Biomass and nutrient flux measurements on Holothuria atra populations on windward reef flats at Enewetak, Marshall Islands
C. F. D'Elia and K. L. Webb
Although it is reasonably well established that corals symbiotic with endozoic algae (the zooxanthellae) are capable of "multitrophic" (1) existence, we are still a long way away from evaluating the relative importance of the various trophic pathways (2). There is no question that zooplankton provide a source of materials and energy to corals, yet there is doubt whether zooplankton are plentiful enough to supply all the needs for these essentials (3). There appear to be alternate sources of supply. Sufficient energy is likely to be available to the symbiotic association by virtue of the photosynthetic capacity of the zooxanthellae (4); thus, energy wise, the effects of the limited quantities of available zooplankton may be obviated. But what of the balance of nutrient elements such as N and P?
Biomass and nutrient flux measurements on Holothuria atra populations on windward reef flats at Enewetak, Marshall Islands
Kenneth L. Webb, William D. DuPaul, and Christopher F. D'Elia
A population survey of the holothurian, Holothuria atra, on the interisland coral reefs of Enewetak revealed average animal densities of about 3 animals m-2 in the zone of small coral heads, Median fresh weighy was 60 g, The size distribution of these animals was negatively correlated with water velocity along the reef.
Ammonia release rates for three species of holothurians, H. atra, H. difficilus and Actinopyga mauritina were weight specific, The release of annnonia by H. atra on Transect II was equivalent to 9% of the total amnnonia exported from the reef proper. The release of phosphorus followed the general rules set for size - metabolism relationships. Nitrogen/phosphorus release ratios are 25:1 for 1 gram fresh weight animals and 42:1 for 60 gram animals, Urea is also released
Bruce J. Neilson
Oysters are filter-feeding organisms which can accumulate substances to concentrations far above those found in the surrounding water. Large acreages of estuary bottoms have been classified .as restricted for shellfish culture due to the presence of pollutants in the water and therefore, the likelihood of high levels of pollutants in oysters grown in those environments. Depuration is a natural process whereby oysters and other shellfish cleanse themselves of accumulated contaminants when they are placed in a clean environment. Recent studies have shown that bacterial depuration of oysters is feasible for the waters of Chesapeake Bay. Depuration is not possible, though, when water temperatures are below 10 °c. Furthermore, both the rate of depuration and the reliability of the process increase with higher temperatures for the 100 to 20 ° C range. Utilization of waste heat from power generating stations would allow depuration to continue during winter months and could improve the quality of the product during the spring and fall.
John V. Merriner
Anadromous fishes have been and continue to be important members of the ichthyofauna within the tidal waters of the Potomac River and other tributaries of Chesapeake Bay in the ecological, commercial, recreational and aesthetic sense. Attributes of the Potomac River basin and its waters have been described in several earlier presentations and shall be cited herein only as they apply to anadromous fishes.
Changes in salinity structure of the James, York and Rappahannock estuaries resulting from the effects of Tropical Storm Agnes
Paul V. Hyer and Evon P. Ruzecki
The peak effect of the flood waters produced by Tropical Storm Agnes was seen on June 25 in the James, June 26 in the Rappahannock, and June 30 in the York. Recovery toward normal salinity conditions after the high runoffs proceeded discontinuously, with alternating periods of vertical stratification and destratification. During strongly stratified stages, saline water advanced upstream along the bottom. In the York and James Rivers, the most dramatic stratification occurred about July 20- 25. This event resulted in bottom salinity values exceeding normal ambient values and, at the river mouths, reaching values hitherto unobserved. This event was apparently controlled by the salinity distribution in the Bay. ~ess pronounced stratification maxima occurred in the James about July 6 and August 18 and in the York during August. These events do not appear to be correlated with stream gauge flow records or local precipitation. These events are possible instances of overshooting of equilibrium by the intruding salt water near the bottom.
Marvin L. Wass
Surveillance of shipworm infestation at Gloucester Point, Va., began in 1958. Borer attack by Bankia gouldi occurred in July to early October each year until the passage of Agnes greatly reduced setting. Populations returned to near normal in 1975. Salinity was shown to vary with watershed rainfall.
C. L. Smith, W. G. MacIntyre, C. A. Lake, and J. G. Windsor Jr.
Nutrient concentrations measured in lower Chesapeake Bay in the summer of 1972 immediately following the flooding associated with Tropical Storm Agnes are compared with those in the summer of 1973, a season of more normal rainfall. The large amount of land runoff produced unseasonably high concentrations of dissolved inorganic nitrogen in the Bay near the mouth of the Potomac River. Phosphate concentrations were essentially unaffected by the flooding. Fluxes of total nitrogen and total phosphorus nutrients through the mouth of Chesapeake Bay were calculated for both summers. The calculated net export of nutrients from the Bay in both August 1972, and June 1973 was found to be small in comparison to nutrient inputs.
George C. Grant, Burton B. Bryan, Fred Jacobs, and John E. Olney
Sampling techniques in use since August 1971 were employed to study effects of Tropical Storm Agnes on lower Chesapeake Bay zooplankton following the storm's passage on June 21, 1972. Mean catches of copepods, cladocerans, barnacle larvae, decapod larvae, chaetognaths, and fish eggs and larvae were calculated for the entire study area and six subareas from 8" bongo net collections. A single subarea was selected for specific identifications within major taxa of zooplankton.
John P. Jacobson and C. S. Fang
During the Agnes flood hourly tidal height data were collected at seven locations along the tidal James River and currents were measured at two transects in the lower James. A comparison between actual tides and currents and the predicted tidal features as given by the tide and tidal current tables of NOAA was made. Results of this comparison show that Agnes did significantly affect water levels in the upper portion of the tidal James, especially near Richmond. However in the lower portion of the James no discernible rise was evident due to the passage of the flood crest. A small storm surge (<2>feet) was noted on the day of the passage of Agnes, 21 June, throughout the tidal James. A phase shift in times of high and low water due to the interaction of the two wave systems was not observed. In the freshwater portion of the tidal James currents continually ebbed during the passage of the flood crest. In the saline portion of the system, the flood effect on the currents was limited to the surface portion of the channel.
R. J. Huggett and M. E. Bender
If one looks back over the past twenty-five years, at the pollution crises which have made the headlines and influenced our decision making policies on both local and national levels, one may note a very interesting aspect. That is, the crises are often associated with the development of new and more sensitive types of analytical instrumentation. Several examples of this readily come to mind: for example, it wasn't until the development of electron capture detection for gas chromatography that DDT really became a crisis. The reason is quite simple: the concentrations which were being accumulated by the organisms were below detection limits of most available instrumentation. Another example is mercury. The advent of atomic absorption spectrophotometry and the refinement of the the flameless method for mercury allowed the mercury problem to be investigated. New instrumentation doesn't cause the crisis, it merely finds it.
Robert L. Ellison and Maynard M. Nichols
Estuaries are highly variable coastal ecosystems. Some of the variation is seasonal and some is longitudinal along the environmental gradient from the river to the sea. Foraminifera are tuned to the periodicity, and a progressive change in the composition and structure of foraminiferal faunas parallels the longitudinal ecocline, identified by the gradient in salinity.
In marshes and tributary estuaries where water is fresh, thecamoebinids comprise the microfauna. Three other marsh faunas are composed chiefly of the agglutinate species: Ammoastuta salsa, Miliammina fusca, Arenoparrella mexicana, Alllmobaculites crassus and species of Haplophragmoides and Trochammina. Their distribution is influenced by salinity and exposure. In the estuaries, where fresh and salt water mix:, two faunas are characterized by: Anmlobaculites crassus, in the middle and upper reaches where salinity is less than about 15 % and the estuary is periodically freshened by river flushing, and by Elphidim clavatum in lower reaches and deeper channels where salinity is higher and mixing is moderate. Elphidium, furthermore, dominates the faunas in the lower part of Chesapeake Bay and, on the inner part of the shelf. At a depth of about 25 m the Elphidium fauna is succeeded by a larger and more diverse fauna that may be partly relict.
The marsh and estuarine faunas shift headward and mouthward with changing river inflow and salinity, and their changes are recorded in cores of estuarine and marsh deposits. Short-term events and paleoclimatic episodes with durations of several hundred years are superimposed on a long-term trend of decreasing salinity during the past 6,000 years as sedimentary infilling exceeded the rise in sea level.
Observations On Dissolved Oxygen Conditions In Three Virginia Estuaries After Tropical Storm Agnes (Summer 1972)
Dissolved oxygen (DO) and salinity levels in the James, York, and Rappahannock estuaries were monitored for approximately two months (June 24-August 31, 1972) following Tropical Storm Agnes. DO depressions developed more rapidly and were more severe in the deep waters of the York and Rappahannock than in the James. Depressions that developed immediately after the storm were followed by recoveries and subsequent, more severe depressions. In late July, bottom water DO concentrations below 1 mg/1 were found at stations covering 15 miles of the York and 25 miles of the Rappahannock. Comparison of river data with Chesapeake Bay data suggests that the rivers contributed oxygen poor water to the Bay during the post-Agnes period. Comparison of 1972 river data with data from other years suggests that the post-Agnes oxygen depressions were more severe than those that occur in normal years.
Patterns of Distribution of Estuarine Organisms and their Response to a Catastrophic Decrease in Salinity
Peter F. Larsen
The occurrence of Tropical Storm Agnes during an ongoing study on the community structure of the macrobenthos associated with the James River oyster reefs provided a unique opportunity to document the responses of this assemblage to such a disturbance. The spatial and temporal patterns of abundance of 18 important taxa are examined in this paper. Eight species exhibited limited up estuary penetration, six were most successful in the upper part of the estuarine segment studied, two were most abundant in the mid-section of the study area, and two were ubiquitous. In the post-Agnes period, .six species exhibited reduced population levels, three experienced population increases, three became relatively more abundant at the down estuary sites and reduced at the up estuary sites, three became relatively more abundant at the up estuary sites and reduced at the down estuary sites, while no significant response was shown by three others. Hypothetical response categories are advanced to explain these responses. The freshet arrived and removed stenotopic species (category 1 response) which allowed others to fill the void in abundance (category 2 response). Other species essentially extended their range downstream where conditions were not optimal but were reduced in their original range because of the physiological stress caused by very .Low salinities (category 3 response). With the return of higher salinities the larvae of more stenohaline species settled where there was open space, i.e., at the up estuary sites (category 4 response). Some species showed no significant changes in abundance (category 5 response).
Public health aspects of Tropical Storm Agnes in Virginia's portion of Chesapeake Bay and its Tributaries
M. P. Lynch and J. Claiborne Jones
All Virginia waters within Chesapeake Bay were closed for the taking of shellfish for direct consumption on 23 June 1972. This initial closing was in anticipation of high microorganism levels accompanying flood waters moving downstream. Various areas beginning with the lower portion of Chesapeake Bay were reopened beginning )n 20 July 1972. By 5 October 1972 all areas closed as a result )f Tropical Storm Agnes were reopened. No increased incidents of infectious diseases caused by waterborne microorganisms were noted in Virginia which could be attributed to Tropical Storm Agnes.
Maynard M. Nichols, Galen Thompson, and Bruce Nelson
Flooding from Tropical Stonn Agnes produced unique hydrographic conditions for transport and dispersal of sediment in the Rappahannock and .James estuaries. Analyses indicate two cycles of response and recovery to the shock of extreme freshwater and sediment influx; one cycle in response to Rappahannock inflow; the other to intense mixing within the estuary. Important stages in the sequence consist of: (1) an initial response and seaward surge of river water and sediment; (2) shock with downstream translation of the salt intrusion head with a near-bottom salinity front and high turbidity in surface and in bottom water; (3) rebound with intense stratification and formation of an enriched turbidity maximum; (4) partial recovery with salinity intrusion strengthened by upstream flow along the bottom; landward migration of the maximum; (5) full recovery and return to partly-mixed state with decay of turbidity maximum over a broad zone 30 days after flooding. Sediment was derived initially from lateral tributaries and then from the main river. The bulk of the load sedimented above the salt intrusion during the first three days of flooding. Sediment dispersed into the estuarine circulation system later was effectively trapped by upstream flow along the bottom. Over the entire event, 91% of the sediment load was trapped.
William H. Kriete and Joseph G. Loesch
This paper reports the present status of the alosine stocks in Virginia and attempts to explain trends of the past 10 years. This paper is a portion of a completion report "The Biology and Management of River Herring and Shad in Virginia," in partial fulfillment of Contract No. 04-5-043-11 with the National Marine Fisheries Service.
The displacement and loss of larval fishes from the Rappahannock and James rivers, Virginia, following a major tropical storm
Walter J. Hoagman and John V. Merriner
Two days after Tropical Storm Agnes, the Virginia Institute of Marine Science (VIMS) established an anchor station at Mile 15 in the Rappahannock and Mile 10 in the James River. Both stations were in mainstream, manned constantly for 10 days, and took continuous current data from meters placed at 0, 6, 8, and 15 min the Rappahannock and 0, 4, 5, 8, and 14 min the James. Concurrently, 0.85 m diameter plankton nets of No. 1 nylon mesh were hung in the flowing surface water for 10 minutes hourly. A small collection of midwater (4 m) plankton samples was obtained from the James station. The shoal areas were not sampled for larval fish. The ichthyoplankton and zooplankton captured were preserved and later identified to species.
The Effect of Tropical Storm Agnes as Reflected in Chlorophyll A and Heterotrophic Potential of the Lower Chesapeake Bay
Paul L. Zubkoff and J. Ernest Warriner III
A hydrographic station (Station Y) at the mouth of the York River (37°14.6'N, 76°23.4'W) was under biological surveillance for one year prior to the arrival of Tropical Storm Agnes. For one full year following this storm, these measurements were continued. In addition, the chlorophyll a and heterotrophic potential measurements were incorporated into an ongoing zooplankton sampling program of the lower Chesapeake Bay below 37°40'N latitude.
The Effect Of Tropical Storm Agnes On Heavy Metal And Pesticide Residues In The Eastern Oyster From Southern Chesapeake Bay
M. E. Bender and R. J. Huggett
The concentrations of cadmium, copper, and zinc in the eastern oyster, Crassostrea virginica, are compared for samples collected before and after Tropical Storm Agnes. The "before" samples consisted of 475 animals from 95 stations collected in January 1971 and the "after" samples of 285 animals from 57 stations collected in January 1973. Shifts in the areal concentrations distributions were observed, apparently due to Agnes. Analyses of hard clams, blue crabs and oysters for chlorinated hydrocarbon pesticides showed influx of these compounds to be minimal as reflected by the residue levels observed. Comparison of residue levels in oysters to pre-Agnes conditions revealed a decrease in pesticide body burden.
The Effect of Tropical Storm Agnes on Oysters, Hard Clams, Soft Clams, and Oyster Drills in VIrginia
D. S. Haven, W. J. Hargis Jr., J. G. Loesch, and J. P. Whitcomb
Tropical Storm Agnes had a major effect on the molluscan fisheries of Virginia. One effect was the direct mortality of oysters, Crassostrea virginiaa, in the upper parts of many estuaries. Typical losses on leased bottoms were: the James River, 10%; the York River, 2%; the Rappahannock River, 50%; and the Potomac River tributaries (Virginia) 70%. Economic loss was in excess of 7.9 million dollars. There was a nearly complete absence of oyster larvae attachment (setting) in 1972. Other effects of Agnes included a nearly complete loss of soft clams, Mya arenaria, in the Rappahannock River. Hard clams, Meraenaria meraenaria, were killed in the upper part of the York River. Oyster drills, Urosalpinx ainerea, were eliminated from the Rappahannock and reduced greatly in numbers in the York and James Rivers.
The effect of Tropical Storm Agnes on the benthic fauna of eelgrass, Zostera marina, in the lower Chesapeake Bay
Robert J. Orth
Tropical Storm Agnes caused major changes in the macroinvertebrate assemblages of both epifauna and infauna in eelgrass, Zostera marina, beds. Species abundance and density of infauna declined by one-third to one-half of values found prior to Agnes. Typical members of the infaunal community such as the amphipods, Ampelisca spp. and Lysianassa alba, the polychaetes Sabella microphthalma and Exogone dispar, ostracods and gastropods were either absent or rare following Agnes. Epifaunal density was much higher than that recorded before Agnes but the number of species was reduced. This high density was attributed to several species, e.g. Molgula manhattensis, which appeared to occupy space left open by the absence of typical members of this community, e.g. Paracereceis caudata and Bittium varium. The abnormally low salinities following Agnes affected various species in different ways. Some species were totally eliminated, severely reduced in abundance or, in a few euryhaline species, not affected at all. In some populations it appeared that adults survived but juveniles suffered high mortalities. Recovery and reestablishment by many species will be complicated by the disappearance of eelgrass in some portions of the Bay.
The effects of the Agnes flood on the salinity structure of the lower Chesapeake Bay and contiguous waters
A. Y. Kuo, E. P. Ruzecki, and C. S. Fang
The transient response of salinity distribution in lower Chesapeake Bay to flood waters from Tropical Storm Agnes is studied in terms of a two-layered, partially mixed estuary. Prior to 30 June 1972, surface salinities were well depressed throughout the Bay while those at the bottom near the Bay mouth were not depressed by 5 July. This resulted in a highly stratified situation normally found in the spring of the year. Stratification decreased when bottom waters were flushed down-bay by the flood (on 5 to 10 July for the region south of New Point Comfort). The "rebound" of salinity structure started immediately after the passage of the flood water which otherwise retarded up-bay movement of bottom waters. This "rebound" began on 13 July near the Bay mouth and progressed up-bay reaching the mouth of the Potomac River by 20 July. During this period, surface salinity remained low, resulting in strong stratification again. The recovery of surface salinity by tidal mixing finally weakened stratification to a near "normal" salinity structure by the end of August. The large mass of flood water leaving the Bay mouth is treated as a natural tracer release. The distribution of flood water on the continental shelf indicates that pulses of freshened surface water left the Bay on ebb tide and were separated from one another by intrusion of saltier shelf water on flood tide. During the period when the wind speed was below 4 m/s, the flood water remained in the upper 10 meters of the water column and traveled southward with a speed of 80 cm/sec.
The Effects of Tropical Storm Agnes on Fishes in the James, York, and Rappahannock Rivers of Virginia
Walter J. Hoagman and Woodrow L. Wilson
Intensive trawl surveys during and after Tropical Storm Agnes were mounted on the James, York, and Rappahannock Rivers to measure the effects of the floodwaters on the distribution and abundance of fish. The direct effect of Agnes on the fish populations was minor and temporary. The normal zone was extended downriver. A substantial portion of the lower-river (marine) species was also displaced downstream and into Chesapeake Bay, but had returned by the follow-up surveys. No adult mortalities due to Agnes were detected. Although we know vast quantities of fish larvae and other plankton were swept into Chesapeake Bay, the overall impact on all fish appears to have been slight.
Robert J. Huggett and Michael E. Bender
The metals copper and zinc were analyzed in bottom sediments (top 1 cm) from the Rappahannock River before and after Tropical Storm Agnes. By extracting the sediments with various techniques (HN03, HCl) the nature of the metal speciation can be estimated. Data show that the inorganic copper was increased by a factor of 2 to 3 in the normally saline portion of the river as a result of Agnes but returned to before-Agnes levels within one year. Metal analyses of suspended sediments collected during the Agnes flooding allows an estimate of sedimentation indicating at least 7.5 mm of new sediments at mile 40, decreasing nearly linearly to 1 mm at mile 15.
Dexter S. Haven
The Potomac and its tributaries support many species of molluscs which are important sources of food for benthic invertebrates, fish, and waterfowl. However, only two species, the American oyster Crassostrea virginica and the soft clam Mya arenaria are harvested for commercial purposes. A third species, the brackish water clam Rangia cuneata occurs locally in tremendous concentrations, but it is not utilized as a food source. Nevertheless, this species is regarded as a potential source of food for people.
During the past six to eight years, commercial landings of both the oyster and the soft clam have dropped from high to extremely low levels. The cause of this decline may be partially explained for oysters, but the reason for the decline for soft clams is not apparent. The brackish water clam Rangia, a recent introduction into the Potomac, remains at high levels of abundance.
In this discussion, emphasis will be placed on the oyster since this species has received the most study.
R. J. Huggett, F. A. Cross, and M. E. Bender
Copper and zinc were analyzed in oysters (Crassostrea virginica) from the Newport River estuary, North Carolina, and the Rappahannock River estuary, Virginia. Results indicated that a concentration gradient existed, higher concentrations of metals being found in animals living in fresher waters as was shown previously for oysters in the James, York, and Rappahannock estuaries in Virginia. Absorbed, precipitatcd-coprecipitated, and organic fractions of copper and zinc in the <63-μm portion of the sediments from the Rappahannock and York rivers and estuaries were estimated from collections made in January 1972 and June 1973. These sediment data are discussed for both estuarine systems and are compared with metal concentrations in oysters. These comparisons indicated that the concentration gradient found in oysters does not appear to be related to the distribution of copper and zinc in the sediments. Alternative explanations for the inverse relationships between concentrations of copper and zinc in oysters and salinity arc given.
Morris L. Brehmer
I have modified the scope of this paper slightly to include urban pollution because the pollution problems produced by the agricultural industry are essentially the same as those produced by municipalities. We know that environmental degradation occurs as a result of the discharge and the byproducts from both sources.
In the case of agricultural activities, the terrain has and still is being modified to convert it to crop production. In the livestock industry, large populations of animals are being concentrated into relatively small areas to meet the economic pressures of meat production. With the human population, the numbers of people are not only increasing but are also concentrating. Probably more important, countless square miles of terrain have been denuded and modified to make way for highway and street construction or for the development of housing units. From both sources the results are the same. We have increased siltation; we have increased organic loading; and we have increased nutrient loading
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.
William J. Hargis Jr.
This paper discusses the tidal tributaries of the ocean and the coastal areas of the mid-Atlantic Bight and the ecological significance of engineering projects. While occasional reference may be made in this paper to remote sensing of problems engendered by engineering works on maritime environments and resources, principal efforts along those Jines are reserved for the group discussion to follow.
The Chesapeake Bay drainage basin_encompasses.almost 65,000 miles and provides space and partial resources for over 11 million people ( 1960) in New York, Pennsylvania, Maryland, Virginia, and the District of Columbia. Two other states, Delaware and West Virginia, to a lesser extent are part of this basin. Major residential, industrial and commercial, military, and recreational activities in the mid-Atlantic area make their demands on the environment and resources and contribute to the economic and social well-being of the populace. Certain social and economic disbenefits often accompany these activities. Population growth in the basin is increasing as are economic and social activities and other user activities.
Reference 1 includes many of the vital statistics on the Chesapeake Bay drainage basin. Numerous other studies in all fields have reported upon many natural, economic and social features of the Bay. Still others on these and additional subjects are in process.
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.
Age Composition and Magnitude of Striped Bass Winter Gill-net Catches in the Rappahannock River, 1967-1970
George C. Grant, Victor G. Burrell Jr., and William H. Kriete
The dependence of commercial fishing success on strong yearclasses of striped bass is demonstrated, using four years of catch records from a small group of cooperating gill-net fishermen. Age analysis of these Rappahannock River catches during 1969 and 1970 revealed that an approximate tripling of landings in 1970 was a result of selection for the dominant 1966 year-class.
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