Loading, Transformation, and Retention of Nitrogen and Phosphorus in the Tidal Freshwater James River (Virginia)

A nutrient mass balance for the tidal freshwater segment of the James River was used to assess sources of nutrients supporting phytoplankton production and the importance of the tidal freshwater zone in mitigating nutrient transport to marine waters. Monthly mass balances for 2007–2010 were based on riverine inputs, local point sources (including sewer overflow events), ungauged inputs, riverine outputs, and tidal exchange. The tidal freshwater James River received exceptionally high areal loads (446 mg TN m^?2 day^?1 and 55 mg TP m^?2 day^?1) compared to other estuaries in the region and elsewhere. P inputs were principally from riverine sources (84 %) whereas point sources contributed appreciably (54 %) to high N loads. Despite high loading rates and short water residence time, areal mass retention was high (143 mg TN m^?2 day^?1 and 33 mg TP m^?2 day^?1). Retention of particulate fractions occurred during high discharge, whereas dissolved inorganic fractions were retained during low discharge when chlorophyll-a concentrations were high. On an annualized basis, P was retained more effectively (59 %) than N (32 %). P was retained by abiotic mechanisms via trapping of particulate forms, whereas N was retained through biological assimilation of dissolved inorganic forms. Results from a limited suite of stable isotope determinations suggest that DIN from point sources was preferentially retained. Combined inputs from diffuse and point sources accounted for only 20 % and 36 % (respectively) of estimated algal N and P demand, indicating that internal nutrient recycling was important to sustaining high rates of phytoplankton production in the tidal freshwater zone.     

The asiatic clam,Corbicula fluminea (Müller), in the tidal Potomac River,Maryland

The Asiatic clam,Corbicula fluminea (Müller), has extended its range to include the tidal fresh-water portion of the Potomac River, Maryland. Though patchily distributed, the clams have attained densities of 665 m^?2. Size-class distributions indicate that the clams first appeared in 1975. About 90% of the population belong to year-class I and were less than 12 mm in length. Elsewhere, this species has created severe water quality problems; it should be closely watched in the Potomac.     

The asiatic clam (Corbicula fluminea) invasion and system-level ecological change in the Potomac River Estuary near Washington,D.C.

The exotic freshwater clam speciesCorbicula fluminea (Asiatic clam) was first reported in the tidal freshwater Potomac estuary near Washington, D.C., in 1977, and was found in benthic surveys, conducted in 1978, 1982, 1984, 1986, and 1992. In 1981 a tripling of water clarity was reported in the region of the clam beds, followed in 1983 by reapperance of submerged aquatic vegetation (SAV) absent for 50 yr. Submerged aquatic vegetation (SAV) has been surveyed and mapped over the entire Potomac estuary region in almost every year from 1976 to 1993 by aerial photography, as part of the United States Environmental Protection Agency's Chesapeake Bay program. Fish surveys in 1986 found populations increased up to 7× in beds of SAV. Starting in 1984, the Washington, D.C. Christmas Bird Census reported significant increases in several aquatic bird populations both nonmigratory and migratory. An extensive benthic survey in September 1986 estimated a spring-summer population of 8.7×10⁶ kg Asiatic clams (wet weight including shell) in the 5-km region of the Potomac below Washington, D.C. This population was calculated as having the capacity to filter one-third to all of the water in this region of the estuary daily, depending on river flow. The 1986 clam population was smaller than that of 1984 and the 1992 population was 25% of that in 1986. Since 1986, SAV acreage has been decreasing in this area of the Potomac. Aquatic bird populations have declined. Yearly nuisance algae (Microcystis) blooms, which had been absent since 1983, reappeared in 1993. This paper presents evidence to support the theory the invasive Asiatic clam population in the 10 km below Washington, D.C., was responsible for SAV resurgence through filtration affecting turbidity. It suggests the clam populations triggered system-level changes in biota, including increase and decrease in local Potomac estuary populations (SAV, bird, fish, algae) over 10 yr, from 1983 to 1993. Major changes in the Asiatic clam population took place approximately 2 yr before parallel changes in SAV acreage were observed.     

Boccardia hamata (Polychaeta:spionidae): A potential pest of the American oyster in the James River,Virginia

During a comprehensive survey of fauna associated with James River oyster reefs in 1971–72, a large population of the spionid polychaeteBoccardia hamata was found. The abundance of this boring species was greater than that of the well-known oyster pestPolydora websteri. B. hamata has not been reported previously in the Chesapeake Bay system and its occurrence on the east coast of the United States is not common. The large numbers in 1971–72 and continued presence ofB. hamata in 1975 suggests that the species may be well established in the James River. This may have some implications to the oyster industry if the species is as harmful as other boring spionids.     

The geomorphic development of wreck shoal,a subtidal oyster reef of the James River,Virginia

Wreck Shoal is a subtidal oyster reef located in the James River estuary, Virginia. This estuary has moved upstream and landward in response to rising sea level. The recent geomorphic history of Wreck Shoal is analyzed based on bathymetric records from the 1850’s to the 1980’s. The data indicate that the shallow oyster reef areas have lost elevation in the last 130 yr. This is attributed to intense harvesting activity during the last century. The late Holocene evolution of Wreck Shoal is developed based on the results of sub-bottom profiles and coring data. These suggest that the Wreck Shoal oyster reef has developed on the ridge and swale topography of a point-bar formed during the late Pleistocene epoch. Contemporary biodeposition processes on Wreck Shoal are evaluated. The results indicate that sediments of biogenic origin (fecal and shell material) potentially accumulate at rates in excess of 50 cm 100 years^?1. A model for subtidal oyster reef development is proposed that accounts for sea level rise, biodeposition, and the harvesting activity of man. The model is verified with field observations of reef elevation and radiocarbon dates of oyster shell material. The implications of these results are that oyster reefs should be considered a renewable natural resource, and therefore managed accordingly in concert with the oysters.     

Growth of juvenile spot,Leiostomus xanthurus Lacépède,in the nursery region of the James River,Virginia

Thein situ growth of juvenile spot,Leiostomus xanthurus Lacépède, was investigated in a nursery area of the James River, Virginia. Juvenile growth was fairly rapid (11.34 mm standard length per month) during their first spring and summer in the nursery grounds, and leveled off in the autumn. The mean size of spot after the first year was calculated to be 148 mm standard length. The weight-length relationship was log₁₀W = ?5.018+3.246 (log₁₀L), where W = wet weight in grams and L = standard length in millimeters. The weight-length relationship of juvenile spot of the James River estuary differed statistically from that reported from South Carolina, Mississippi, Louisiana, and Texas. Spot of the James River, Virginia, were found to be heavier per unit length than those of other regions, which suggests that the Chesapeake Bay system provides an optimum environment for growth of 0-year class fish.     

A modeling study of a tidal intrusion front and its impact on larval dispersion in the James River estuary, Virginia

A tidally-induced frontal system regularly develops in a small area off Newport News Point in the lower James River, one of the tributaries of the Chesapeake Bay. In conjunction with the front, a strong counter-clockwise eddy develops on the shoals flanking the northern side of the channel as the result of tidal interaction with the local bathymetry and estuarine stratification. A three-dimensional hydrodynamic model was applied to simulate the eddy evolution and front development, and to investigate time-varying circulation and material transport over a spring-neap tidal cycle. The model results show that variation of tidal range, together with periodic stratification-destratification of the estuary, has a significant impact on the residual circulation of the lower James River. The net surface water circulation, which takes the form of a counterclockwise eddy on the Hampton Flats, is stronger during neap tide than during spring tide. Strong stratification and weak flood current during neap tide results in a dominant ebb flow at the surface, which delays flooding within the channel and advances the phase lead of flood tide on shoals adjacent to the channel, thus increasing both period and intensity of the eddy. Front development in the area off Newport News Point provides a linkage between shoal surface water and channel bottom water, producing a strong net upriver bottom transport. The existence of the vertical transport mechanism was independently demonstrated through tracer experiments. The impact of the dynamics on larval dispersion was investigated through a series of model simulations of the movement of shellfish larvae over multiple tidal cycles following their release at selected bottom sites. These results show that eddy-induced horizontal circulation and vertical transport associated with the frontal system are important mechanisms for the retention of larval organisms in the James River.     

Spatial and temporal variabilities of hypoxia in the Rappahannock River,Virginia

Hypoxia/anoxia in bottom waters of the Rappahannock River, a tributary estuary of Chesapeake Bay, was observed to persist throughout the summer in the deep basin near the river mouth; periodic reoxygenation of bottom water occurred on the shallower sill at the river mouth. The reoxygenation events were closely related to spring tide mixing. The dissolved oxygen (DO) in surface waters was always near or at the saturation level, while that of bottom waters exhibited a characteristic spatial pattern. The bottom DO decreased upriver from river mouth, reaching a minimum upriver of the deepest point of the river and increasing as the water becaume shallower further upriver. A model was formulated to describe the longitudinal distribution of DO in bottom waters. The model is based on Lagrangian concept—following a water parcel as it travels upriver along the estuarine bottom. The model successfully describes the characteristic distribution of DO and also explains the shifting of the minimum DO location in response to spring-neap cycling. A diagnostic study with the model provided insight into relationships between the bottom DO and the competing factors that contribute to the DO budget of bottom waters. The study reveals that both oxygen demand, either benthic or water column demand, and vertical mixing have a promounced effect on the severity of hypoxia in bottom waters of an estary. However, it is the vertical mixing which controls the longitudinal location of the minimum DO. The strength of gravitational circulation is also shown to affect the occurrence of hypoxia. An estuary with stronger circulation tends to have less chance for hypoxia to occur. The initial DO deficit of bottom water entering an estuary has a strong effect on DO concentration near the river mouth, but its effect diminishes in the upriver direction.     

A model study of turbidity maxima in the York River estuary,Virginia

A three-dimensional numerical model is used to investigate the mechanisms that contribute to the formation of the turbidity maxima in the York River, Virginia (U.S.). The model reproduces the basic features in both salinity and total suspended sediments (TSS) fields for three different patterns. Both the prominent estuary turbidity maximum (ETM) and the newly discovered secondary turbidity maximum (STM) are simulated when river discharge is relatively low. At higher river inflow, the two turbidity maxima move closer to each other. During very high river discharge event, only the prominent turbidity maximum is simulated. Diagnostic model studies also suggest that bottom resuspension is an important source of TSS in both the ETM and the STM, and confirm the observed association between the turbidity maxima and the stratification patterns in the York River estuary. The ETM is usually located near the head of salt intrusion and the STM is often associated with a transition zone between upriver well mixed and downriver more stratified water columns. Analysis of the model results from the diagnostic studies indicates that the location of the ETM is well associated with the null point of bottom residual flow. Convergent bottom residual flow, as well as tidal asymmetry, is the most important mechanisms that contribute to the formation of the STM. the STM often exists in a region with landward decrease of bottom residual flow and net landward sediment flux due to tidal asymmetry. The channel depth of this region usually decreases sharply upriver. As channel depth decreases, vertical mixing increases and hence the water column is better mixed landward of the STM.     

Compound paleovalley fills in the Lower Pennsylvanian New River Formation, West Virginia, USA

Ancient paleovalley fills are typically interpreted in the rock record using over-generalized models without carefully considering modern analogs, especially in light of recent discoveries. It is now known that many Quaternary paleovalleys are compound in origin, exhibit considerable stratigraphic complexity, contain multiple incisions, and can be orders of magnitude larger than their putative ancient counterparts. Compound paleovalley fills in the Lower Pennsylvanian New River Formation (NRF) are directly comparable to these Quaternary analogs, stimulating a paradigm shift in the interpretation of ancient paleovalleys. In the NRF, multiple laterally- and vertically-juxtaposed fill successions, separated by incision surfaces, record high-frequency fluvial responses to external controls within lower-order sequences. Lowstand incision and sediment bypass, as predicted in sequence stratigraphy, is largely discounted by the available evidence and the definition of regional sequence boundaries is not straightforward. The identification of genetic sequences may be the most effective approach to understanding the NRF and, by inference, many other ancient paleovalleys. Results from this study of the NRF promote a revised model for ancient paleovalleys that incorporates: 1) the pre-eminence of compound architecture, 2) periodic episodes of incision and subaerial exposure occurring in response to high-frequency changes in climate or relative sea level, 3) fluvial downcutting as the primary cause of paleovalley incision, although some sediments are still preserved in a net-erosional regime, and 4) composite, time-transgressive sequence boundaries that may be difficult or impossible to correlate regionally.     

Resurgence of submersed aquatic macrophytes in the tidal Potomac River,Maryland, Virginia,and the District of Columbia

A 1978–81 survey of submersed aquatic macrophytes in the tidal Potomac River showed that there were virtually no plants in the freshwater tidal river between Chain Bridge and Quantico, Virginia, decades after the disappearance of plants in the late 1930’s. Plant populations were monitored in subsequent years (1983–85) using qualitative shoreline surveys and quantitative resampling of the original 1978–81 transects. In 1983, 12 species of submersed aquatic macrophytes were found in the tidal river. Population increases were dramatic; by fall 1985, plants had colonized all shallow areas between Alexandria and Gunston Cove, Virginia.Hydrilla verticillata dominated in Dyke Marsh-Hunting Creek and Swan Creek. Most other areas contained a variable mixture ofHeteranthera dubia, Myriophyllum spicatum, Ceratophyllum demersum, Vallisneria americana, Najas guadalupensis andHydrilla verticillata. No plants were found along the main river or in tidal embayments in the reach between Gunston Cove and Quantico, Virginia. Total dry weight collected in the upper tidal river in fall 1985 was 14.5 times that of spring 1985, and four times that of fall 1984.     

Population dynamics of spot,leiostomus xanthurus,in polyhaline tidal creeks of the York River estuary,Virginia

Most populations of estuarine-dependent, early life stages of marine fishes are open. As a result, it has been difficult to apply conventional population models to most systems. In this study, a marked population of young-of-year spot (Leiostomus xanthurus) was released into a polyhaline tidal creek within the Guinea Marshes of the York River estuary, Virginia. Over a 90-day study period, 221 marked fishes were recaptured. Plots of the ratio of marked to unmarked individuals (m_i/n_i) in subsequent samples indicated that the population was resident in the creek for up to 162 days with the average individual present for 81 days. When this population turnover rate was compared to the total population decay rate (marked plus unmarked fish), it was determined that exchange between habitats (immigration/emigration) accounted for about 36.4% of the total decay rate, with the remainder attributed to natural mortality. By correcting the overall disappearance rate for population turnover due to emigration and using this adjusted value as a measure of instantaneous mortality (z), the estimated production (over 90 days) in this population was 23,630 cal (98,870 J) per m². This figure agrees with a previously derived estimate for spot in the Guinea marshes and is nearly two orders of magnitude higher than other reported values for this species for all size classes over the entire growing season.     

Spatial and Temporal Distributions of Live and Dead Copepods in the Lower Chesapeake Bay (Virginia,USA)

Hydrography and copepod abundances (Acartia tonsa, Eurytemora affinis, and nauplii) were regularly monitored for 2 years in sub-estuaries of the lower Chesapeake Bay. Copepod vital status was determined using neutral red. Abundances of A. tonsa copepodites and nauplii peaked in late summer and were related to water temperature. E. affinis was present in early fall and winter–spring. Copepod carcasses were a persistent feature in the plankton from 2007 to 2009, with similar annual patterns of occurrence during both years. The relative abundance of carcasses varied among species and developmental stages, with means of 30% dead for stages NI–NIII copepod nauplii, 12–15% for stages NIV–NVI nauplii and A. tonsa copepodites, and 4–8% for E. affinis copepodites. Percent dead was also higher for adult male than female A. tonsa. No strong relationships were found between measured hydrographic variables and percent dead, but the higher percent dead in young nauplii and adult male A. tonsa may indicate greater susceptibility of these stages to death from environmental stressors.     

Contribution of allochthonous carbon to American shad production in the Mattaponi River,Virginia, using stable isotopes

Our objective was to quantify the contribution of autochthonous, locally-produced phytoplankton, and allochthonous, terrestrial-derived organic matter (OM) to the production of young-of-year (YOY) American shad(Alosa sapidissima) using stable isotopes. We measured the carbon and nitrogen stable isotope composition of YOY American shad in the tidal fresh water of the Mattaponi River, a tributary in the York River estuary, during three consecutive years. The isotopic ratios of larval American shad varied among years, indicating a switch from reliance on a primarily autochthonous food web pathway during low and moderate discharge years (50–90%; 2002, 2004) to a primarily allochthonous pathway during a high discharge year (< 35% phytoplankton; 2003). Reliance on phytoplankton by larval fish declined exponentially with increasing Mattaponi River discharge. In 2003, juvenile production was also supported by allochthonous OM, though autochthonous phytoplankton accounted for an increasingly large fraction during June through August, up to 40–55%. We also found a long-term, positive relationship between the duration of above average flow during April through June in the Mattaponi River and a corresponding index of juvenile American shad abundance. The largest American shad cohort recorded since 1967 was observed in 2003, a high discharge year. The production of this cohort was largely supported by allochthonous OM. The results suggest an important link between river discharge, energy flow, and recruitment, wherein high discharge favors reliance on terrestrial carbon by YOY American shad, owing to changes in zooplankton diet, macroinvertebrate abundance, or both, and also favors high American shad abundance.     

Chloride loading in the South Fork of the Shenandoah River,Virginia, U.S.A.

Loading trends and sources of CI^– in the South Fork of the Shenandoah River, Virginia were analyzed for the period 1929–1982. CI^– has increased from approximately 2 mg/L (2,776 tons/yr) to over 10 mg/L (14,256 tons/yr). Natural CI^– is estimated to be 1.01 mg/L (1,388 tons/yr) with precipitation providing 0.99 mg/L and rocks 0.02 mg/L. From 1929 to 1949 CI^– concentrations were relatively constant and independent of discharge, conforming to the Type II curve of Davis and Zobrist (1978), indicative of natural or relatively uncontaminated streams. Since 1952 CI^– concentrations increased exponentially as river discharge decreases conforming to the Type I curve of Davis and Zobrist for polluted streams. Since 1965 anthropogenic CI^– loading at 12,868 tons/yr has remained relatively constant. Four major sources contribute 92.2 percent (11,871 tons/yr) of the anthropogenic CI^–: (1) deicing salts—4,149 tons/yr, (2) domestic sewage—3,015 tons/yr, (3) livestock and poultry wastes—2,458 tons/yr, and (4) commercial fertilizers—2,249 tons/yr.     

Preference ratios for mercury and other chemical elements in the Madeira River, Brazil

Samples of water, suspended solids, and bottom sediments from the Madeira River, Rondônia state, Brazil, were physically and chemically analyzed to investigate the actual Hg mobilization in the aquatic environment and compare it with that of other heavy metals and elements in the area. Two dimensionless Hg preference ratios were defined, expressing (1) the ratio of Hg and other elements in the liquid phase divided by the ratio of Hg and other elements in bottom sediments (P_l.phase) and (2) the ratio of Hg and other elements in the particulate matter divided by the ratio of Hg and other elements in bottom sediments (P_s.solids). These preference ratios are useful for comparing Hg transport in three different phases (liquid, particulate matter, and bottom sediments). They also were applicable to any analyzed element in the area studied, because they generated an almost constant value when the maximum calculated was divided by the minimum (P_l.phase=2931; P_s.solids=84) and because of their sensitivity to the dominance of sorption processes by Fe oxides and hydroxides. Mercury could be transported preferentially to other analyzed elements in the particulate phase only if its concentration reached values at least 10⁴-fold higher than those expected or quantified in the area.     

Delineation of mafic intrusions near Bedford (Virginia,USA) using geological and geophysical methods

In the present work, geological and geophysical methods were used to delineate the locations of multiple mafic intrusions at the Claytor Nature Study Center (CNSC) near Bedford, VA. An investigation of the groundwater hydrology of CNSC was launched in 2007. As a first step in that project a preliminary geological survey revealed sparse evidence of a number of diabase intrusions in the area. While diabase intrusions are not particularly permeable features, contact metamorphism of the host rock could provide conduits for groundwater due to stress fractures and joints and high-temperature recrystallization of the rock matrix. Following the geological survey, geophysical surveys including seismic refraction, ground penetrating radar, and magnetic ground measurements were conducted to determine the location and extent of these multiple igneous intrusions. Seismic and radar surveys proved to be inconclusive, but the magnetic surveys showed strong magnetic anomalies. The magnetic data were obtained using a Geometrics G-856 proton precession magnetometer and were interpreted using the Mag2dc algorithm and SGeMS geostatistical software. The results show that the intrusions are dikes that cut across nearly perpendicular to the regional metamorphic structures and trend generally north–south with a dip of approximately 75°–90° to the west. These findings are consistent with one of the general directions of stresses associated with the North Atlantic seafloor spreading in late Triassic or early Jurassic period. Subsequent hydrologic testing and groundwater modeling confirm the role of the dikes in the overall hydrogeology of the site.     

Impact of major organophosphate pesticides used in agriculture to surface water and sediment quality (Southern Caspian Sea basin,Haraz River)

Organophosphate pesticides are compounds that are not only toxic to both humans and wildlife but also difficult to degrade under natural environmental conditions. In Iran, agricultural practices are strongly dependent on the use of pesticides due to climatic and soil conditions, thus posing a potential risk to groundwater quality standards. Evaluating the concentration of organophosphate pesticides namely diazinon, fenitrothion, dichlorvos, ethion, profenofos, malathion and azinphos methyl in water samples in May (low precipitation rate), December (high precipitation rate) as well as the concentration in sediments along the Haraz River is taken into consideration in this study. Generally the pesticides concentration in water samples are relatively higher in May in comparison with that in December; this fact may be due to two major reasons: the first reason is attributed to the prompt raining after the treatment period of most orchards and dry farming lands that will terminate in more wash out of such pesticides towards the branches and main river channel, while the second reason may be considered as the less river water dilution rate in May because of lower precipitation rate. Furthermore, the relatively higher concentrations in downstream stations may be contributed to more intensified agricultural (specially rice paddies), urban and rural land uses in this region in comparison with upstream areas which contain mainly dry farming, grazing lands and orchards with relatively lower loads of pesticides. Additionally, as the Henry’s law constant of all pesticides considered in this study are relatively low, volatilization may not be regarded as an important route of dissipation. Diazinon, azinphos methyl and dichlorvos showed the highest water concentrations in comparison with other pesticides that may be justified by their extended use within the basin during last decades. Based on the chemical properties as well as remarkably higher values in sediment samples in comparison with water ones, it is concluded that the two pesticides, ethion and fenitrothion, persist in the environment due to non-degradable tendencies. Although the concentration of mentioned pesticides is not so high in the water samples, more precautions must be considered in their future use. Regarding the sorption coefficient variation alongside the river, the higher values in upstream and central parts may be attributed to the higher potential of different types of erosion regarding deeper slopes and also sand, gravel and carbonate mining activities at the banks and also river bed in such regions which is considered as an anthropogenic disturbance. In case of central parts, in addition to mentioned reasons, the existence of coal outcrops in the geologic texture of the study area may also be considered as a key role in augmentation of the sorption coefficient. Finally, the dominant clayey and loamy soils containing more organics may be attributed as the major reason of sorption tendency in downstream.