Chesapeake Bay Subestuaries Research Articles

A subaqueous soil‐landscape conceptual model to guide soil survey in Chesapeake Bay subestuaries

AbstractRhodeRiver, a subestuary on the western shore of Chesapeake Bay, contains a diverse array of subaqueous soils that range from submerged paleosols to fine‐textured fluid soils and organic soils. A subaqueous soil survey was completed for the Rhode River subestuary by collecting bathymetric data, delineating landforms, and sampling and describing soils across the submerged landscape. Soil map units were developed by correlating soil properties and taxonomic classification with delineated landforms, resulting in the development of seven proposed soil series corresponding to new soil map units. Geologic maps and other supporting information about the dominant factors of soil formation in this landscape were used with the soil survey of Rhode River to develop a conceptual subaqueous soil‐landscape model of soil genesis to explain the origin and distribution of soils in Rhode River. This is the first time that a subaqueous soil‐landscape model has been developed for the flooded river valley geomorphic setting of Chesapeake Bay, and it will assist subaqueous soil surveys in other western shore Chesapeake Bay subestuaries and similar environmental settings where upland environments have been or are currently submerging.

Contributions of Organic and Mineral Matter to Vertical Accretion in Tidal Wetlands across a Chesapeake Bay Subestuary

Persistence of tidal wetlands under conditions of sea level rise depends on vertical accretion of organic and inorganic matter, which vary in their relative abundance across estuarine gradients. We examined the relative contribution of organic and inorganic matter to vertical soil accretion using lead-210 (210Pb) dating of soil cores collected in tidal wetlands spanning a tidal freshwater to brackish gradient across a Chesapeake Bay subestuary. Only 8 out of the 15 subsites had accretion rates higher than relative sea level rise for the area, with the lowest rates of accretion found in oligohaline marshes in the middle of the subestuary. The mass accumulation of organic and inorganic matter was similar and related (R2 = 0.37). However, owing to its lower density, organic matter contributed 1.5–3 times more toward vertical accretion than inorganic matter. Furthermore, water/porespace associated with organic matter accounted for 82%–94% of the total vertical accretion. These findings demonstrate the key role of organic matter in the persistence of coastal wetlands with low mineral sediment supply, particularly mid-estuary oligohaline marshes.

Phenotypic plasticity in life-history characteristics of invasive blue catfish, Ictalurus furcatus

Phenotypic plasticity in life-history characteristics of invasive blue catfish, Ictalurus furcatus

Spatial and Temporal Variation in Brackish Wetland Seedbanks: Implications for Wetland Restoration Following Phragmites Control

Chesapeake Bay tidal wetlands are experiencing a broad-scale, aggressive invasion by the non-native, clonal grass Phragmites australis. The grass is often managed with herbicides in efforts to restore native plant communities and wildlife habitat. Management efforts, however, can act as a disturbance, resulting in increased light availability, potentially fostering reinvasion from soil seedbanks. If native vegetation establishes quickly from seedbanks, the site should have greater resiliency against invasion, while disturbed sites where native plants do not rapidly establish may be rapidly colonized by P. australis. We surveyed the soil seedbank of three vegetation cover types in five Chesapeake Bay subestuaries: areas where P. australis had been removed, where P. australis was left intact, and with native, reference vegetation. We determined the total germination, the proportion of the seedbank that was attributable to invasive species, the richness, the functional diversity, and the overall composition of the seedbanks in each of the cover types (i.e., plots). After 2 years of herbicide treatment in the P. australis removal plots, vegetation cover type impacted the total germination or the proportion of invasive species in the seedbank. In contrast, we also found that seedbank functional composition in tidal brackish wetlands was not influenced by vegetation cover type in most cases. Instead, plots within a subestuary had similar seedbank functional composition across the years and were composed of diverse functional groups. Based on these findings, we conclude that plant community recovery following P. australis removal is not seed-limited, and any lack of native vegetation recruitment is likely the result of yet-to-be-determined abiotic factors. These diverse seedbanks could lead to resilient wetland communities that could resist invasions. However, due to the prevalence of undesirable species in the seedbank, passive revegetation following invasive plant removal may speed up their re-establishment. The need for active revegetation will need to be assessed on a case-by-case basis to ensure restoration goals are achieved.

Tidal Habitats Support Large Numbers of Invasive Blue Catfish in a Chesapeake Bay Subestuary

The introduction of a non-native freshwater fish, blue catfish Ictalurus furcatus, in tributaries of Chesapeake Bay resulted in the establishment of fisheries and in the expansion of the population into brackish habitats. Blue catfish are an invasive species in the Chesapeake Bay region, and efforts are underway to limit their impacts on native communities. Key characteristics of the population (population size, survival rates) are unknown, but such knowledge is useful in understanding the impact of blue catfish in estuarine systems. We estimated population size and survival rates of blue catfish in tidal habitats of the James River subestuary. We tagged 34,252 blue catfish during July–August 2012 and 2013; information from live recaptures (n = 1177) and dead recoveries (n = 279) were used to estimate annual survival rates and population size using Barker’s Model in a Robust Design and allowing for heterogeneity in detection probabilities. The blue catfish population in the 12-km study area was estimated to be 1.6 million fish in 2013 (95% confidence interval [CI] adjusted for overdispersion: 926,307–2,914,208 fish). Annual apparent survival rate estimates were low: 0.16 (95% CI 0.10–0.24) in 2012–2013 and 0.44 (95% CI 0.31–0.58) in 2013–2014 and represent losses from the population through mortality, permanent emigration, or both. The tagged fish included individuals that were large enough to exhibit piscivory and represented size classes that are likely to colonize estuarine habitats. The large population size that we estimated was unexpected for a freshwater fish in tidal habitats and highlights the need to effectively manage such species.

Oyster (Crassostrea virginica [Gmelin, 1791]) Mortality at Prolonged Exposures to High Temperature and Low Salinity

ABSTRACT Mortality of two size classes ( 35 mm) of eastern oysters Crassostrea virginica when exposed to combinations of low salinity (1, 2, 3, and 4) for extended periods (up to 30 days) at summer water temperatures typical of the Virginia Chesapeake Bay subestuaries was examined. A critical salinity-temperature combination of less than two at greater than 28°C for more than 1 wk exposure for oyster mortality is suggested. A review of limited historical salinity-temperature tolerance data suggest selection of local populations of oysters having differing salinity tolerances. Such selection may prove critical to persistence of low-salinity populations in the Chesapeake Bay subestuaries with projected climate change.

Phosphorus Sequestration in Sediments Along the Salinity Gradients of Chesapeake Bay Subestuaries

We used a sequential extraction technique to compare the forms and amounts of particulate phosphorus (PP) deposited in the top meter of sediment spanning salinities from 0 to 10 in three Chesapeake Bay subestuaries: the Potomac, Choptank, and Bush Rivers. Fe-bound P (i.e., citrate-bicarbonate-dithionite (CDB)-extractable P) was the most dynamic fraction of PP, dominating oligohaline (salinity <3) sediments, but declining to near zero with depth in the most saline sediments of all three subestuaries. In contrast, we previously found Fe-P dominating the sediment PP at salinities ranging 0–11 in the Patuxent subestuary. Particulate organic P was relatively constant with depth and salinity and became the dominant form of PP in the most saline sediments. Fe-P was not replaced with diagenetic authigenic carbonate fluorapatite. In all the subestuaries, Fe-P generally persisted in the oligohaline sediments to 1 m depth, where sediment ages ranged from 60 to 200 years, based on 210Pb dating. At one site, PP burial reflected changing P loads from a nearby wastewater treatment plant. About 67% of the PP discharged from the Potomac watershed at the head of tide was buried in the sediments in the upper reaches of that subestuary. The form and amount of PP buried depended on the amount of CDB-extractable Fe in the sediments and on the distribution of sediment deposition along the salinity gradient. Even though CDB targets iron oxides, the predominant form of Fe in the sediments was ferrous iron suggesting that the predominant form of PP in oligohaline sediments may be ferrous phosphate.

Tidal Marshes across a Chesapeake Bay Subestuary Are Not Keeping up with Sea-Level Rise.

Sea-level rise is a major factor in wetland loss worldwide, and in much of Chesapeake Bay (USA) the rate of sea-level rise is higher than the current global rate of 3.2 mm yr-1 due to regional subsidence. Marshes along estuarine salinity gradients differ in vegetation composition, productivity, decomposition pathways, and sediment dynamics, and may exhibit different responses to sea-level rise. Coastal marshes persist by building vertically at rates at or exceeding regional sea-level rise. In one of the first studies to examine elevation dynamics across an estuarine salinity gradient, we installed 15 surface elevation tables (SET) and accretion marker-horizon plots (MH) in tidal freshwater, oligohaline, and brackish marshes across a Chesapeake Bay subestuary. Over the course of four years, wetlands across the subestuary decreased 1.8 ± 2.7 mm yr-1 in elevation on average, at least 5 mm yr-1 below that needed to keep pace with global sea-level rise. Elevation change rates did not significantly differ among the marshes studied, and ranged from -9.8 ± 6.9 to 4.5 ± 4.3 mm yr-1. Surface accretion of deposited mineral and organic matter was uniformly high across the estuary (~9–15 mm yr-1), indicating that elevation loss was not due to lack of accretionary input. Position in the estuary and associated salinity regime were not related to elevation change or surface matter accretion. Previous studies have focused on surface elevation change in marshes of uniform salinity (e.g., salt marshes); however, our findings highlight the need for elevation studies in marshes of all salinity regimes and different geomorphic positions, and warn that brackish, oligohaline, and freshwater tidal wetlands may be at similarly high risk of submergence in some estuaries.

Archaeal rhizosphere communities differ between the native and invasive lineages of the wetland plant Phragmites australis (common reed) in a Chesapeake Bay subestuary

Phragmites australis is a common wetland plant species worldwide and best known in North America as a persistent invasive species. Only in recent decades was a native lineage, Phragmites australis subsp. americanus, confirmed in North American wetlands. This study investigated whether the two lineages support unique microbial communities in the rhizosphere. Paired stands of the native and invasive lineages were sampled at four locations across a Chesapeake Bay subestuary to assess differences in soil chemical parameters and the microbial community. Microbial characterization included quantification of genes coding for bacterial and archaeal 16S rRNA, archaeal ammonia-monooxygenase, methane co-enzyme A, and particulate methane-monooxygenase. These gene targets helped quantify bacterial biomass, archaeal biomass, ammonia-oxidizers, methanogens, and methane oxidizers, respectively. Bacterial and archaeal community composition was determined by creating terminal restriction fragment length polymorphism profiles of the 16S rRNA, and bacterial and fungal composition was assessed using phospholipid fatty acid profiles. Bacterial biomass and composition did not differ between the two lineages, but significant differences were observed in archaeal community composition. There were significantly more copies of archaeal 16S rRNA genes in invasive (1.8 × 108 g−1 soil) compared to native (7.2 × 107 g−1 soil) lineages (P = 0.04) and composition profiles were significantly different according to PerMANOVA (P = 0.006). Phospholipid fatty acid profiles also differed between invasive and native lineages (P = 0.023). These findings support other studies that have noted the ability of native and invasive Phragmites to differentially affect soil biogeochemistry and suggest that archaea in the rhizosphere of wetland plants may be more sensitive to plant variation than bacteria.

Stand Age is Associated with Clonal Diversity, but Not Vigor, Community Structure, or Insect Herbivory in Chesapeake Bay Phragmites australis

Invasions are dynamic as both the invading organism and the invaded ecosystem change. Intrinsic changes to the invader (invasion process) can involve population level genetic and reproductive changes. Extrinsic changes (invasion effect) occur to the environment that is invaded (e.g., alterations to the physical environment), to the invaded plant community (e.g., changes in species diversity and composition or evolutionary changes), or to insect herbivory. To investigate how invasions change through time, we investigated both the process and effect of a Phragmites australis invasion by comparing young and old P. australis stands within two Chesapeake Bay subestuaries. We quantified clonal richness of P. australis stands, vigor of the invader, herbivore damage, and plant community composition. Our results indicate that only the population-scale genetics (clonal richness, genetic distance) changed over the course of 40 years. Clonal richness was lower in the old P. australis stands, likely due to intraspecific competition and/or initial colonization by fewer genotypes. The mean genetic distance among clones within old stands was lower than within young stands, suggesting that clones within old stands were mostly closely-related, while young stands were likely established by seeds from nearby stands and so were more representative of the local area. Clones in different old stands were genetically more distant from each other than those in young stands were from clones in other young stands. This pattern suggested that old stands were established by independent colonization events, while young patches were established by a mixture of seeds from local stands, which generated the lower average genetic distance between clones across young stands. We found that community composition, plant vigor, and herbivore damage to stems were similar across different age stands, which indicated that the effect of P. australis invasion becomes stable within a few decades. Over longer periods, the intrinsic invasion process may be more dynamic than the invasion effect.

Wetland plant species richness across estuarine gradients: The role of environmental factors and the mid-domain effect

Wetland plant species richness across estuarine gradients: The role of environmental factors and the mid-domain effect

Nitrogen and Phosphorus Differentially Affect Annual and Perennial Plants in Tidal Freshwater and Oligohaline Wetlands

To test species composition and biomass responses to excess nutrients, herbaceous plants of tidal freshwater and oligohaline wetlands in a Chesapeake Bay subestuary were fertilized with nitrogen (N), phosphorus (P), both N and P (N + P), or not fertilized (Control) for 4 years. In marshes, the N treatment increased abundance measures of perennials but decreased those of annuals while the P treatment increased annuals and decreased perennials. In swamps, however, perennials increased in response to P. Total herbaceous aboveground biomass production was not limited by N, P, or N + P in marshes or swamps. These findings suggest that annual species are more susceptible than perennials to P limitation, possibly due to lack of a large perenniating root organ and lower susceptibility to mycorrhizal inoculation. Furthermore, eutrophication effects are likely to vary between swamp and marsh habitats and depend on whether the dominant nutrient supplied is nitrogen or phosphorus.

Impervious Surface, Summer Dissolved Oxygen, and Fish Distribution in Chesapeake Bay Subestuaries: Linking Watershed Development, Habitat Conditions, and Fisheries Management

Abstract We estimated target and limit impervious surface reference points (ISRPs) based on Chesapeake Bay dissolved oxygen (DO) criteria, and we examined associations and relationships among the percentage of watershed in impervious surface (IS), summer DO, and the presence of indicator species (blue crab Callinectes sapidus, white perch Morone americana, striped bass M. saxatilis, and spot Leiostomus xanthurus) in bottom waters of nine brackish subestuaries of Chesapeake Bay. Ideally, a target ISRP represented a level of development that maintained mean bottom DO at 5 mg/L or greater, while an ISRP threshold represented development that degraded mean bottom DO to less than 3 mg/L. The proportion of bottom trawls containing each indicator species rapidly declined from about 0.40 to 0.10 when DO fell below 3 mg/L, whereas the proportion remained at about 0.50 when DO was above 5 mg/L. The IS percentage had a significant negative influence on mean bottom DO and the odds that indicator species were present in midchannel bottom waters (0.8–7.0 m deep). Watersheds at or below a target IS of 5.5% (rural watershed) maintained mean bottom DO above 3 mg/L, but mean DO was only occasionally at or above 5 mg/L. Mean DO seldom exceeded 3 mg/L in watersheds with an IS value above 10% (suburban threshold). Comprehensive watershed management will be needed to offset significant degradation of bottom-water fish habitat in brackish subestuaries if rural lands are converted to suburban areas. Received July 6, 2010; accepted March 23, 2011

Demographics and Parasitism of American Eels in the Chesapeake Bay, USA

AbstractThe Chesapeake Bay supports the largest U.S. harvest of American eels Anguilla rostrata, yet little is known about the underlying demographics and production rates that sustain these harvests. Chesapeake Bay subestuaries (Sassafras, Chester, Choptank, Patuxent, Potomac, and James rivers) are expected to provide productive but varying growth habitats for yellow‐stage American eels due to differences in land use, prey availability, and salinity. We compared length, age, growth, mortality, condition, and health (prevalence and incidence of parasitism by the swim bladder nematode Anguillicola crassus) of yellow American eels (n = 850) in the six principal subestuaries. As has been observed for other systems, female American eels in the Chesapeake Bay were larger, older, and heavier and had higher growth rates than male, intersexual, or undifferentiated American eels. Prevalence of male and intersexual individuals in the upper bay region was higher than that in the lower bay and higher than that reported for estuaries in South Carolina, Quebec, and the Hudson River, New York. American eel demographic attributes (gender, length, weight, condition, age, growth, and parasitism) differed substantially among the subestuaries. Individual growth rates ranged from 26.7 to 149.3 mm/year; the Choptank River had the highest mean growth rate (72.7 mm/year), and the Chester River had the lowest (60.2 mm/year). Estimated instantaneous loss rates (0.52–1.01 per year) did not vary systematically among subestuaries. Prevalence of parasitized American eels ranged from 17.8% to 72.0% and was higher in the upper bay subestuaries than in the lower bay. Swim bladder damage and Anguillicola crassus presence were not associated with American eel age or growth rate. In summary, female prevalence, growth rates, and condition were lower and parasite prevalence and intensity were higher in the less‐saline upper bay than in the lower bay, suggesting fundamental differences in the productivity and spawning contributions between these two regions.

Effects of hypoxia on Mnemiopsis leidyi, ichthyoplankton and copepods: clearance rates and vertical habitat overlap

Differences in predator and prey tolerances to low dissolved oxygen (DO) concentra- tions influence planktonic food web interactions in seasonally hypoxic environments. We examined low-DO effects on predation by hypoxia-tolerant ctenophores, Mnemiopsis leidyi, on less hypoxia- tolerant ichthyoplankton prey in laboratory experiments. We also examined the relationship between bottom DO concentrations and vertical distributions, and considered potential effects of biotic (pre- dators and competitors) versus abiotic factors (low DO concentration, temperature, salinity, time of day) on vertical habitat overlaps between gelatinous zooplankton and their prey during summer in a Chesapeake Bay subestuary. Laboratory clearance rates of M. leidyi feeding on bay anchovy (Anchoa mitchilli) eggs and yolk sac larvae, and naked goby (Gobiosoma bosc) larvae were as high at low DO (1.5 mg l -1 ) as at high DO concentrations (7 mg l -1 ). Multiple years of field sampling at 2 sites revealed that the proportions of motile species in the bottom layer of the water column, espe- cially fish larvae and copepods, increased with increasing bottom DO concentrations during both day and night. Ctenophore densities remained high in the bottom layer even at low DO concentrations. Vertical habitat overlaps among 4 motile predator-prey pairs (M. leidyi-copepods, M. leidyi-naked goby larvae, bay anchovy larvae-copepods, and naked goby larvae-copepods) increased with increasing bottom DO concentrations (1st- or 2nd-order regression models). Vertical overlap between M. leidyi and fish eggs was significantly higher during the day than at night. Statistical models sug- gest that both bottom DO concentration and scyphomedusan predators influenced M. leidyi-fish egg nighttime and M. leidyi-copepod combined day-night overlaps. Hypoxia influences food web inter- actions more through altered habitat use and encounter rates than by directly affecting predation. Strong behavioral responses of larval fish and copepods to low bottom DO concentrations may decrease habitat overlap between M. leidyi predators and their prey.

Evaluating the photoalteration of estuarine dissolved organic matter using direct temperature-resolved mass spectrometry and UV-visible spectroscopy

Direct temperature-resolved mass spectrometry (DT-MS) was used to evaluate the molecular-level photodegradation of dissolved organic matter (DOM) isolated from three sites in a Chesapeake Bay subestuary (swamp- and marsh-influenced up-river, midestuarine, and bay mouth). From each site, filtered (<0.1 or <0.2 μm) water samples were irradiated in solarsimulated ultraviolet light followed by isolation of the DOM using C18-solid-phase extraction and subsequent DT-MS analysis. To provide background DOM photoreactivity data for the water samples, we also determined dissolved inorganic carbon (DIC) photoproduction and chromophoric dissolved organic matter (CDOM) photobleaching. DIC photoproduction was correlated with initial DOM light absorbance, initial dissolved organic carbon (DOC) concentration, and photobleaching. Changes in DT-MS characteristics within the extracts (in particular, the loss of an “aromatic” signal believed to be from reworked terrestrial material) were found to correlate linearly with the absorbance of the corresponding water samples. A relationship between photobleaching and DT-MS characteristics was also observed, with the upstream samples asymptotically approaching a constant “molecular-level” value as photobleaching increased. Both relationships appeared to be independent of absorbance wavelength in the ultraviolet. Following irradiation, the swamp/marsh-dominated upstream samples resembled the down-stream samples in terms of absorption spectra and MS-determined molecular-level characteristics. These shifts indicate that terrestrially-derived DOM may be more difficult to differentiate from marine DOM upon photodegradation, which has implications regarding evaluating the terrestrial impact within the marine DOM pool.

Effects of watershed and estuarine characteristics on the abundance of submerged aquatic vegetation in Chesapeake Bay subestuaries

Watershed land use can affect submerged aquatic vegetation (SAV) by elevating nutrient and sediment loading to estuaries. We analyzed the effects of watershed use and estuarine characteristics on the spatial variation of SAV abundance among 101 shallow subestuaries of Chesapeake Bay during 1984–2003. Areas of these subestuaries range from 0.1 to 101 km2, and their associated local watershed areas range from 6 to 1664 km2. Watershed land cover ranges from 6% to 81% forest, 1% to 64% cropland, 2% to 38% grassland, and 0.3% to 89% developed land. Landscape analyses were applied to develop a number of subestuary metrics (such as subestuary area, mouth width, elongation ratio, fractal dimension of shoreline, and the ratio of local watershed area to subestuary area) and watershed metrics (such as watershed area). Using mapped data from aerial SAV surveys, we calculated SAV coverage for each subestuary in each year during 1984–2003 as a proportion of potential SAV habitat (the area mixed-agricultural > agricultural > developed. Change-point analyses indicated strong threshold responses of SAV abundance to point source total nitrogen and phosphorus inputs, the ratio of local watershed area to subestuary area, and septic system density in the local watershed.

Watershed Land Use Is Strongly Linked to PCBs in White Perch in Chesapeake Bay Subestuaries

We related total PCBs (t-PCBs) in white perch (Morone americana), an abundant estuarine resident that supports a valuable recreational and commercial fishery in the mid-Atlantic region, to the amount and spatial arrangement of developed land in watersheds that discharge into 14 subestuaries of Chesapeake Bay. We considered the intensity of development in watersheds using four developed land-use measures (% impervious surface, % total developed land, % high-intensity residential + commercial [%high-res/comm], and % commercial) to represent potential source areas of PCBs to the subestuaries. We further evaluated the importance of source proximity by calculating three inverse-distance weighted (IDW) metrics of development, an approach that weighted developed land near the shoreline more heavily than developed land farther away. Unweighted percentages of each of the four measures of developed land explained 51-69% of the variance in t-PCBs. However, IDWs markedly improved the relationships between % developed land measures and t-PCBs. Percent commercial land, weighted by its simple inverse distance, explained 99% of the variance in t-PCBs, whereas the other three measures explained as much as 93-97%. PCBs historically produced or used in commercial and residential areas are apparently persisting in the environment atthe scale of the watersheds and subestuaries examined in this study, and developed land close to the subestuary has the greatest unit effect on t-PCBs in fish. These findings provide compelling evidence for a strikingly strong linkage between watershed land use and t-PCBs in white perch, and this relationship may prove useful for identifying unsampled subestuaries with a high risk of PCB contamination.

Nitrogen, phosphorus, and sulfur dynamics in a low salinity marsh system dominated by Spartina alterniflora

In the upper reaches of estuaries, the pulsing of nutrient inputs and salinity occurs on fine temporal and spatial scales. We investigated the supply and exchanges of N, P, and S along a salinity gradient in a Chesapeake Bay subestuary. Interactions among nutrients in surface water, sediment porewater, and shoots of the dominant marsh macrophyte Spartina alterniflora were also examined. The system was characterized by a spatial gradient in nutrient availability, with the low salinity region representing abundant allochthonous nitrogen and phosphorus inputs and minimal sulfur availability. Autochthonous production and consumption of nutrients were most important at the midpoint of the salinity gradient. These patterns were reflected in the N, P, and S content of S. alterniflora over one growing season. This brackish tidal creek system also had a gradient of temporal variability in levels of inorganic nitrogen, phosphorus, and sulfur. The higher salinity portion experienced the damping effect of waters of more constant nutrient composition, whereas the upper portion of the system was characterized by highly pulsed nutrient availability. Interchanges between nutrient pools were apparent throughout the system. Sediment, and even surface-water, concentrations of nutrients seemed to respond to plant root zone oxidation and uptake and release of nutrients. Porewater biogeochemical processes were linked to surface-water nutrient dynamics as well.

Giardia duodenalis cysts of genotype A recovered from clams in the Chesapeake Bay subestuary, Rhode River.

Filter-feeding molluscan shellfish can concentrate zoonotic and anthroponotic waterborne pathogens. Cysts of Giardia sp. were detected by immunofluorescent antibodies in tissues of the clams Macoma balthica and M. mitchelli from Rhode River, a Chesapeake Bay (Maryland) subestuary. Molecular tests identified the cysts as Giardia duodenalis Genotype A, the most common genotype recovered from humans. Macoma clams are burrowers in mud or sandy-mud substrata and preferentially feed on the surface sediment layer. Waterborne Giardia cysts settle rapidly to the bottom in slow-moving waters and contaminate the sediment. Macoma clams do not have economic value, but can serve as biologic indicators of sediment contamination with Giardia sp. cysts of public health importance. These clams can be used for sanitary assessment of water quality.