Potomac River Estuary Research Articles

Variable contribution of wastewater treatment plant effluents to downstream nitrous oxide concentrations and emissions

Abstract. Nitrous oxide (N2O), a potent greenhouse gas and ozone-destroying agent, is produced during nitrogen transformations in both natural and human-constructed environments. Wastewater treatment plants (WWTPs) produce and emit N2O into the atmosphere during the nitrogen removal process. However, the impact of WWTPs on N2O emissions in downstream aquatic systems remains poorly constrained. By measuring N2O concentrations at a monthly resolution over a year in the Potomac River estuary, a tributary of the Chesapeake Bay in the eastern United States, we found a strong seasonal variation in N2O concentrations and fluxes: N2O concentrations were larger in fall and winter, but the flux was larger in summer and fall. Observations at multiple stations across the Potomac River estuary revealed hotspots of N2O emissions downstream of WWTPs. N2O concentrations were higher at stations downstream of WWTPs compared to other stations (median: 21.2 nM vs. 16.2 nM) despite the similar concentration of dissolved inorganic nitrogen, suggesting the direct discharge of N2O from WWTPs into the aquatic system or a higher N2O production yield in waters influenced by WWTPs. Meta-analysis of N2O measurements associated with WWTPs globally revealed variable influence of WWTPs on downstream N2O concentrations and emissions. Since wastewater production has increased substantially with the growing population and is projected to continue to rise, accurately accounting for N2O emissions downstream of WWTPs is important for constraining and predicting future global N2O emissions. Efficient N2O removal, in addition to dissolved nitrogen removal, should be an essential part of water quality control in WWTPs.

Alkalinity in Tidal Tributaries of the Chesapeake Bay

AbstractDespite the important role of alkalinity in estuarine carbon cycling, the seasonal and decadal variability of alkalinity, particularly within multiple tidal tributaries of the same estuary, is poorly understood. Here we analyze more than 25,000 alkalinity measurements, mostly from the 1980s and 1990s, in the major tidal tributaries of the Chesapeake Bay, a large, coastal‐plain estuary of eastern North America. The long‐term means of alkalinity in tidal‐fresh waters vary by a factor of 6 among seven tidal tributaries, reflecting the alkalinity of nontidal rivers draining to these estuaries. At 25 stations, mostly in the Potomac River Estuary, we find significant long‐term increasing trends that exceed the trends in the nontidal rivers upstream of those stations. Box model calculations in the Potomac River Estuary indicate that the main cause of the estuarine trends is a declining alkalinity sink. The magnitude of this sink is consistent with a simple model of calcification by the invasive bivalve Corbicula fluminea. More generally, in tidal tributaries fed by high‐alkalinity nontidal rivers, alkalinity is consumed, with sinks ranging from 8% to 27% of the upstream input. In contrast, tidal tributaries that are fed by low‐alkalinity nontidal rivers have sources of alkalinity amounting to 34% to 171% of the upstream input. For a single estuarine system, the Chesapeake Bay has diverse alkalinity dynamics and can thus serve as a laboratory for studying the numerous processes influencing alkalinity among the world's estuaries.

Sources and transformations of anthropogenic nitrogen along an urban river–estuarine continuum

Abstract. Urbanization has altered the fate and transport of anthropogenic nitrogen (N) in rivers and estuaries globally. This study evaluates the capacity of an urbanizing river–estuarine continuum to transform N inputs from the world's largest advanced (e.g., phosphorus and biological N removal) wastewater treatment facility. Effluent samples and surface water were collected monthly along the Potomac River estuary from Washington D.C. to the Chesapeake Bay over a distance of 150 km. In conjunction with box model mass balances, nitrate stable isotopes and mixing models were used to trace the fate of urban wastewater nitrate. Nitrate concentrations and δ15N-NO3− values were higher down-estuary from the Blue Plains wastewater outfall in Washington D.C. (2.25 ± 0.62 mg L−1 and 25.7 ± 2.9 ‰, respectively) compared to upper-estuary concentrations (1.0 ± 0.2 mg L−1 and 9.3 ± 1.4 ‰, respectively). Nitrate concentration then decreased rapidly within 30 km down-estuary (to 0.8 ± 0.2 mg L−1), corresponding to an increase in organic nitrogen and dissolved organic carbon, suggesting biotic uptake and organic transformation. TN loads declined down-estuary (from an annual average of 48 000 ± 5000 kg day−1 at the sewage treatment plant outfall to 23 000 ± 13 000 kg day−1 at the estuary mouth), with the greatest percentage decrease during summer and fall. Annually, there was a 70 ± 31 % loss in wastewater NO3− along the estuary, and 28 ± 6 % of urban wastewater TN inputs were exported to the Chesapeake Bay, with the greatest contribution of wastewater TN loads during the spring. Our results suggest that biological transformations along the urban river–estuary continuum can significantly transform wastewater N inputs from major cities globally, and more work is necessary to evaluate the potential of organic nitrogen and carbon to contribute to eutrophication and hypoxia.

Spatial patterns of plant litter in a tidal freshwater marsh and implications for marsh persistence

The maintenance of marsh platform elevation under conditions of sea level rise is dependent on mineral sediment supply to marsh surfaces and conversion of above- and belowground plant biomass to soil organic material. These physical and biological processes interact within the tidal zone, resulting in elevation-dependent processes contributing to marsh accretion. Here, we explore spatial pattern in a variable related to aboveground biomass, plant litter, to reveal its role in the maintenance of marsh surfaces. Plant litter persisting through the dormant season represents the more recalcitrant portion of plant biomass, and as such has an extended period of influence on ecosystem processes. We conducted a field and remote sensing analysis of plant litter height, aboveground biomass, vertical cover, and stem density (collectively termed plant litter structure) at a tidal freshwater marsh located within the Potomac River estuary, USA. LiDAR and field observations show that plant litter structure becomes more prominent with increasing elevation. Spatial patterns in litter structure exhibit stability from year to year and correlate with patterns in soil organic matter content, revealed by measuring the loss on ignition of surface sediments. The amount of mineral material embedded within plant litter decreases with increasing elevation, representing an important tradeoff with litter structure. Therefore, at low elevations where litter structure is short and sparse, the role of plant litter is to capture sediment; at high elevations where litter structure is tall and dense, aboveground litter contributes organic matter to soil development. This organic matter contribution has the potential to eclipse that of belowground biomass as the root:shoot ratio of dominant species at high elevations is low compared to that of dominant species at low elevations. Because of these tradeoffs in mineral and organic matter incorporation into soil across elevation gradients, the rate of marsh surface elevation change is remarkably consistent across elevation. Because of the role of plant litter in marsh ecosystem processes, monitoring and assessment of these dynamic geomorphic marsh landscapes might be streamlined through the measurement of plant litter structure, either via LiDAR technologies or field observation.

Spatial and temporal patterns of modern (~100 yr) sedimentation in a tidal freshwater marsh: Implications for future sustainability

To maintain their position within the tidal frame, tidal marshes must accrete upward at approximately the same rate as relative sea-level rise (RSLR), typically accomplished through accumulation of mineral (from fluvial, estuarine, and/or coastal waters) and/or organic (plant-derived) sediments. However, accelerating RSLR and declining fluvial suspended-sediment supplies, along with other direct anthropogenic disturbances, may limit the ability of many marshes to keep pace. This study tests hypotheses addressing the relationship of marsh surface elevation and channel influence to marsh accretion and evaluating marsh response to historic mining disturbance and long-term environmental change, using Dyke Marsh Preserve, a tidal freshwater marsh located along the Potomac River estuary (Chesapeake Bay watershed) as a model system. Variability in sediment characteristics (grain size, organic content, bulk density) and decadal-scale accretion rates (using 210Pb) across the marsh platform is related to ecogeomorphic setting, anthropogenic activities (mining), and long-term changes in river sediment supply and RSLR. In general, sediment on river/tidal-channel banks accumulates faster and is more influenced by fluvially derived sediment than in the marsh interior. Accumulation rates on the banks can be predicted using proxies for sediment supply and availability most of the time; however, these relationships do not hold for the marsh interior, highlighting the role of sediment redistribution and/or complex sediment-vegetation interactions. Development of temporally variable sediment rating curves reveals that suspended-sediment loads of the Potomac River have decreased since the late 1970s, although current loads appear adequate to support accretion on the marsh platform, if net onshore sediment-transport mechanisms exist.

Erratum to: Response to L. Land Comment on Bricker, Rice, Bricker (2014) From Headwaters to Coast: Influence of Human Activities on Water Quality of the Potomac River Estuary. Aquatic Geochemistry 20: 291–324

Erratum to: Response to L. Land Comment on Bricker, Rice, Bricker (2014) From Headwaters to Coast: Influence of Human Activities on Water Quality of the Potomac River Estuary. Aquatic Geochemistry 20: 291–324

Oysters’ Impact on Water Quality in the Potomac River Estuary

Oysters’ Impact on Water Quality in the Potomac River Estuary

Response to L. Land Comment on Bricker, Rice, Bricker (2014) From Headwaters to Coast: Influence of Human Activities on Water Quality of the Potomac River Estuary. Aquatic Geochemistry 20: 291–324

Response to L. Land Comment on Bricker, Rice, Bricker (2014) From Headwaters to Coast: Influence of Human Activities on Water Quality of the Potomac River Estuary. Aquatic Geochemistry 20: 291–324

From Headwaters to Coast: Influence of Human Activities on Water Quality of the Potomac River Estuary

The natural aging process of Chesapeake Bay and its tributary estuaries has been accelerated by human activities around the shoreline and within the watershed, increasing sediment and nutrient loads delivered to the bay. Riverine nutrients cause algal growth in the bay leading to reductions in light penetration with consequent declines in sea grass growth, smothering of bottom-dwelling organisms, and decreases in bottom-water dissolved oxygen as algal blooms decay. Historically, bay waters were filtered by oysters, but declines in oyster populations from overfishing and disease have led to higher concentrations of fine-sediment particles and phytoplankton in the water column. Assessments of water and biological resource quality in Chesapeake Bay and tributaries, such as the Potomac River, show a continual degraded state. In this paper, we pay tribute to Owen Bricker’s comprehensive, holistic scientific perspective using an approach that examines the connection between watershed and estuary. We evaluated nitrogen inputs from Potomac River headwaters, nutrient-related conditions within the estuary, and considered the use of shellfish aquaculture as an in-the-water nutrient management measure. Data from headwaters, nontidal, and estuarine portions of the Potomac River watershed and estuary were analyzed to examine the contribution from different parts of the watershed to total nitrogen loads to the estuary. An eutrophication model was applied to these data to evaluate eutrophication status and changes since the early 1990s and for comparison to regional and national conditions. A farm-scale aquaculture model was applied and results scaled to the estuary to determine the potential for shellfish (oyster) aquaculture to mediate eutrophication impacts. Results showed that (1) the contribution to nitrogen loads from headwater streams is small (about 2 %) of total inputs to the Potomac River Estuary; (2) eutrophic conditions in the Potomac River Estuary have improved in the upper estuary since the early 1990s, but have worsened in the lower estuary. The overall system-wide eutrophication impact is high, despite a decrease in nitrogen loads from the upper basin and declining surface water nitrate nitrogen concentrations over that period; (3) eutrophic conditions in the Potomac River Estuary are representative of Chesapeake Bay region and other US estuaries; moderate to high levels of nutrient-related degradation occur in about 65 % of US estuaries, particularly river-dominated low-flow systems such as the Potomac River Estuary; and (4) shellfish (oyster) aquaculture could remove eutrophication impacts directly from the estuary through harvest but should be considered a complement—not a substitute—for land-based measures. The total nitrogen load could be removed if 40 % of the Potomac River Estuary bottom was in shellfish cultivation; a combination of aquaculture and restoration of oyster reefs may provide larger benefits.

Rates and Probable Causes of Freshwater Tidal Marsh Failure, Potomac River Estuary, Northern Virginia, USA

Dyke Marsh, a distal tidal marsh along the Potomac River estuary, is diminishing rapidly in areal extent. This study documents Dyke Marsh erosion rates from the early-1860s to the present during pre-mining, mining, and post-mining phases. From the late-1930s to the mid-1970s, Dyke Marsh and the adjacent shallow riverbottom were mined for gravel, resulting in a ~55 % initial loss of area. Marsh loss continued during the post-mining phase (1976–2012). Causes of post-mining loss were unknown, but were thought to include Potomac River flooding. Post-mining areal-erosion rates increased from 0.138 ha yr−1 (~0.37 ac yr−1) to 0.516 ha yr−1 (~1.67 ac yr−1), and shoreline-erosion rates increased from 0.76 m yr−1 (~2.5 ft yr−1) to 2.60 m yr−1 (~8.5 ft yr−1). Results suggest the accelerating post-mining erosion reflects a process-driven feedback loop, enabled by the marsh's severely-altered geomorphic and hydrologic baseline system; the primary post-mining degradation process is wave-induced erosion from northbound cyclonic storms. Dyke Marsh erosion rates are now comparable to, or exceed, rates for proximal coastal marshes in the same region. Persistent and accelerated erosion of marshland long after cessation of mining illustrates the long-term, and potentially devastating, effects that temporally-restricted, anthropogenic destabilization can have on estuarine marsh systems.

Multi-decade Responses of a Tidal Creek System to Nutrient Load Reductions: Mattawoman Creek, Maryland USA

We developed a synthesis using diverse monitoring and modeling data for Mattawoman Creek, Maryland, USA to examine responses of this tidal freshwater tributary of the Potomac River estuary to a sharp reduction in point-source nutrient loading rate. Oligotrophication of these systems is not well understood; questions concerning recovery pathways, threshold responses, and lag times remain to be clarified and eventually generalized for application to other systems. Prior to load reductions Mattawoman Creek was eutrophic with poor water clarity (Secchi depth <0.5 m), no submerged aquatic vegetation (SAV), and large algal stocks (50–100 μg L−1 chlorophyll-a). A substantial modification to a wastewater treatment plant reduced annual average nitrogen (N) loads from 30 to 12 g N m−2 year−1 and phosphorus (P) loads from 3.7 to 1.6 g P m−2 year−1. Load reductions for both N and P were initiated in 1991 and completed by 1995. There was no trend in diffuse N and P loads between 1985 and 2010. Following nutrient load reduction, NO2 + NO3 and chlorophyll-a decreased and Secchi depth and SAV coverage and density increased with initial response lag times of one, four, three, one, and one year, respectively. A preliminary N budget was developed and indicated the following: diffuse sources currently dominate N inputs, estimates of long-term burial and denitrification were not large enough to balance the budget, sediment recycling of NH4 was the single largest term in the budget, SAV uptake of N from sediments and water provided a modest seasonal-scale N sink, and the creek system acted as an N sink for imported Potomac River nitrogen. Finally, using a comparative approach utilizing data from other shallow, low-salinity Chesapeake Bay ecosystems, strong relationships were found between N loading and algal biomass and between algal biomass and water clarity, two key water quality variables used as indices of restoration in Chesapeake Bay.

Modeling the pH in the tidal fresh Potomac River under conditions of varying hydrology and loads

The pH of the freshwater portion of the Potomac River estuary attains 9–10.5, driven by photosynthesis during cyanobacteria blooms. Processes which contribute to elevated pH are examined by adding a mass-balance model of the carbonate cycle to an existing eutrophication model. Four new variables are added to the model suite: alkalinity, total inorganic carbon, total calcium, and calcium carbonate. The pH is computed from these four quantities via equilibrium kinetics. The model is employed in a continuous simulation of the years 1994–2000. Emphasis in examination of model results is placed on the tidal fresh portion of the system where elevated pH is an environmental concern. Model sensitivity analysis indicates hydrology has the greatest influence on pH. During low-flow periods, residence time is lengthy allowing ample time for algal production to occur. The production stimulates net uptake of TIC, and results in enhanced pH.

Growing Zostera marina (eelgrass) from Seeds in Land-Based Culture Systems for Use in Restoration Projects

The use of aquaculture systems to grow the seagrass Zostera marina (eelgrass) from seeds for restoration projects was evaluated through laboratory and mesocosm studies. Along the mid-Atlantic coast of North America Z. marina seeds are shed from late spring through early summer, but seeds typically do not begin to germinate until the late fall. Fall is the optimal season to plant both seeds and shoots in this region. We conducted studies to determine if Z. marina seeds can be induced to germinate in the summer and seedlings grown in mesocosms to a size sufficiently large enough for out-planting in the fall. Seeds in soil-less culture germinated in the summer when held at 14°C, with percent germination increasing with lower salinities. Cold storage (4°C) of seeds prior to planting in sediments enhanced germination and seedling survival. Growth rates of seedlings were significantly higher in nutrient enriched estuarine sediments. Results from preliminary studies were used in designing a large-scale culture project in which 15,000 shoots were grown and out-planted into the Potomac River estuary in the Chesapeake Bay and compared with an equal number of transplanted shoots. These studies demonstrate that growing Z. marina from seeds is an alternative approach to harvesting plants from donor beds when vegetative shoots are required for restoration projects.

Transport and Fate Model for PCB Total Maximum Daily Loads (TMDLs) in the Potomac River Estuary

Transport and Fate Model for PCB Total Maximum Daily Loads (TMDLs) in the Potomac River Estuary

Evaluating a Large-Scale Eelgrass Restoration Project in the Chesapeake Bay

Approximately 90,000 shoots of eelgrass (Zostera marina) were planted over 3 years (2003–2005) at Piney Point (PP) in the lower Potomac River estuary in the Chesapeake Bay (mid-Atlantic coast of North America) following 3 years of habitat evaluation using a Preliminary Transplant Suitability Index (PTSI) and test plantings. Initial survival was high for the 2003 and 2004 plantings; however, most of the eelgrass died during the summer following the fall planting. Habitat quality and restoration success were monitored for the 2005 plantings and compared to a nearby restoration site (St. George Island [SGI]). Eelgrass planted at PP in the fall of 2005 declined through the summer of 2006 with some recovery in the spring of 2007, but was gone by the end of the summer of 2007. The summer decline from late July to mid-August of 2006 coincided with water temperatures greater than 30°C, hypoxic oxygen (0–3 mg/L) concentrations, and low percent light at leaf level (PLL < 15%). Epiphyte loads were much heavier at PP than at SGI, despite similar water quality. We suggest that this was the result of higher wave exposure at PP. All of these factors are likely to have contributed to the mortality of the 2005 plantings. Submerged aquatic vegetation habitat quality based on the PTSI, median PLL during the growing season, and test plantings did not explain the decline of the plantings. Restoration site selection criteria should be expanded to include the effects of wave exposure on self-shading and epiphyte loads, and the potential for both short-term exposures to stressful conditions and long-term changes in habitat quality.

The use of antibiotics to reduce bacterioplankton uptake of phytoplankton extracellular organic carbon (EOC) in the Potomac River estuary

Phytoplankton production and accumulation of extracellular organic carbon (EOC) was tracked during diel intervals in microcosms by inhibiting bacterioplankton assimilation of EOC with streptomycin and kanamycin. Bacterioplankton production ( 3H-thymidine incorporation) and metabolism ( 14C-glucose incorporation) were monitored in samples collected from the Potomac River estuary to determine the effect of the antibiotics. Particulate (i.e., raw water) primary production and EOC (i.e., water passing through 1.0 μm glass fiber filter) production rates were monitored to determine the impact of antibiotics on phytoplankton. In preliminary experiments, neither streptomycin nor kanamycin alone significantly inhibited bacterioplankton activity compared to controls, but when both were present secondary production and metabolism were reduced up to 90%, and remained as such for 45 h. During field evaluations using a streptomycin and kanamycin mixture (50 μM each) particulate primary production and EOC production were not statistically different in control and antibiotic treated samples indicating that the antibiotics did not negatively influence phytoplankton production rates. In the presence of antibiotics dissolved free amino acids (DFAA) and, to a lesser extent, monosaccharides were significantly elevated compared to controls. This study demonstrates that streptomycin and kanamycin are capable of inhibiting bacterioplankton metabolism and uptake of dissolved organic carbon (DOC) in the samples tested so that the contribution of EOC to the DOC pool and to bacterioplankton metabolism could be measured and assessed.

Optimal Wasteload Allocation Procedure for Achieving Dissolved Oxygen Water Quality Objectives. II: Optimal Load Control

This paper presents the development of an efficient strategy for achieving in-stream dissolved oxygen (DO) water quality standards (WQSs) via optimized point-load control strategies using the adjoint method. To this end, a least-squares-type objective function is formulated that measures the difference between desired (WQSs) and current DO concentrations at strategically selected monitoring points in the domain. The goal is to minimize the difference between actual DO concentration and the WQS, hence allowing time-variant loadings to utilize the assimilative capacity of the receiving water body at an optimal level. Time-variant discharge rates for a number of discharge locations are considered as control parameters, while different zone-specific critical DO levels are imposed as constraints. The selection of the control is kept flexible and a number of different scenarios are tested. First, only carbonaceous biochemical oxygen demand is used, which allows for a reduction of the number of equations that need to be solved. In the other tests, all constituents are switched on and different variables at each load node are selected as a control by first varying the concentrations individually, and then linking them through control of the volumetric flow rate. Optimization is achieved using a conjugate gradient search method, for which the gradients are computed through the solution of both the direct and adjoint problems. It is shown that the large amount of gradient information (parameter space has a dimension of several thousands) can be computed very efficiently using the adjoint, and that optimized results are achieved after only a few iterations irrespective of the initial guess. Computations are carried out using both two-dimensional model formulation applied to a long rectangular channel with varying width and slope and a model for the upper Potomac River estuary.

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.

Zooplankton Grazing in a Potomac River Cyanobacteria Bloom

During summer, bloom-forming cyanobacteria, including Anacystis, Aphanizomenon, and Microcystis aeruginosa, dominate tidal-fresh waters of the upper Potomac River estuary with densities exceeding 108 cells l−1. In an attempt to determine the importance of these high cyanobacteria densities to planktonic herbivory in the system, short-term grazing experiments were conducted in July and August 1987. Using size-fractionated river phytoplankton assemblages, zooplankton grazing rates were determined for dominant or subdominant planktonic microzooplankton and mesozooplankton feeding on 14C-labeled river assemblages, 14C-labeled river assemblages enriched with unlabeled cyanobacteria, and unlabeled river assemblages enriched with 14C-labeled cyanobacteria. Grazing rates were estimated for the rotifers Polyarthra remata, Hexarthra mira, Asplanchna brightwelli, Brachionus angularis, Epiphanes sp., Trichocerca similis, and the cyclopoid copepod Cyclops vernalis. Neither rotifers nor the copepod grazed heavily on Microcystis. Rotifer grazing rates on labeled cyanobacteria ranged from 4 to 1,650 nl· [individual · h]−1 while copepod rates ranged from undetectable to 135 μl · [copepod · h]−1. Grazing rates on labeled river phytoplankton assemblages were 4–100 times higher than noted for zooplankton feeding on cyanobacteria. The addition of the colonial alga to labeled river phytoplankton assemblages resulted in mixed zooplankton responses, that is, lower and higher grazing rates than observed on river assemblages with no added cyanobacteria. Total zooplankton demand for cyanobacteria and river phytoplankton assemblages was estimated for the study period July–August 1987. Rotifer plus C. vernalis herbivory would have removed 1–5% and 49%, respectively, of the standing stock of the two autotroph pools each day. Literature-derived clearance rates for Bosmina indicate, however, that herbivory by this cladoceran could increase demand to 24% and 60%, respectively, in bloom and nonbloom assemblages. These data suggest that the majority of cyanobacterial production remains ungrazed and may be transported to the lower estuary for salinity-induced aggregation and sedimentation.

Effects of increasing salinity on a cyanobacteria bloom in the Potomac River estuary

From June through August, 1985, a bloom of the cyanobacterium Microcyslis aeruginosa was observed in the upper Potomac River reaching densities of 193 × 106 cells 1−1 and 83% of total cells in the surface mixed layer. However, in regions typified by salinities of 1–2 p.p.t. immediately down-river, the alga disappeared from the phytoplankton assemblage, never contributing >17% of total phytoplankton numbers. In an attempt to determine the effect of increasing salinity on limiting horizontal distributions of cyanobacteria blooms in the Potomac River estuary, three laboratory experiments were conducted. Bloom samples collected from the river were exposed to daily salinity increases of 1–2 p.p.t. through the addition of NaCl or a complement of full sea salts. Relative to samples receiving no salt supplement, densities of Microcyslis spp. declined in salinity-stressed samples. However, total cyanobacteria remained constant or increased due to rapid growth of Aphanizomenon. The addition of a mixture of full sea salts favored aggregation of cyanobacteria, yielding an average of 132 cells colony−1; ≤36 cells colony−1 were observed in populations maintained in river water and NaCl-supplemented samples. Chlorophyll concentrations declined slightly relative to assemblages receiving no salt additions while carbon fixation was reduced in salinity-stressed assemblages. These results suggest that salinities from 0.5 to 7 p.p.t., typical of the oligohaline region of the Potomac River, could limit the distribution of Microcystis-dominated blooms down-estuary. In addition, increasing salinities should result in the aggregation of photosynthetically inhibited cells and, through prolonged exposure to increasing osmotic stress, produce large, rapidly sinking detrital particles supporting microbial decomposition in oligohaline and mesohaline regions of the lower Potomac River estuary, perhaps perpetuating high microbial oxygen demand and anoxia in this portion of the river/estuary.