Tidal Potomac River Research Articles

Hydrophobic organochlorine compounds sequestered in submersed aquatic macrophytes ( Hydrilla verticillata (L.F.) Royle) from the tidal Potomac River (USA)

The potential for hydrophobic organochlorine contaminants to be sequestered in submersed aquatic vegetation was evaluated by determining the concentrations of cis- and trans-chlordane, dieldrin, and polychlorinated biphenyls (PCBs) in feral aquatic macrophytes ( Hydrilla verticillata (L.f.) Royle) collected from the tidal Potomac River. Similarities in mean dry-weight concentrations of the identified organochlorine compounds in H. verticillata and surrounding alluvial sediments indicated that the extent of sequestration in H. verticillata was of the same magnitude as sorption of these compounds to river sediments, but some qualitative differences in PCB congener profiles existed. The results imply that to some degree H. verticillata can influence downstream fluxes of organic contaminants in fluvial transport in the Potomac River, and, furthermore, identify this species as a viable candidate organism for hydrophobic organochlorine contaminant biomonitoring in the Chesapeake Bay estuary.

Invasions and Declines of Submersed Macrophytes in the Tidal Potomac River and Estuary, the Currituck Sound-Back Bay System, and the Pamlico River Estuary

ABSTRACT Long-term changes in biomass, species composition, and distribution of submersed aquatic macrophytes have been documented and studied at two sites in the mid-Atlantic region: the tidal Potomac River and Estuary in Maryland, Virginia, and the District of Columbia, and the Currituck Sound-Back Bay system in Virginia and North Carolina. Additional information based on a shorter time period is available for the Pamlico River Estuary in North Carolina. This paper briefly describes the study areas and summaries the history of declines and increases in each area and factors implicated in these changes. The remainder of the paper is devoted to a discussion of factors influencing invasion/establishment success and the current status of submersed macrophytes in the three areas.

Role of Weather and Water Quality in Population Dynamics of Submersed Macrophytes in the Tidal Potomac River

Weather and water-quality data from 1980 to 1989 were correlated with fluctuations in submersed macrophyte populations in the tidal Potomac River near Washington, D.C., to elucidate causal relationships and explain population dynamics. Both reaches were unvegetated in 1980 when mean growing-season Secchi depths were 0.65 were associated with increases in plant coverage. Changes in mean seasonal Secchi depth were related to changes in mean seasonal TSS and chlorophyll a concentration; mean Secchi depths >0.65 generally occur when seasonal mean TSS is <19 mg l−1 and seasonal mean chlorophyll a concentration is ≤15 μg l−1. Secchi depth is highly correlated with plant growth in the upper tidal river and chlorophyll a and TSS with plant growth in the lower tidal river. Wind speed is an important influence on plant growth in both reaches.

Effects of Submersed Macrophytes on Dissolved Oxygen, pH, and Temperature under Different Conditions of Wind, Tide, and Bed Structure

ABSTRACT Seasonal data on diurnal dissolved-oxygen concentration (DO), pH, temperature and chlorophyll-a were collected and species composition and vertical structure of macrophyte beds were analyzed in the tidal Potomac River during the 1987 growing season. The relationships among these variables and physical and climatic factors were analyzed. Elevated surface temperatures, DO and pH were found in macrophyte beds in June and August; surface temperatures were also elevated in the dense Hydrilla verticillata dominated bed in October-November after senescence had begun. Bottom DO, pH and temperature were lower than surface values. Bottom temperatures in vegetated sites were highly variable compared with bottom temperatures in unvegetated sites. Tide, wind, vegetative structure and available sunshine interacted in a complex fashion to control the magnitude of the diurnal DO, pH and temperature and the stratification of DO, pH and temperature with depth in vegetated sites. Fluctuations in DO and pH at unvege...

Spatial and temporal patterns in a cyanobacterial phytoplankton bloom in the tidal freshwater Potomac River, USA

Spatial and temporal patterns in a cyanobacterial phytoplankton bloom in the tidal freshwater Potomac River, USA

Light Attenuation and Submersed Macrophyte Distribution in the Tidal Potomac River and Estuary

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

Distribution and Abundance of Fishes Associated with Submersed Aquatic Plants in the Potomac River

The distribution and abundance of fishes in submersed aquatic plants of three relative densities (no plants, intermediate plant density, high plant density) were estimated in the tidal Potomac River near Alexandria, Virginia. Fish were sampled with a boat-mounted electroshocker at night in May (when plants were emerging), August (peak plant densities), and November (plant senescence) of 1986. Mean densities of all plants ranged from 9 to 33 g/m2 (dry-weight basis) in May, and 400 to greater than 1,000 g/m2 in August and November. Hydrilla verticillata was usually the dominant aquatic plant. In May, overall mean fish abundance was highest in areas of high plant density (36 fish/5 min shocking), whereas in August and November fish abundance was highest in areas of intermediate plant densities (100 and 62 fish/5 min electroshocking, respectively). Areas without plants contained a relatively high number of filter-feeding fishes, including Atlantic menhaden Brevoortia tyrannus and blueback herring Alosa aestivalis. The fish assemblage in the vegetated sites comprised mainly brown bullhead Ictalurus nebulosus, banded killifish Fundulus diaphanus, pumpkinseed Lepomis gibbosus, largemouth bass Micropterus salmoides, and yellow perch Perca flavescens. The bay anchovy Anchoa mitchilli, white perch Morone americana, and inland silverside Menidia beryllina were distributed throughout all three sites during the study. Fish also were sampled with pop nets in aquatic plants and with seines between shore and the plant beds. More than five times more fish (9.8/m2) were collected with pop nets in areas with intermediate plant density, where there were several codominant plant species, than in areas with dense hydrilla (1.8 fish/m2). Shore-zone fish densities estimated with seine hauls were higher in areas adjacent to dense hydrilla beds (9 fish/m2) than in areas with no plants (1.5/m2) or near intermediate plant densities (3.3/m2), but the number of fish species was lowest near hydrilla.

Effects of Submersed Macrophytes on Water Quality in the Tidal Potomac River, Maryland

ABSTRACT In August, 1986, diurnal measurements of temperature, conductivity, dissolved oxygen and pH, and tide-related measurements of suspended-particulate matter and chlorophyll-a were made at three stations within a dense macrophyte bed and at a fourth unvegetated station outside the bed. Light penetration and current velocity were measured at a hole within the bed and at the unvegetated station. Water temperature, dissolved oxygen and pH increased during the day with the steepest vertical gradients and maximum readings occurring within the bed during midafternoon at low tide. Current velocities within the beds were about one-third of those at the open-water site and water clarity was considerably greater inside the bed. Chlorophyll-a and suspended-particulate concentrations were significantly lower at vegetated stations than at the unvegetated open-water station.

The distribution of nitrogen species and adsorption of ammonium in sediments from the tidal Potomac River and estuary

The distribution of dissolved ammonium, adsorbed ammonium and residual, organic and total nitrogen was measured in Potomac River tidal, transition zone and lower estuary sediments to a depth of 66 cm. For these sediments, exchangeable ammonium, and thereby adsorbed ammonium concentrations, were determined directly using an ammonia electrode in alkaline sediment suspensions. Ammonia electrode data were comparable to data obtained by KCl extraction of fresh sediment. The conventional unitless ammonium adsorption coefficient, calculated as the slope of the regression line drawn when sediment-adsorbed ammonium (μmol g −1 dry wt of sediment) is plotted against interstitial water ammonium (μmol g −1 dry wt sediment), is 1·5 for this system. When a modified ammonium adsorption coefficient is calculated from sediment-adsorbed ammonium concentrations and a ratio of interstitial water ammonium and potassium concentrations, the regression equation through the data has a zero intercept and is more nearly linear than the regression equation of data based on conventional calculations. The use of a ratio including ammonium and potassium concentrations in the interstitial water term takes into account ionic strength variations in the estuary and competition between ammonium and potassium for adsorption sites.

87:6256 Loads of suspended sediment and nutrients from local nonpoint sources to the tidal Potomac River and Estuary, Maryland and Virginia, 1979–1981 water years: Hickman, R.E., 1987. A water-quality study of the tidal Potomac River and Estuary. Wat.-Supply Pap., USGS, 2234-G:35pp

87:6256 Loads of suspended sediment and nutrients from local nonpoint sources to the tidal Potomac River and Estuary, Maryland and Virginia, 1979–1981 water years: Hickman, R.E., 1987. A water-quality study of the tidal Potomac River and Estuary. Wat.-Supply Pap., USGS, 2234-G:35pp

Resurgence of submerged aquatic macrophytes in the tidal Potomac River, Maryland, Virginia, and the District of Columbia : Carter, Virginia and Nancy Rybicki, 1986 Estuaries, 9(4B):368–375. USGS, 430 Natl. Center, Reston, VA 22092, USA

Resurgence of submerged aquatic macrophytes in the tidal Potomac River, Maryland, Virginia, and the District of Columbia : Carter, Virginia and Nancy Rybicki, 1986 Estuaries, 9(4B):368–375. USGS, 430 Natl. Center, Reston, VA 22092, USA

Availability of Ferric Iron for Microbial Reduction in Bottom Sediments of the Freshwater Tidal Potomac River

The distribution of Fe(III), its availability for microbial reduction, and factors controlling Fe(III) availability were investigated in sediments from a freshwater site in the Potomac River Estuary. Fe(III) reduction in sediments incubated under anaerobic conditions and depth profiles of oxalate-extractable Fe(III) indicated that Fe(III) reduction was limited to depths of 4 cm or less, with the most intense Fe(III) reduction in the top 1 cm. In incubations of the upper 4 cm of the sediments, Fe(III) reduction was as important as methane production as a pathway for anaerobic electron flow because of the high rates of Fe(III) reduction in the 0- to 0.5-cm interval. Most of the oxalate-extractable Fe(III) in the sediments was not reduced and persisted to a depth of at least 20 cm. The incomplete reduction was not the result of a lack of suitable electron donors. The oxalate-extractable Fe(III) that was preserved in the sediments was considered to be in a form other than amorphous Fe(III) oxyhydroxide, since synthetic amorphous Fe(III) oxyhydroxide, amorphous Fe(III) oxyhydroxide adsorbed onto clay, and amorphous Fe(III) oxyhydroxide saturated with adsorbed phosphate or fulvic acids were all readily reduced. Fe(3)O(4) and the mixed Fe(III)-Fe(II) compound(s) that were produced during the reduction of amorphous Fe(III) oxyhydroxide in an enrichment culture were oxalate extractable but were not reduced, suggesting that mixed Fe(III)-Fe(II) compounds might account for the persistence of oxalate-extractable Fe(III) in the sediments. The availability of microbially reducible Fe(III) in surficial sediments demonstrates that microbial Fe(III) reduction can be important to organic matter decomposition and iron geochemistry. However, the overall extent of microbial Fe(III) reduction is governed by the inability of microorganisms to reduce most of the Fe(III) in the sediment.

Effect of sediment depth and sediment type on the survival of Vallisneria americana Michx grown from tubers

Sedimentation resulting from storms may have been one of the reasons for the elimination of submersed aquatic vegetation from the tidal Potomac River in the late 1930's. Laboratory studies were conducted to investigate the effects of different depths of overlying sediment and composition of sediment on the survival of Vallisneria americana Michx (wildcelery) grown from tubers. Survival of plants grown from tubers decreased significantly with increasing sediment depth. Survival of tubers declined from 90% or more when buried in 10 cm to no survival in greater than 25 cm of sediment. Survival with depth in sand was significantly lower than in silty clay. Field investigation determined that the majority of tubers in Vallisneria beds are distributed between 10 and 20 cm in depth in silty clay and between 5 and 15 cm in depth in sand. Based on the field distribution of tubers and on the percent survival of plants growing from tubers at each depth in the laboratory experiment, we suggest that the deposition of 10 cm or more of sediment by severe storms such as occurred in the 1930s could contribute to the loss of vegetation in the tidal Potomac River.

Effect of Discharge on the Chlorophyll a Distribution in the Tidally-Influenced Potomac River

In the tidal Potomac River, high river discharges during the spring are associated with high chlorophylla concentrations in the following in the following summer, assuming that summertime light and temperature conditions are favorable. Spring floods deliver large loads of particulate N and P to the tidal river. This particulate N and P could be mineralized by bacteria to inorganic N and P and released to the water column where it is available for phytoplankton use during summertime. However, during the study period relatively low concentrations of chlorophylla (less than 50 μg l−1 occurred in the tidal river if average monthly discharge during July or August exceeded 200 m3s−1. Discharge and other conditions combined to produce conditions favorable for nuisance levels of chlorophylla (greater than 100 μg l−1 approximately one year out of four. Chlorophylla maxima occurred in the Potomac River transition zone and estuary during late winter (dinoflagellates) and spring (diatoms). Typical seasonal peak concentrations were achieved at discharges as high as 970 m3 s−1, but sustained discharges greater than 1,100 m3 s−1 retarded development. Optimum growth conditions occurred following runoff events of 10 to 15 d duration which produced transit times to the transition zone of 7 to 10 d. Wet years with numerous moderate-sized runoff events, such as 1980, tend to produce greater biomass in the transition zone and estuary than do dry years such as 1981.

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.

The effects of grazers and light penetration on the survival of transplants of Vallisneria americana Michs in the tidal Potomac River, Maryland

Poor light penetration and grazing are among the factors potentially responsible for the lack of submersed aquatic macrophytes in the tidal Potomac River. Between 1980 and 1983, plugs, springs and tubers of Vallisneria americana Michx were transplanted from the oligohaline Potomac Estuary to six sites in the freshwater tidal Potomac River. Transplants made in 1980 and 1981 were generally successful only when protected by full exclosures which prevented grazing. Grazing resulted in the removal of whole plants or clipping off of plant leaves in unprotected plots. Plants protected in the first year were permanently established, despite the occurrence of grazing in subsequent years, at Elodea Cove and Rosier Bluff, where light penetration was high (average 1% light level was 1.6–1.7 m). Plants were not permanent;y established at Goose Island, where light penetration was lower (average 1% light level was 1.4 m) and grazing occurred, or Neabsco Bay where light penetration was very low (average 1% light level was 1.0 m) and grazing may not have occurred. In 1983, Secchi depth transparencies in the upper tidal river were improved significantly compared to 1978–1981. Both protected and unprotected transplants thrived in 1983.

The effect of the Asiatic clam, Corbicula fluminea, on phytoplankton of the Potomac River, Maryland

In the summers of the 1960s and 1970s, phytoplankton in the fresh, tidal Potomac River showed a downstream gradient of low to high abundance. Phytoplankton abundance in a 6–8‐km segment of the river, in summer 1980 and 1981, was 40–60% below that upstream and downstream, forming a “sag.”In 1980, the highest densities of the Asiatic clam Corbicula fluminea were in the same reach as the phytoplankton sag. Filtration rates indicate that the volume of the sag reach could be pumped through the Corbicula population in 3–4 days. Corbicula removed 30% of the phytoplankton chlorophyll a from a river water sample in 2 h. Pheophytin a in surficial sediments correlated well with clam biomass as a consequence of phytoplankton excreted in pseudofeces.The hypotheses that peak discharges, zooplankton, toxic substances, and nutrient limitation induced the sag were not supported.

Distribution of submersed aquatic macrophytes in the tidal Potomac River

Results of a 3-year survey (1978–1980) and review of historic trends have shown a major decline in the number of species and the distribution of submersed aquatic macrophytes in the tidal Potomac River since the early 1900's. The freshwater tidal river is essentially devoid of plants and only very sparse populations remain in the mesohaline section of the estuary. Present plant populations are largely confined to the transition-zone region where salinity instability at the fresh-to-brackish water interface is believed to reduce biotic stress on submersed vegetation. Many factors may be implicated in the loss of vegetation over major regions of the tidal Potomac River; however, long-term conditions of excessive nutrients appear to be primarily responsible for the present distribution.

Authigenic Vivianite in Potomac River Sediments: Control by Ferric Oxy-Hydroxides

ABSTRACT Sand-size particles of vivianite (Fe3(PO4)2 · 8H2O) have been identified by X-ray diffraction and scanning electron microscopy in surficial sediments of the tidal Potomac River near a major sewage treatment plant. Vivianitc particles are concentrated in a surface layer of coarse-sand dredge spoil, which overlies much finer sediment similar to that found in adjacent areas. Although saturation indices indicate that the pore waters of virtually all the sediments in the study area are supersaturated with respect to vivianite, it is found only in association with dredge spoil. Pore-water profiles of Fe, P, and Eh, as well as the size and morphology of individual grains, indicate that the vivianite is authigenic and was not introduced with he dredge spoil. It is suggested that the major control on the occurrence of the mineral is the presence or absence of amorphous ferric oxy-hydroxides. which apparently react with pore-water phosphorus to form stable ferric hydroxy-phosphates preferentially to the formation of vivianite.

Nutrient exchange across the sediment-water interface in the Potomac River estuary

The flux of ammonia, phosphate, silica and radon-222 from Potomac tidal river and estuary sediments is controlled by processes occurring at the sediment-water interface and within surficial sediment. Calculated diffusive fluxes range between 0·6 and 6·5 mmol m −2 day −1 for ammonia, 0·020 and 0·30 mmol m −2 day −1 for phosphate, and 1·3 and 3·8 mmol m −2 day −1 for silica. Measured in situ fluxes range between 1 and 21 mmol m −2 day −1 for ammonia, 0·1 and 2·0 mmol m −2 day −1 for phosphate, and 2 and 19 mmol m −2 day −1 for silica. The ratio of in situ fluxes to diffusive fluxes (flux enhancement) varied between 1·6 and 5·2 in the tidal river, between 2·0 and 20 in the transition zone, and from 1·3 to 5·1 in the lower estuary. The large flux enhancements from transition zone sediments are attributed to macrofaunal irrigation. Nutrient flux enhancements are correlated with radon flux enhancements, suggesting that fluxes may originate from a common region and that nutrients are regenerated within the upper 10–20 cm of the sediment column. The low fluxes of phosphate from tidal viver sediments reflect the control benthic sediment exerts on phosphorus through sorption by sedimentary iron oxyhydroxides. In the tidal river, benthic fluxes of ammonia and phosphate equal one-half and one-third of the nutrient input of the Blue Plains sewage treatment plant. In the tidal Potomac River, benthic sediment regeneration supplies a significant fraction of the nutrients utilized by primary producers in the water column during the summer months.