With ongoing sea level rise (SLR), tidal freshwater marshes (TFMs) eventually will be flooded with more brackish water. The impact of more water and salt on the plant community of TFMs, however, is unknown. With SLR, both the invasive reed Phragmites australis and the native salt marsh grass Spartina alterniflora could become dominant species in TFMs. I am looking at determining how increases in salinity and inundation caused by sea level rise will impact the relative distribution of Phragmites and Spartina in tidal freshwater marshes in Southeastern Virginia. Using GIS, I summarized past expansion patterns by mapping the current and historical distribution of Phragmites and Spartina. With soil samples collected from 6 TFMs in James City County with established Phragmites stand, I tested the effects of salinity and flooding on the germination of Phragmites and Spartina seeds and the subsequent effects of competition with these conditions. Inundation positively impacted the abundance of Phragmites and Spartina, while competition form Phragmites and Spartina decreased native species richness. Based on germination success and historical distributions, SLR-caused range shifts were predicted for Phragmites and Spartina and suggest Phragmites and Spartina will be more abundant in number within TFMs and in more TFMs in James City County. TFM area across James City County will diminish if accretion rates cannot keep pace with sea level rise and current TFMs will transition to oligohaline marshes, causing significant community changes.

Tidal marshes are a major ecological resource in Virginia and a driver of many estuarine functions. Therefore, the long term sustainability of tidal marsh ecosystems is a question of great interest in the research community. Sea level is rising at an unusually high rate in the Chesapeake Bay relative to most of the Atlantic coastline, putting Bay marshes at high risk from drowning and erosion. Sea level rise-driven salinity changes communities and alters ecosystem services. Understanding the patterns of change and the importance of different drivers of change is critical to tidal marsh sustainability. The overarching goal of this research is to examine how changes in natural and anthropogenic factors interact to affect tidal wetland distribution, extent and plant composition with the intent of promoting coastal resiliency to sea level rise impacts through informed coastal management. I quantified changes in marsh extent over the past 40 years and related changes in marsh extent to sea level rise and other drivers of change. Then I examined shifts in plant community composition throughout the Chesapeake Bay, VA, looking for signals of increased inundation and salinity. In small headwater systems, I explored the utility of these changes in plant composition for predicting soil sulfur content (an early signal of salinity intrusion). These changes in marshes from the past 40 years were used to elucidate results from an elevation-based model of future marsh persistence under accelerating sea level rise. Several lessons emerged from this dissertation: 1. Analyses of changes in tidal marsh extent and plant communities are complementary, clarifying vulnerabilities and prognosis under future conditions. 2. Human shoreline use (e.g., development, shoreline hardening, boating activity) can dominate physical processes to alter the marsh response to sea level rise. 3. Defining sediment availability for a given marsh may not be sufficient to determine its potential for expansion or persistence under sea level rise. 5. Marsh plant communities can be an early signal of change, showing shifts in inundation frequency before there is any change in marsh extent. 6. Tidal marshes will continue to decline over the next 100 years. However, most of the loss will be in low salinity, riverine marshes. Some high salinity, Bayfront marshes will expand if the land they need to migrate is preserved. 7. Tidal marsh response to sea level rise has, and will continue to, vary by marsh form, geologic setting, location in the estuary, and surrounding land use decisions. 9. Targeted land use decisions coupled with active restoration may help minimize future marsh loss.

Molecular genetic techniques were used to elucidate genetic population structure in three species of sharks, the coastal sandbar shark (Carcharhinus plumbeus) and Atlantic sharpnose shark (Rhizoprionodon terraenovae), and the pelagic shortfin mako (Isurus oxyrinchus). Allozyme analysis and analysis of restriction fragment length polymorphisms (RFLP) of mitochondrial DNA (mtDNA) were used to test the null hypothesis that the mid-Atlantic Bight and the Gulf of Mexico sandbar sharks consist of a single gene pool. RFLP analysis of mtDNA was used to determine the pattern and level of genetic divergence in the sandbar shark between the western North Atlantic and the Eastern Indian Ocean, and within the entire species range of the cosmopolitan shortfin mako and the Atlantic sharpnose shark. No significant genetic divergence was detected in the sandbar shark between the mid-Atlantic Bight and Gulf of Mexico. Genetic variation was extremely low but homogeneously distributed. A significant degree of genetic divergence was detected between North Atlantic and Australian sandbar sharks. All Australian sandbar shark mtDNAs were fixed for alleles other than those detected in the North Atlantic. The hypothesis that the shortfin mako comprises a single panmictic population was rejected. The overall probability of drawing samples with such disparate allele frequencies from a single gene pool was &<&0.001. The only barrier to gene flow detected appeared to be the equatorial Atlantic. Samples from Brazil, Australia, and California were not significantly different from each other, however all three were significantly different from the North Atlantic sample. The shortfin mako exhibited a considerably higher level of genetic variation than the sandbar shark. The Atlantic sharpnose shark did not exhibit significant differences in allele frequency throughout its range. The level of genetic variation detected in mtDNA was intermediate to that of the sandbar shark and the shortfin mako.

The sources and fates of carbon in a tidal freshwater marsh (Sweet Hall marsh; Pamunkey River, Virginia) were determined to understand the role that these marshes play with respect to estuarine carbon cycling. A carbon gas flux model, based on measured carbon dioxide and methane fluxes, was developed to calculate annual rates of macrophyte and microalgal photosynthesis and community and belowground respiration. Because carbon fluxes out of marsh sediments may underestimate true belowground respiration if sediment-produced gases are transported through plant tissues, gross nitrogen mineralization was used as a proxy for belowground carbon respiration. Annual community respiration exceeded gross photosynthesis, suggesting an allochthonous input of organic carbon to the marsh. Sediment deposition during tidal flooding was measured as a potential exogenous carbon source. Short term deposition rates (biweekly to monthly) were spatially and temporally variable, with highest rates measured near a tidal creek during summer. Annual deposition on the marsh was sufficient to balance relative sea level rise and measured respiration rates. Sediment inventories of 7Be indicated that spatial patterns of sedimentation were not due to sediment redistribution within the marsh. Accretion rates calculated from 137Cs (decadal scale) and 14C (centuries to millennia) were substantially less than annual deposition rates. The concentration and isotopic composition of dissolved and particulate inorganic and organic carbon (DIC, DOC, POC) were measured in a marsh creek which drained the study site. Seasonal isotopic variations in DIC were explained by marsh porewater drainage and decomposition of marsh-derived carbon. A model linking DIC concentrations and water transport showed that DIC export from tidal marshes could explain a significant portion of excess DIC production in the adjacent estuary. Isotopic mixing models indicated seasonal variability in the importance of phytoplankton as a source of DOC and POC although there was no evidence for a net flux of these materials between the marsh and estuary. Annually, the marsh carbon budget was closely balanced, with sources exceeding sinks by approximately 5 percent. This similarity suggests that those processes which were not quantified (e.g. consumption by marsh and riverine fauna) were quantitatively unimportant with respect to the entire marsh carbon budget.

No description available.

Parasite communities of spot, Leiostomus xanthurus, and Atlantic croaker, Micropogonias undulatus, were examined to determine if (1) their structure changed with host age, (2) geographical location affected community structure, (3) food habits affected community structure, and (4) their parasite communities are predictable. Juvenile fish were collected monthly from Chesapeake Bay and Pamlico Sound. Adult fish were collected offshore north of Cape Hatteras in fall, and offshore south of Cape Hatteras in spring and fall. A total of 21 parasitic species occurred in juvenile spot and 19 in juvenile croaker from Chesapeake Bay and Pamlico Sound. The parasite communities of juvenile fishes varied with size, season, and area. Although juvenile spot and croaker shared eight and six parasites between estuaries, respectively, many nonspecific parasites (generalists) were more common in both spot and croaker from one estuary than the other. The estuary of residence was as important as host species identity in determining parasite community structure. Twenty-three species of metazoan parasites were recorded from adult spot and 26 from adult croaker. of the 33 parasitic species found, 17 occurred in both spot and croaker. All parasites had overdispersed or clumped distributions among hosts. Adult spot and croaker collected offshore had much greater species-richness, diversity, and total number of individual parasites than juvenile fishes collected in adjoining estuaries. The number of species and diversity of parasites in adult fish was greater in croaker than spot. However, when only gastrointestinal helminths were considered, spot had greater species-richness as well as greater numbers of individual helminths. Comparison of adult spot and croaker parasite faunas collected offshore indicated that their respective parasite component communities were distinct and that similar infracommunity variability existed in both hosts. Although the parasite dominance hierarchy in adults of both species varied slightly between areas and seasons, there appeared to be predictable dominant species. The core species were accompanied by subordinate, less predictable species. Variability in both relative intensities and presence absence of parasites within communities resulting from their diverse diets made them less predictable than those of other vertebrates with less diverse diets.

The temporal spectral light environment of shallow areas of the lower Chesapeake Bay was characterized by cosine collection of downwelling diffuse irradiance at 12 wavelengths between 400 and 700 nm. An extensive monthly and site comparison of spectral attenuation coefficients is presented and compared with previous measurements of the light quality environment of the Chesapeake Bay and other estuaries and marine waters. Spectral irradiance and attenuation of light in a mangrove creek and Thalassia testudinum bed of Laguna de Terminos, Campeche, Mexico and their relationships to wind-driven suspension of particulate matter, and dissolved substances from the mangrove swamp is also discussed. A review of the physics of spectral attenuation in estuaries is included. Particular attention was given to the relationship between the occurrence of seagrasses (Zostera marina mainly) in the Chesapeake Bay and spectral attenuation. Potential losses of photosynthetically storable radiation (PSR) due to reduced light quality in non-vegetated areas is calculated. A theoretical homeostatic relationship between seagrass leaf baffling of the water column, light quality and critical bed size is presented. The logic and output of a digital ecosystem simulation model of theoretical estuarine photosystem responses to simulated varying underwater light quality is presented. The model assumes that photosystems adapt to maximize power. Conclusions include: a dramatic pattern of seasonal spectral attenuation in the lower Chesapeake Bay; seasonal differences in spectral attenuation between vegetated and unvegetated sites; 63% less violet light was able to pass through a meter of water at unvegetated sites in May than vegetated sites.

No description available.

No description available.

No description available.