangrove and fecal markers in recent organic matter in a tropical tidal flat estuary

Three sediment cores (50 cm depth) were collected at three different sites from a tidal flat estuary at Passagem Channel (Vitória, Espírito Santo State-Brazil) to evaluate the influence of recent urbanization processes on the deposition of organic matter (OM) in a complex polluted tropical estuary. In addition to geochronology (by excess ²¹⁰Pb), the sources of natural and anthropogenic OM to the sediments were evaluated by total organic C (TOC – 14.29 ± 8.73, 30.43 ± 14.71 and 48.70 mg g⁻¹ ± 25.46, respectively, for P1, P2 and P3), C/N molar ratio and lipid biomarkers (sterols and terpenoids). Taraxerol (3.10 ± 4.85, 9.71 ± 3.85 and 16.10 mg gTOC⁻¹ ± 32.48 for P1, P2 and P3, respectively) and sitosterol (1.71 ± 2.72, 2.94 ± 6.41 and 4.07 mg gTOC⁻¹ ± 4.41 for P1, P2 and P3, respectively) were the most abundant compounds in all cores, suggesting a major contribution of terrestrially-derived OM to the study region. Coprostanol levels and selected sterol index indicated significant contamination by fecal material. The organic geochemical indicators suggest that changes of OM reflect occupation and urbanization alteration processes around the Passagem Channel over the last 70 a, mainly the conversion of mangrove forest into urban areas, bridge building and Treatment Plant Station installation.     

伴随数据同化法反演涌潮河口开边界

构造了非线性守恒型浅水方程的伴随方程,并给出无结构三角网格的开边界校正表达式,利用Godunov格式和Riemann间断解Roe通量格式的设计思想,建立了无结构三角网格有限体积法的伴随开边界反演模型。进行了概化涌潮河口和钱塘江涌潮河口M2分潮开边界反演的数值实验,经过同化得到开边界上振幅的平均误差分别为0.000 6 m和0.053 3 m,证实了本文构造有限体积伴随模型的可行性,也表明本模型能够适应间断解问题。