mid-Atlantic Salt Marshes Research Articles

Genetic Diversity and Clonal Structure of Spartina alterniflora in a Virginia Marsh

We asked how genetic diversity of Spartina alterniflora (Smooth Cordgrass) partitions across multiple spatial scales in mid-Atlantic salt marshes. We analyzed samples from 5 Virginia marshes, spanning ∼ 35 km of coastline, using microsatellite markers to examine genetic diversity and genotype clustering. In a single marsh, nested spatial analysis of the genotypes allowed indirect evaluation of colonization dynamics. Samples collected ≥10 m apart had clonal and allelic diversity levels similar to those of other geographic locations; however, genotypic richness and evenness of samples collected 0.2 and 1.0 m apart were reduced. Sampling scale had little effect on allelic diversity. Expected heterozygosity exceeded observed heterozygosity values at all sites and spatial scales, suggesting Smooth Cordgrass inbreeding is common in these marshes. We hypothesize that the observed spatial patterns indicate there is genetic dominance of a few, well-adapted clones balanced by sexual reproduction and recruitment, especially after disturbance, thereby creating genetically diverse and potentially resilient marshes.

Global change accelerates carbon assimilation by a wetland ecosystem engineer

The primary productivity of coastal wetlands is changing dramatically in response to rising atmospheric carbon dioxide (CO2) concentrations, nitrogen (N) enrichment, and invasions by novel species, potentially altering their ecosystem services and resilience to sea level rise. In order to determine how these interacting global change factors will affect coastal wetland productivity, we quantified growing-season carbon assimilation (≈gross primary productivity, or GPP) and carbon retained in living plant biomass (≈net primary productivity, or NPP) of North American mid-Atlantic saltmarshes invaded by Phragmites australis (common reed) under four treatment conditions: two levels of CO2 (ambient and +300 ppm) crossed with two levels of N (0 and 25 g N added m−2 yr−1). For GPP, we combined descriptions of canopy structure and leaf-level photosynthesis in a simulation model, using empirical data from an open-top chamber field study. Under ambient CO2 and low N loading (i.e., the Control), we determined GPP to be 1.66 ± 0.05 kg C m−2 yr−1 at a typical Phragmites stand density. Individually, elevated CO2 and N enrichment increased GPP by 44 and 60%, respectively. Changes under N enrichment came largely from stimulation to carbon assimilation early and late in the growing season, while changes from CO2 came from stimulation during the early and mid-growing season. In combination, elevated CO2 and N enrichment increased GPP by 95% over the Control, yielding 3.24 ± 0.08 kg C m−2 yr−1. We used biomass data to calculate NPP, and determined that it represented 44%–60% of GPP, with global change conditions decreasing carbon retention compared to the Control. Our results indicate that Phragmites invasions in eutrophied saltmarshes are driven, in part, by extended phenology yielding 3.1× greater NPP than native marsh. Further, we can expect elevated CO2 to amplify Phragmites productivity throughout the growing season, with potential implications including accelerated spread and greater carbon storage belowground.

Sexual selection and the physiological consequences of habitat choice by a fiddler crab.

In mid-Atlantic salt marshes, reproductively active male sand fiddler crabs, Uca pugilator, use a single greatly enlarged major claw as both a weapon to defend specialized breeding burrows from other males and an ornament to attract females for mating. During the summer breeding season, females strongly prefer to mate with males controlling burrows in open areas high on the shore. Food availability decreases while temperature and desiccation stress increase with increasing shore height, suggesting that the timing and location of fiddler crab mating activity may result in a potential trade-off between reproductive success and physiological condition for male crabs. We compared thermal preferences in laboratory choice experiments to body temperatures of models and living crabs in the field and found that from the perspective of a fiddler crab, the thermal environment of the mating area is quite harsh relative to other marsh microhabitats. High temperatures significantly constrained fiddler crab activity on the marsh surface, a disadvantage heightened by strongly reduced food availability in the breeding area. Nevertheless, when the chance of successfully acquiring a mate was high, males accepted a higher body temperature (and concomitantly higher metabolic and water loss rates) than when the chances of mating were low. Likewise, experimentally lowering costs by adding food and reducing thermal stress in situ increased fiddler crab waving display levels significantly. Our data suggest that fiddler crabs can mitigate potential life history trade-offs by tuning their behavior in response to the magnitude of both energetic and non-energetic costs and benefits.

The effects of crab bioturbation on Mid-Atlantic saltmarsh tidal creek extension: Geotechnical and geochemical changes

Understanding saltmarsh response to sea-level rise is critical for management and mitigation of these valuable coastal areas. However, comprehensive field studies of sea-level driven changes to the marsh landscape that consider combined biological, geological, and hydrodynamic interactions are rare. This study analyzes ecophysical feedbacks from crab colonization and bioturbation on geotechnical and geochemical properties of the soil in a Mid-Atlantic Spartina alterniflora saltmarsh. The study area is within a marsh that is experiencing creek extension due to accelerated sea-level rise and increasing periods of marsh inundation. Measurements of redox potential, pH, belowground biomass, and soil strength reveal that intense crab bioturbation by Sesarma reticulatum significantly changes the biogeochemical properties of the soil. Oxidized conditions in the upper 10–15 cm of the marsh induced by burrowing causes enhanced degradation of S. alterniflora belowground biomass (roots and rhizomes, reduction from 1.9 ± 0.6 kg/m2 to 1.1 ± 0.4 kg/m2), which reduces the structural integrity of the soil. This process ultimately increases the erosion potential of the sediment in creek head areas (documented by a reduction in shear strength from 10 ± 7 kPa to 2 ± 1 kPa), facilitating creek extension in order to accommodate tidal flows. The pervasiveness of similar tidal creek morphology in southeast Atlantic saltmarshes suggests this process is occurring in other marshes with a moderate tidal range undergoing sea-level rise.

Nitrogen uptake by the shoots of smooth cordgrass Spartina alterniflora

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 433:43-52 (2011) - DOI: https://doi.org/10.3354/meps09117 Nitrogen uptake by the shoots of smooth cordgrass Spartina alterniflora T. J. Mozdzer1,3,*, M. Kirwan1,2, K. J. McGlathery1, J. C. Zieman1 1Department of Environmental Sciences, University of Virginia, 291 McCormick Rd, Charlottesville, Virginia 22904, USA 2Patuxent Wildlife Research Center, US Geological Survey, 12100 Beech Forest Rd, Laurel, Maryland 20708, USA 3Present address: Smithsonian Environmental Research Center, 647 Contees Wharf Rd, Edgewater, Maryland 21037, USA *Email: mozdzert@si.edu ABSTRACT: The smooth cordgrass Spartina alterniflora is the foundation species in intertidal salt marshes of the North American Atlantic coast. Depending on its elevation within the marsh, S. alterniflora may be submerged for several hours per day. Previous ecosystem-level studies have demonstrated that S. alterniflora marshes are a net sink for nitrogen (N), and that removal of N from flooding tidal water can provide enough N to support the aboveground biomass. However, studies have not specifically investigated whether S. alterniflora plants assimilate nutrients through their aboveground tissue. We determined in situ foliar and stem N uptake kinetics for 15NH4, 15NO3, and 15N-glycine by artificially flooding plants in a mid-Atlantic salt marsh. To determine the ecological importance of shoot uptake, a model was created to estimate the time of inundation of S. alterniflora in 20 cm height intervals during the growing season. Estimates of inundation time, shoot mass, N uptake rates, and N availability from long-term data sets were used to model seasonal shoot N uptake. Rates of aboveground N uptake rates (leaves + stems) were ranked as follows: NH4+ > glycine > NO3–. Our model suggests that shoot N uptake may satisfy up to 15% of the growing season N demand in mid-Atlantic salt marshes, with variation depending on plant elevation and water column N availability. However, in eutrophic estuaries, our model indicates the potential of the plant canopy as a nutrient filter, with shoot uptake contributing 66 to 100% of plant N demand. KEY WORDS: Foliar uptake · Stem uptake · Ammonia · Glycine · Nitrate · DON Full text in pdf format PreviousNextCite this article as: Mozdzer TJ, Kirwan M, McGlathery KJ, Zieman JC (2011) Nitrogen uptake by the shoots of smooth cordgrass Spartina alterniflora. Mar Ecol Prog Ser 433:43-52. https://doi.org/10.3354/meps09117 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 433. Online publication date: July 18, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Effects of Open Marsh Water Management on the Reproductive Success and Nesting Ecology of Seaside Sparrows in Tidal Marshes

Open marsh water management (OMWM), a method of mosquito reduction through habitat alteration, is widely practiced in Mid-Atlantic salt marshes. The effect of these habitat modifications on obligate salt marsh breeding birds is poorly understood. The present study was conducted to quantify the extent of OMWM and determine if the technique affected Seaside Sparrow (Ammodramus maritimus) reproductive success and nesting ecology. Seaside Sparrow territory density, nest density, nest survival, productivity and vegetation cover was estimated on 19 plots (1–3 ha) within tidal marshes in Sussex County, Delaware in May–August 2006–2007. Plots were characterized as limited (N = 10) or extensive OMWM (N = 9) based on the amount of alterations and time since manipulations. Seaside Sparrow territory density and nest density were two times greater on limited OMWM plots than extensive OMWM plots. Also, the number of Seaside Sparrow eggs/ha and fledglings/ha were greater on limited OMWM plots. Seaside Sparrow nest survival rates did not differ between limited and extensive OMWM plots, but reproductive output (# nests/ha, eggs/ha, and fledglings/ha) was greater on areas with limited OMWM. OMWM may be an alternative means of mosquito control but should not be considered a method of habitat enhancement for Seaside Sparrows and possibly other obligate salt marsh bird species.

Predation by the black-clawed mud crab,Panopeus herbstii, in Mid-Atlantic salt marshes: Further evidence for top-down control of marsh grass production

Although top-down control of plant growth has been shown in a variety of marine systems, it is widely thought to be unimportant in salt marshes. Recent caging experiments in Virginia and Georgia have challenged this notion and shown that the dominant marsh grazer (the periwinkle,Littoraria irrorata) not only suppresses plant growth, but can denude marsh substrate at high densities. In these same marshes, our field observations suggest that the black-clawed mud crab,Panopeus herbstii, is an abundant and potentially important top-down determinant of periwinkle density. No studies have quantitatively examinedPanopeus distribution or trophic interactions in marsh systems, and its potential impacts on community structure remained unexplored. We investigated distribution and feeding habits ofPanopeus in eight salt marshes along the Mid-Atlantic seashore (Delaware-North Carolina). We found that mud crabs were abundant in tall (4–82 ind m−2), intermediate (0–15 ind m−2), and short-form (0–5 ind m−2)Spartina alterniflora zones in all marshes and that crab densities were negatively correlated with tidal height and positively correlated with bivalve density. Excavation of crab lairs r⇍utinely produced shells of plant-grazing snails (up to 36 lair−1) and bivalves. Lab experiments confirmed that mud crabs readily consume these abundant marsh molluscs. To experimentally examine potential community effects of observed predation patterns, we manipulated crab and periwinkle densities in a 1-mo field experiment. Results showed thatPanopeus can suppress gastropod abundance and that predation rates increase with increasing snail density. In turn, crab suppression of snail density reduces grazing intensity on salt marsh cordgrass, suggesting presence of a trophic cascade. These results indicate that this previously under-appreciated consumer is an important and indirect determinant of community structure and contribute to a growing body of evidence challenging the long-standing notion that consumers play a minor role in regulating marsh plant growth.

Tidal and seasonal variations in the quantity and composition of seston in a North American, mid-Atlantic saltmarsh

We determined the concentration of seston, particulate organic matter, and biological components (chlorophyll a, bacteria, and heterotrophic nanoflagellates) for <25 μm size fraction seston over five seasons in Canary Creek saltmarsh, Delaware Bay, USA. This material is the potential food resource for suspension-feeding ribbed mussels, Geukensia demissa, that inhabit the marsh intertidal zone. For eight tidal cycles each season we collected water six times at hourly intervals from mid-flood tide to mid-ebb tide. Although the concentration of seston did not vary seasonally, there were significant seasonal variations (analysis of variance, P<0.05) in seston components, with chlorophyll a concentration being highest in May and bacteria and heterotrophic nanoflagellates most abundant in August. Seston composition also varied within each tidal cycle with a magnitude as great as the seasonal variation. We conclude that ribbed mussels are subject to an unpredictable food supply that varies in composition and concentration on the order of hours and days. In contrast to the pronounced temporal changes, seston characteristics did not differ significantly among sampling locations within the marsh, or between samples collected close to the sediment surface and from the upper water column. Resuspension of sediment particles caused by tidal flow was not evident in tidal creeks and there were no dominant patterns in total seston concentration corresponding to tidal stages (flood tide, high slack water, and ebb tide) over the five sampling months. The abundance of biological components in the seston, including chlorophyll a, bacteria, and heterotrophic nanoflagellates, were significantly greater during high flood tide and high slack water than during ebb tide. The decline of biological components, particularly chlorophyll a in the ebb tide, indicates that this temperate saltmarsh imported organic material produced in the Delaware estuary.

Seston available as a food resource for the ribbed mussel ( Geukensia demissa) in a North American, mid-Atlantic saltmarsh

We studied the composition of the <25 μm seston size fraction as a food resource potentially available to suspension feeding ribbed mussels, Geukensia demissa, over an annual cycle in Canary Creek saltmarsh, Delaware Bay. There were significant seasonal variations in the concentration of particulate organic carbon (POC), particulate organic nitrogen (PON), and total carbohydrate, but not cellulose. The concentration of cellulose, measured by hydrolytic cellulase enzyme assay, was relatively low (seasonal range 24 to 35 μg l −1) and only comprised from 3% of total carbohydrate in May 1996 to 13% in November 1995. We used the biomass of microalgae, estimated from chlorophyll a, and abundance of free-living bacteria and heterotrophic nanoflagellates to calculate each component's equivalent carbon content. Microalgae were the most dominant carbon source (62% annually) among the four identified components (phytoplankton, bacteria, heterotrophic nanoflagellates, and cellulose) in all seasons except in August 1995 when carbon from bacteria was most abundant (55%). The annual average carbon equivalents of heterotrophic nanoflagellates and cellulose were relatively small (2 and 4%, respectively). The total concentration of POC in the seston was much greater than the carbon derived from the four identified components. The proportions that these identified components contributed to POC varied seasonally and combined only accounted for 8–24% of POC. Based on these estimates, the bulk of the POC in Canary Creek marsh was not associated with any of the four components we identified. We suggest that this uncharacterized material was some type of non-lignocellulosic, amorphous detritus of unknown utility as a food resource for ribbed mussels.

Symbiosis between an Alpheid Shrimp and a Xanthoid Crab in Salt Marshes of Mid-Atlantic States, U.S.A.

Instances of mutualism and commensalism between alpheid shrimp and other marine invertebrates and fish are common in tropical waters. In this study, we present evidence that the temperate alpheid big-clawed snapping shrimp (Alpheus heterochaelis) participates in a symbiosis with the black-clawed mud crab (Panopeus herbstii), which constructs and maintains burrows in salt marshes of mid-Atlantic states, U.S.A. We surveyed eight mid-Atlantic salt marshes and found that 11% of occupied crab burrows |$(n = 1042)$| also housed Alpheus. Feeding trials showed that whereas Panopeus readily consumes other marsh Crustacea (i.e., grass shrimp and fiddler crabs), it will not prey on snapping shrimp. Alpheus caged in the marsh without access to mud crab lairs could not construct burrows. These results suggest that alpheid shrimps, thought to be limited in distribution to unvegetated mudflats and oyster reefs, may expand their intertidal range by living commensally with Panopeus herbstii in salt marsh habitats.

Seasonal variations in the rates of aquatic and aerial respiration and ammonium excretion of the ribbed mussel, Geukensia demissa (Dillwyn)

Aquatic and aerial rates of oxygen consumption and ammonium excretion of ribbed mussels, Geukensia demissa (Dillwyn), collected from the mid-intertidal zone of a mid-Atlantic salt marsh, were measured under ambient conditions of food, temperature, and salinity over five seasons. Rates of aquatic respiration covaried with body size and season, as the rates were high and strongly related to mussel tissue weight in spring and summer but low and weight independent in winter. There was a significant interannual difference between summer of 1995 and 1996. Rates of aerial respiration also varied seasonally, with high rates of oxygen consumption in summer and low rates in winter. The magnitude of these seasonal variations were greater than those for aquatic respiration, and as a result, the ratio of aerial to aquatic respiration was higher in summer (0.61 and 0.52) than in winter (0.11). This indicates that G. demissa was able to actively regulate aerial respiration, thereby permitting high aerobic metabolism during prolonged periods of air exposure in summer. We hypothesize that such high rates of aerial respiration, during the seasons of high metabolic activity, are required to provide sufficient energy for mussels to facilitate food digestion during air exposure at low tide. Rates of ammonium excretion varied seasonally and increased with mussel weight in all seasons. The atomic ratio of oxygen to nitrogen (O:N), calculated from aquatic respiration vs. ammonium excretion, was significantly lower in autumn (26) than in other seasons (46–60) among which the O:N did not vary significantly.

Seasonal utilization of different seston carbon sources by the ribbed mussel, Geukensia demissa (Dillwyn) in a mid-Atlantic salt marsh.

Seston in salt marshes contains a temporally and spatially complex mixture of natural microparticulate organic material, including phytoplankton, vascular plant detritus, bacteria, heterotrophic nanoflagellates and benthic diatoms. Quantitative information is available concerning how suspension-feeding consumers, such as the ribbed mussel, Geukensia demissa (Dillwyn), utilize some of these components to satisfy their carbon demands. Despite this information there is still a limited understanding of how the relative nutritive contribution of these different dietary items may shift during the year associated with variations in both seston composition and the mussel's physiological condition. To investigate if the mussel's ability to use specific constituents of natural seston varies seasonally, we ran a series of pulse-chase 14C feeding experiments under ambient conditions in March, May, August and November 1996. Phytoplankton, cellulosic detritus, bacteria, heterotrophic nanoflagellates and benthic diatoms were radiolabeled and supplemented in small amounts to natural marsh water for feeding to mussels. The fate of 14C in mussel tissues, feces, respiration and excretion was quantified and contrasted among the different diet types and seasons. Microcapsules containing radiolabeled carbohydrate and protein were used as standards to differentiate possible between-experiment variations in seston composition from seasonal changes in the mussel's feeding and digestive physiology. Mussel clearance rates for all diets were highest in summer and autumn and lowest in winter and spring. In contrast, seasonal shifts in digestive physiology were only found for certain diets. The seasonal range of assimilation efficiencies for microcapsule standards (18-29%) and field-collected microheterotrophs (bacteria 76-93% and heterotrophic nanoflagellates 87-94%) did not differ significantly during the year, whereas summer and autumn assimilation efficiencies for cellulosic detritus (22-24%), phytoplankton (71-79%) and benthic diatoms (89-93%) were up to twofold greater than those in winter and spring (13%, 40-59% and 45-81%, respectively). We conclude that the digestive physiology (e.g., digestive enzyme production) of mussels responds to shifts in dietary components during the year.

Mid-Atlantic Salt-Marsh Shorelines: Mathematical Commonalities

A fractal is a geometric figure that characteristically has great complexity in its boundaries. Fractals occur everywhere in nature: in plants and crystals, and some shorelines. Fractal dimension quantifies the degree of complexity of a fractal and ranges from 1 (a one-dimensional object) to 2 (a solid object). A higher fractal dimension indicates greater complexity. In this study we tested the hypothesis that the shapes of the shorelines of mid-Atlantic salt marshes have a uniform complexity. Fractal dimension was employed as a method of measuring the shoreline's complexity to make comparison possible. We found that marine beaches, particularly those without creeks, had dimensions near 1. Shores that were resistive (i.e., highly irregular and rocky) had low fractal dimensions. Salt marshes from disparate regions had fractal dimensions that were similar to each other and also distinctly higher than the dimensions of all other fractal and non-fractal shores measured in this study. The constancy of the fractal dimension of the salt marshes suggests a common geomorphological determinant. *** DIRECT SUPPORT *** A01BY069 00015

Selective Herbivory and Plant Community Structure in a Mid‐Atlantic Salt Marsh

Factors affecting the distribution of the grasses Spartina alterniflora and Distichlis spicata in a mid—Atlantic salt marsh were examined. A series of eight shallow wells, four in patches of each grass type, was used to describe physicochemical conditions known to limit the distribution of both grasses. Tidal amplitude, surface of subsurface salinity, and subsurface oxidation—reduction potential were found to be within range, but suboptimal, for both grasses. Evidence of selective grazing upon S. alterniflora was found through examination of grazing sign within the grass patches, observations of feral horse feeding behavior, and examination of feral horse feces for grass epidermal fragments. An exclosure experiment simulated preferential grazing and showed that S. alternifora responded negatively while D. spicata responded positively to simulated preferential grazing of S. alterniflora. These results point to a competitive relationship between the grasses under suboptimal conditions for dominance of either species. Selective herbivory (analogous to predation) upon S. alterniflora was shown to be a plausible factor impacting the competition relationship to favor D. spicata. Location along physical gradients, interspecies competition, and herbivory are discussed in relation to salt marsh plant communities.

The Importance of Dead and Young Live Shoots of Spartina alterniflora (Poaceae) in a Mid-Latitude Salt Marsh for Overwintering and Recoverability of Underground Reserves

In a mid-Atlantic salt marsh Spartina alterniflora overwinters with two types of aerial biomass, dead culms from the preceding growing season, and young live shoots that emerged in the fall. The importance of both shoot types for the overwintering capability and the quantity of recoverable underground reserves (RUR) available at the beginning of the following growing season was studied. No reduction in the amount of RUR in the plants' underground parts during the winter period was detected. Mobilization of RUR in March is faster than in December. Apparently, the physiological state of S. alterniflora at the onset of winter does not favor a rapid reconversion of the reserves. When both live and dead shoots were removed in December, the plants did not survive the winter. The presence of either live or dead shoots enabled the plants to survive. Though their biomass was much less, the young live shoots performed this function equally as well as the old dead shoots. The live shoots did not add significantly to the RUR through photosynthesis during the winter period, and a role of young live shoots was likely to be the conduction of oxygen to the underground plant parts to support aerobic respiration.

Habitat Use by Postfledging American Black Ducks in Maine and New Brunswick

We examined habitat use by 112 postfledging American black ducks (Anas rubripes) in eastern Maine and southwestern New Brunswick from September through early December of 1985, 1986, and 1987. Ducks were captured on Moosehorn National Wildlife Refuge (NWR), Maine. Palustrine Emergent Wetland was the most preferred habitat type. Riverine habitats were avoided in September, but were used more than, or in proportion to, their availability in November as ice formed on lentic habitats. Black ducks used a greater variety of habitat types during the day than at night, when ducks used mostly large (>30 ha) Emergent Wetland marshes. Managed impoundments of Moosehorn NWR were the most used wetlands (66% of all diurnal, 90% of all nocturnal locations). For black duck management, we propose maintaining large (30-50 ha) marshes containing dense emergent vegetation that are located near a complex of diverse wetland types. J. WILDL. MANAGE. 54(3):451-459 Habitat use by breeding American black ducks has been documented for northeastern North America (Coulter and Mendall 1968, Coulter and Miller 1968, Reed 1975, Ringelman 1980, Diefenbach 1988) and for mid-Atlantic saltmarshes (Stotts and Davis 1960). Most research on postbreeding ducks has involved adult birds during wing molt (Gilmer et al. 1977, Bowman 1987), whereas little is known of habitat use by young waterfowl after fledging. Data on habitat use by postfledging black ducks are lacking, and this period of the annual cycle might be critical to survival (Ringelman and Longcore 1980). Our objective was to describe daily and seasonal habitat use by postfledging black ducks. We thank field assistants B. E. Benedict, J. S. Henderson, R. A. Houston, J. D. Sayre, R. T. Speer, and D. B. Stotts. G. E. Horton, M. K. Meiman, and E. P. Schrading assisted with data entry. W. A. Halteman and S. M. Arthur provided statistical advice. D. G. Jorde and J. A. Spendelow reviewed the manuscript. Financial support was provided by the U.S. Fish and Wildlife Service through Cooperative Agreement No. 14-16-0009-1557, Research Work Order No. 2 with the University of Maine.

Spawning Site Utilization in a Delaware Population of Fundulus heteroclitus (Pisces: Cyprinodontidae)

Populations of Fundulus heteroclitus in mid-Atlantic salt marshes are known to deposit eggs on the marsh surface in empty shells of the Atlantic ribbed mussel (Geukensia demissa). Experimental placement of eggs in this location was associated with normal development and hatching, whereas placement of eggs in tidal ditches resulted in developmental arrest or death of the eggs. Eggs placed in the ditches were subject to siltation and extended periods of immersion in water of less than 2.0 ml/L dissolved oxygen. We have quantified the distribution of both eggs and G. demissa shells in seven 10 m deep plots along ditches in a Delaware marsh. Empty shells were unevenly distributed but were found over the entire 10 meters at densities as great as 10 per M2, with the greater numbers usually occurring beyond 2 m from the ditch. Shells located 1-10 m from ditches were used as spawning sites with a maximum mean of 2.7 ? 1.1 spawns/m2 occurring in the fifth meter of a heavily used plot. Our results suggest that the distribution of spawning in shells is influenced by height of the mussel shell in relation to tidal inundation, as well as size and condition of the shell, and possibly by plant distribution.