Roger Mann, Daphne M. Munroe, Eric N. Powell, Eileen E. Hoffmann, and John M. Klinck
Bivalve mollusks store a complete history of their life in the growth lines in their valves. Through sclerochronology, in combination with isotope signatures, it is possible to reconstruct both post-recruitment growth history at the individual level and commensurate environmental records of temperature and salinity. Growth patterns are integrators of local primary productivity; spatial and temporal changes in growth illustrate commensurate patterns of food availability. Mactrid clams are long-lived, benthic dominant species found on inner continental shelves throughout the Northern Hemisphere where they variously support major fisheries (Spisula solidissima in the Mid-Atlantic Bight, Mactromeris polynyma in eastern Canada, Spisula sachalinensis in Japan) and recreational fisheries (Mactromeris polynyma in Alaska), and serve as dietary items for charismatic species such as bearded seals (Erignathus barbatus) and walrus Odobenus rosmarus divergens). Ongoing studies, employing sophisticated adult growth and larval dispersal models of the response of Spisula solidissima to climate change in the Mid-Atlantic Bight, suggest the general use of mactrids as barometers of climate change over broader geographic footprints. Mactromeris polynyma is a candidate species for shallow arctic marine systems, having a pan-arctic distribution from the Gulf of Maine in the Atlantic to the Bering Sea and Gulf of Alaska in the northern Pacific. The longevity of extant individuals (≤25 years) provides opportunity for detailed reconstruction of the benthic environment and food regimes at the decadal level.
Ryan W. Schloesser, Mary C. Fabrizio, Robert J. Latour, Greg C. Garman, Bob Greenlee, Mary Groves, and James Gartland
Rapid increase in abundance and expanded distribution of introduced blue catfish Ictalurus furcatus populations in the Chesapeake Bay watershed have raised regional management concerns. This study uses information from multiple surveys to examine expansion of blue catfish populations and document their role in tidal river communities. Originally stocked in the James, York, and Rappahannock River systems for development of commercial and recreational fisheries, blue catfish have now been documented in adjacent rivers and have expanded their within-river distribution to oligo- and mesohaline environments. Range expansions coincided with periods of peak abundance in 1996 and 2003 and with the concurrent decline in abundance of native white catfish I. catus. Blue catfish in these systems use a diverse prey base; various amphipod species typically dominate the diet of smaller individuals ([FL]), and fishes are common prey for larger blue catfish (>300 mm FL). Recent studies based on stable isotope analyses suggest that adult blue catfish in these systems are apex predators that feed extensively on important fishery resources, including anadromous shads and herrings Alosa spp. and juvenile Atlantic menhaden Brevoortia tyrannus. Minimizing effects on Chesapeake Bay communities by controlling high densities of blue catfish populations is a primary goal of management, but conflicting demands of the commercial and recreational sectors must be resolved. Further, low market demand and human consumption concerns associated with purported accumulation of contaminants in blue catfish pose additional complications for regulating these fisheries.
Six Fish and 600,000 Thirsty Folks—A Fishing Moratorium on American Shad Thwarts a Controversial Municipal Reservoir Project in Virginia, USA
J. E. Olney, D. M. Bilkovic, C. H. Hershner, L. M. Varnell, H. Wang, and R. L. Mann
Moratoria on fishing directly impact fishers, distributors and marketers of product and can have serious socio-economic implications. Moratoria can impact communities but usually populations closely linked to the banned activity. In an unprecedented example, a moratorium on fishing in Virginia has directly impacted a nonfishing citizenry by thwarting plans for a public utility. In May 2003, a panel empowered to regulate marine resources denied permission to withdraw raw water from a pristine freshwater river, the Mattaponi. The controversial action spoiled a multi-million dollar plan to establish the King William Reservoir, a water source considered essential to future growth and development in the region. The facility was designed to serve a projected 600,000 people in 2040 but the Mattaponi Indians, environmentalists, local citizens and commercial fishers opposed the plan. A central issue was conservation of American shad Alosa sapidissima, an anadromous clupeid native to the U.S. east coast. An inriver moratorium on fishing for American shad imposed in 1994 remains in effect. In the reservoir debate, scientists advised the panel that the project would withdraw water in the center of the larval nursery area for this species and in a river that accounted for the highest statewide production of juveniles. Scientists recommended relocating the intake since losses of larvae to withdrawal could be counter to restoration goals of the moratorium. Using quantitative models, municipal authorities argued that only six American shad would be lost annually to impingement or entrainment. The panel rejected this argument and proposals to mitigate losses.
J. D. Boon
Tidal conditions fail to explain a paradoxical similarity in water level extremes induced by Hurricane Isabel on 18 September 2003, and the 23 August 1933 storm of record at Hampton Roads, Virginia. Storm surge peaks occurred near astronomical high tide during both storms, but Isabel arrived during neap tides while tides during the 1933 storm were nearer to spring. In addition, Isabel produced a lesser storm surge, yet she yielded a storm tide, or high-water mark, roughly equal to that of the 1933 hurricane. The answer to the paradox lies in observed sea level—water level measured relative to the land—and its movement during the 70 years between these events. Water level analysis shows that the sea level change observed can be divided into three categories at three different time scales: daily (astronomical tides), monthly (seasonal change), and yearly (secular trend in sea level). At Hampton Roads, a secular rise rate of 4.25 mm⋅yr-1 (1.39 ft/century) predicted an increase of 29.8 cm in 70 years; mean sea level for the month of September stood an additional 21.9 cm above the annual mean for 2003. These numbers are comparable to the mean semirange of tide (37.0 cm) at Hampton Roads. Thus seasonal and secular change are both factors of key importance in evaluating storm tide risk at time scales attributable to major hurricanes (100 years). Adoption of a new vertical reference, projected monthly mean sea level, is proposed to facilitate their inclusion in storm tide predictions at decadal time scales.
L. H. Brasseur, A. C. Trembanis, J. M. Brubaker, and C. T. Friedrichs
After making landfall on the North Carolina coast on the morning of 18 September 2003, Category 2 Hurricane Isabel tracked northward parallel to and slightly west of the Chesapeake Bay. At Gloucester Point, near the mouth of the York River estuary, strong onshore winds with speeds in excess of 20 m⋅s-1 persisted for over 12 hours and peak winds reached over 40 m⋅s-1, causing a sustained up-estuary wind stress. Storm surge exceeded 2 m throughout most of the lower Chesapeake Bay. A 600 kHz acoustic Doppler current profiler (ADCP), deployed at a depth of 8.5 m off Gloucester Point, provided high-quality data on waves, storm surge, currents, and acoustic backscatter throughout the water column before, during, and after the storm. Pressure and salinity sensors at three additional sites further up the estuary provided information on water surface slope and saltwater excursion up the estuary. A first-order estimate of three terms of the along-channel momentum equation (barotropic pressure gradient, acceleration, and friction) showed that the pressure gradient appeared to be balanced by the wind stress and the acceleration during the storm. The storm’s path and slow speed were the primary causes of the extremely high storm surge relative to past storms in the area.
An Unprecedented Scientific Community Response to an Unprecedented Event: Tropical Storm Agnes and the Chesapeake Bay
M. P. Lynch
In June 1972, the remnants of Hurricane Agnes brought destructive floods to the watershed of the Chesapeake Bay basin. Unlike Hurricane Isabel, Agnes did not strike Chesapeake Bay directly, but deposited a record amount of rainfall on the watershed. The evening that the Agnes rainfall began in earnest coincided with a meeting of the Citizens Program for the Chesapeake Bay. The directors of the three largest Chesapeake Bay research institutions, Drs. Donald W. Pritchard, L. Eugene Cronin, and William J. Hargis Jr., were in attendance at this meeting. The potential magnitude of the Agnes rainfall was readily apparent at the meeting as one of the planned evening events had to be moved due to a foot of water in the meeting room. The following morning at breakfast, the three directors committed their institutions to “Operation Agnes,” extensive studies of the biological, chemical, and physical impacts of this event. Hargis, Cronin, and Pritchard were good friends and strong competitors of long standing. Since 1949, Pritchard had been the first full-time director of the Chesapeake Bay Institute (CBI); Cronin had headed the Chesapeake Biological Laboratory (CBL) since 1951; and Hargis had been director of the Virginia Institute of Marine Science (VIMS) (and its predecessor the Virginia Fisheries Laboratory-VFL) since 1959. In 1964, the three directors had set up an informal Chesapeake Bay Research Council (CBRC) to coordinate some of their Chesapeake Bay research activities. They used the CBRC mechanism to coordinate “Operation Agnes,” a commitment that was made without any assurance of financial support for these studies. The gamble taken by the three laboratory directors was successful, eventually resulting in a peer-reviewed book published by The Johns Hopkins University Press entitled The Effects of Tropical Storm Agnes on the Chesapeake Bay Estuarine System. Operation Agnes was the last project undertaken by the CBRC. Reorganization by two of the parent institutions and incorporation of the Chesapeake Research Consortium (CRC) resulted in a realignment of Chesapeake Bay scientific leadership and the leadership of Operation Agnes moved from CBRC to CRC. The scientific community’s response to Tropical Storm Agnes—an unprecedented event— was in itself unprecedented. A number of coincidences came into play: recent (1969) experience with flooding from Hurricane Camille; fortuitous attendance of the leaders of the three largest Chesapeake Bay research institutions at a meeting directly affected by Agnes; and the prior mobilization of the three institutions to conduct extensive hydrographic studies throughout Chesapeake Bay. The most important factor, however, was the strong commitment of three laboratory directors to the understanding of the Chesapeake Bay system.
M. M. Montane and H. M. Austin
Few studies have focused on the effects of climatic perturbations, such as hurricanes, on finfish recruitment and behavior. The Virginia Institute of Marine Science (VIMS) Trawl Survey has sampled continuously throughout the Virginia portion of Chesapeake Bay for 50 years. While hurricanes have impacted Chesapeake Bay during this time, three periods of hurricane activity— September and November 1985 (hurricanes Gloria and Juan), September 1989 (Hurricane Hugo), and September 2003 (Hurricane Isabel)—coincided with the largest spikes in juvenile recruitment of Atlantic croaker (Micropogonias undulatus) for half a century. The fall (October–December) croaker young-of-year indices for 1985, 1989, and 2003 were seven, five, and eight times greater, respectively, than the 50-year average. Typically Atlantic croaker display great interannual variability in Chesapeake Bay, with these fluctuations shown to be weather related. The timing of Atlantic croaker recruitment to Chesapeake Bay is such that late summer/fall hurricanes are most likely to affect them, as opposed to other shelf spawners. Understanding the effects of hurricanes on species, such as croaker, that have enormous ecological, commercial, and recreational importance is essential for prudent fisheries management.
W. G. Reay and K. A. Moore
Water quality impacts from Tropical Cyclone Isabel on the York River estuary were assessed based on long-term, near-continuous, shallowwater monitoring stations along the York River proper (poly- and mesohaline regimes) and its two tidal tributaries—the Mattaponi and Pamunkey rivers (oligohaline and tidal freshwater regimes). Regional rainfall from 18 to 19 September 2003 ranged from 5.8 to 11.7 cm. Peak mean daily stream flow occurred on 21 September 2003 and represented a 20- and 30-fold increase over prestorm conditions on the Mattaponi and Pamunkey rivers, respectively. Isabel produced a storm surge of 1.7 m near the mouth of the estuary and 2.0 m in the upper tidal freshwater regions. The tidal surge resulted in a short-term (12- to 36-hour) pulse of high salinity water (approximately 10 ppt greater than pre-storm conditions) within the oligohaline portion of the estuary. In comparison, salinity levels within the upper tidal fresh water and down-river poly-and mesohaline regions remained relatively unchanged. Following the storm surge, salinity levels within lower portions of the estuary declined 1.5 to 4.5 ppt for an extended period in response to freshwater runoff. Elevated turbidity—in some cases extreme—was in direct response to the storm surge and waves associated with Tropical Cyclone Isabel. With the exception of a single station, maximum storm-associated turbidity levels varied between 192 and >1000 NTUs (nephelometric turbidity units). Turbidity levels returned to prestorm conditions within a 24- to 30-hour period at most stations. Perhaps the most significant environmental impact associated with the passage of Isabel was the persistent low dissolved oxygen (DO) levels (3–4 mg⋅L-1) that occurred at the tidal freshwater stations. Low DO at these stations coincided with increased freshwater inflow to the Mattaponi and Pamunkey rivers, suggesting augmented loadings of readily degradable organic material from the watershed. Mean daily DO levels took approximately two weeks to return to prestorm levels at these sites. Dissolved oxygen levels at the poly- and mesohaline stations within the York River proper remained at or above 5 mg⋅L-1 prior to, during, and after the storm’s passage.
J. Shen, W. Gong, and H. Wang
Hurricane Isabel made landfall near Drum Inlet, about 240 km south of the Chesapeake Bay mouth, on the Outer Banks of North Carolina at 17:00 UTC (GMT 12:00), 18 September 2003. Hurricane Isabel is considered one of the most significant tropical cyclones to affect portions of northeastern North Carolina and east-central Virginia. The ADvanced CIRCulation Model (ADCIRC) model was applied to the Chesapeake Bay to simulate Hurricane Isabel. High-resolution grids were placed inside the Bay and tributaries; coarse grids were placed outside the Bay. The spatial grid resolution in the Bay mainstem is about 200–1000 m and the spatial grid resolution in the tributaries ranges from 50–700 m. A parametric wind model was used to drive the model. The model results show that, with the use of a parametric wind model, the model can predict the peak surge and storm tide histories along the Bay mainstem and tributaries. The model was used to analyze the impact of sea level rise on surge and inundation prediction.
H. Wang, J. Cho, J. Shen, and Y. P. Wang
An unstructured grid hydrodynamic model was used to study storm surge in the Chesapeake Bay during Hurricane Isabel. The model-simulated, storm-induced water level compared reasonably well with the measured data collected around the Bay. Calibrated water level was extracted from the model to further analyze the dynamics of the surge as it formed and propagated along the mainstem Chesapeake. Based on time-series analysis, formation of the surge due to the pumping of coastal waters (hereafter called the primary surge) into the Chesapeake was first identified at the Bay mouth with a peak height of 1.5 m above mean sea level (MSL). Once formed, it propagated northward with gradually diminishing amplitude at a speed of about 5 m⋅sec-1 until reaching Windmill Point, near the mouth of the Rappahannock River in Virginia. Beyond Windmill Point, the surge height increased monotonically toward the northern part of the Chesapeake Bay. Spatial analysis of surge height revealed that a second-stage surge was induced directly by the southerly wind following Hurricane Isabel’s passage inland. The persistent southerly wind induced a setup and a set-down in the upper and lower Chesapeake respectively, with the dividing line near Windmill Point where the water level stayed at approximately 0.5 m above MSL during the event. Space-time analysis provided further evidence that the abnormally high water in the upper Chesapeake Bay was the result of the primary surge wave as well as the second-stage surge caused by the southerly wind-induced setup.
A Global Perspective On The Effects Of Eutrophication And Hypoxia On Aquatic Biota And Water Quality
Robert J. Diaz, Janet Nestlerode, and Minnie L. Diaz
Development associated with human populations has led to the globalization of many environmental problems. In marine systems, the most serious of these problems are directly related to the process of eutrophication. The increased production of organic matter in these marine systems associated with eutrophication is the primary factor impacting species abundance and composition and dissolved oxygen budgets. Oxygen, which is essential to maintaining balance in ecosystem processes through its role in mediating microbial and metazoan activities, has declined to critically low levels in many systems, which has led to the development of hypoxia (<2 ml O2>/l) and anoxia (0 ml O2/l). Currently, most oxygen depletion events are seasonal, but trends toward longer periods that could eventually lead to persistent hypoxic or anoxic conditions are emerging. Over the last 50 years, there has been an increase in the number of systems reporting problems associated with low dissolved oxygen. Currently there are over 100 hypoxic/anoxic areas around the globe, ranging in size from <1 km2 to 70000>km2, that exhibit a graded series of responses to oxygen depletion, ranging from no obvious change to mass mortality of bottom fauna. Ecosystems currently severely stressed by eutrophication induced hypoxia continue to be threatened with the loss of fisheries, loss of biodiversity, alteration of food webs, and simplification of energy flows.
John A. Musick and Julia K. Ellis
The factors that may either constrain or contribute to sustainable marine fisheries were examined by reviewing and analyzing the history and current status of several U.S. fisheries. Among major factors under consideration are inherent vulnerability (vulnerability in some species is high because of low intrinsic rates of increase and/or naturally infrequent recruitment); environmental degradation (fisheries may collapse because of anthropogenic habitat destruction); availability of data (information necessary co conduce accurate stock assessments may be inadequate for some species); quality of the scientific advice (inappropriate models or scientifically inaccurate assessments may be used); and effectiveness of management decisions (managers may disregard recommendations from scientific committees, and/or implement management measures chat are risk-prone). Fisheries that are examined include the Atlantic Coast striped bass Morone saxatilis fishery, the New England groundfish fishery, the Atlantic shark fishery, the Atlantic and Gulf reef fish fisheries, and the Pacific rockfish fishery. Although many of the factors listed above contributed co declines in these fisheries, the root cause in all cases was harvesting at rates that were much higher than could be sustained by recruitment. Management was largely ineffective because management decisions were risk-prone and motivated by short-term economic considerations rather than long-term sustainability. Only after passage of legislation not only authorizing but specifying mandatory stock rebuilding, has most management been sufficiently precautionary to allow sustainability.
Michael C. Newman
Fish are suddenly exposed to hypoxic conditions during diverse events such as seiche- or turnover-related water movements, bottom water release from reservoirs, ice-over of eutrophic arctic lakes, and rapid shifts in respiration: photosynthesis associated with cultural eutrophication. In each case, chemical equilibria established under hypoxic conditions that result in metal dissolution and accumulation suddenly shift toward chemical equilibria of oxic conditions. Critical changes in speciation include those determining the free ion activity that, as expressed by the Free Ion Activity Model (FIAM), is often the most bioactive form of a dissolved metal. Metal phase can also change rapidly and, in some cases, result in a precipitate on respiratory surfaces. Exposure of fish gills to metal (and integument of larval or small fish) changes O2 exchange dynamics. Changes in mucus quality and production and lamellae morphology decrease the amount of effective gill exchange surface and increase the diffusive layer thickness. These changes exacerbate those associated with the reduced O2 partial pressure gradient. Consequent shifts in blood chemistry (e.g., pH and ion composition) and ventilation also affect metal transport and deposition within fish tissues. Some of these changes have immediate consequences, but others can continue for long periods after the hypoxic conditions pass. Long-term metal effects can influence fish tolerance during future hypoxic episodes.
A joint, similar action model can be applied if the parsimonious assumption is made that asphyxiation constitutes a common mode-of-action for both acute metal effects and hypoxia. Joint action models are applicable based on either conventional dose-effect or survival time approaches. Expansion of such models to a physiologically-based toxicokineticstoxicodynamics framework (e.g., framed around the Fick equation) would be desirable, provided that model parameter requirements remain realistic. Long-term effects may be better addressed with models such as the binary logistic models used by epidemiologists.
Patrick K. Baker and Roger Mann
Large populations of soft shell clams persist only in relatively shallow, sandy, mesohaline portions of the Chesapeake Bay. These areas are mostly in Maryland, but also occur in the Rappahannock River, Virginia. In some other portions of the Bay, especially polyhaline portions, low populations of soft shell clams persist subtidally. Restricted populations persist intertidally.
Soft shell clams grow rapidly in the Chesapeake Bay, reaching commercial size in two years or less. They reproduce twice per year, in spring and fall, but probably only fall spawnings are important in maintaining population levels. Major recruitment events do not occur in most years, despite heavy annual sets. Soft shell clams are important food for many predators. Major predators on juveniles include blue crabs, mud crabs, flatworms, mummichogs, and spot. Major predators on adults include blue crabs, eels, and cownose rays. Some other species that may depend heavily on soft shell clams include ducks, geese, swans, muskrats, and raccoons.
Diseases may play an important role in regulating adult populations of soft shell clams; hydrocarbon pollution is linked to increased frequency of disease. Oil pollution does the most widespread and persistent damage to soft shell clams through toxicity, aside from its role in inducing disease. Heavy metals, pesticides, and similar pollutants can be extremely toxic, but the harmful effects to clams do not last if the pollution abates. The main concern with the latter toxicants is bioaccumulation by soft shell clams, with the potential for passing toxic contaminants on to predators or to humans.
Siltation caused by storm events, dredging operations, or erosion, can smother clam populations. Eutrophication, enhanced by nutrient inputs from sewage or agriculture, is not known to have affected soft shell clam populations.
G. Curtis Roegner and Roger Mann
The hard clam is found along the eastern coast of North America from the Gulf of St. Lawrence to Texas. In Chesapeake Bay, the hard clam is restricted to salinities above approximately 12 ppt. An extensive survey of hard clam resources is overdue.
Statements concerning long term trends in populations are not feasible. Hard clams ·grow to a maximum shell length of about 120 mm. There are few documented cases of diseases in wild hard clam populations. Parasitic infestations are also slight. The life cycle of the hard clam includes a pelagic larval phase and a relatively sedentary benthic juvenile and adult phase. In Chesapeake Bay, ripe gametes can be found between May and October, and spawning commences when temperatures rise above 20-23 ·c. The larvae are planktotrophic (feeding). Metamorphosis usually commences at a shell length of 200-210 mm. Predation on new recruits is very high; dense aggregations of hard clams have been found in the absence of predators. Aside from predation and fishing pressure, the natural mortality of larger clams appears very low.
Hard clams are important suspension-feeding infauna, thus they are important in grazing of primary production, transfer of carbon and nitrogen to benthic food chains, and, through excretion, rapid recycling of particulate nitrogen as ammonia. The major food source for hard clams is planktonic microalgae. In Chesapeake Bay, growth occurs in spring and fall, when optimum water temperatures coincide with abundant food.
Clams are capable of living in a variety of sediment types, but higher abundances are found in coarse-grained sediments. Hard clam stocks are susceptible to overfishing. Recruitment rates are poorly understood, as are possible reestablishment periods if areas are depleted through commercial harvesting, and factors influencing larval settlement rates. Hard clam mariculture is well established and could easily be expanded into sites within the Bay. Given the ability of clams to bioaccumulate toxic substances, adequate monitoring should be maintained. The sub lethal effects of toxic material readily found in the lower James River should be examined
Ann Hayward Rooney-Char and Maurice P. Lynch
Harold J. Humm
The Men All Singing is a folk history as well as economic. It reaches back almost four centuries to tell of a little-known fishery that shares with cod the vista of our coastal life. Now the last decades of the twentieth century see great changes in all fisheries and our treatment of the waters that give so much of our food.
Dexter S. Haven and Reinaldo Morales-Alamo
Filter feeders, such as mollusks, tunicates, and barnacles, ingest particles as small as 1 micron during their feeding process and void them in fecal pellets which range from 500 to 3,000 microns in length; these pellets settle at a much faster rate than their component particles. Feces and pseudofeces that settle to the bottom are termed biodeposits. Oyster biodeposits contain 77 to 91 percent inorganic matter, 4 to 12 percent organic carbon, and about 1.0 gram per kilogram of phosphorus. Fecal pellets are alternately deposited and resuspended by tidal currents. They settle and accumulate in areas of estuaries where the fine particles themselves would not. A portion of the biodeposits settling on sediment surfaces is mixed into subsurface deposits and may alter the textural and chemical properties of the original sediments.
Maynard M. Nichols
The effects of channel deepening on the salinity and density flow in the James River estuary, Virginia, were studied to predict changes that might affect oyster production. A hydraulic model with 1: 1,000 horizontal and l: 100 vertical scales was employed to integrate three-dimensional changes in salinity and velocity through reaches of variable bottom geometry. After natural characteristics of the tide, current, and salinity were reproduced in the model, tests were run at three levels of steady river inflow, before and after a 3-meter channel deepening. Results were combined with corollary field observations to evaluate changes in present-day ecological conditions.
Deepening produced the greatest salinity change in the middle estuary where the major cut was performed. The lower water layer located mainly in the channel became saltier by about 0.5 part per thousand, whereas the upper layer over the oyster shoals became fresher by about 0.2 part per thousand. Changes in bottom water salinity were greatest at intermediate inflow and least at very low inflow. High fresh-water inflow created the greatest change in vertical salinity gradient. With greater stratification, tidal velocities were less effective in promoting vertical mixing between lower and upper estuarine water layers, and the net volume transport in each layer was reduced.
Since the changes in salinity and flow pattern due to channel deepening were small, no effects inimical to the oyster fishery were predicted. Similarly the prospective changes in sedimentary regime will not offset the beneficial effects of the proposed deepening project.
Maynard M. Nichols
The James River estuary of the Chesapeake Bay region follows the course of a former river valley drowned within the last 9,000 years by the most recent rise of sea level. The floor is shaped into a central channel bordered by submerged shoals. Observations show suspended sediment is transported mainly by alternating tidal currents and secondarily by the net nontidal estuarine circulation. Transport results in a sequence of grain size distributions reflecting the mixing of two textural end members, clay and sand.
Silty clay is deposited in the river and upper estuary, whereas sand occurs near the mouth. Transitional types, clayey sand and sand-silt-clay, predominate in the middle estuary. Additionally, biogenic materials, oyster shells and fecal pellets, and small amounts of residual components eroded from older deposits are mixed into the sediments by currents, waves, and organisms. Bottom sediment types vary widely according to local relief, to varying intensity of environmental processes, and to changing rates of supply from different sources:
Deposition is greatest in the middle estuary where salinity ranges from 5 to 14 parts per thousand. An elongate zone of relatively high deposition in the lower estuary corresponds to the intersection of the level of no-net-motion with the bottom. Despite substantial infilling, it is believed the estuary is maintained by the continued rise of sea level and by currents that flush part of the river-borne load through the estuary.
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