Our Students and Their Research


You may search for students by class year and keywords found in the student's name, university, project title or abstract. Our first class was in 1989.

Our Students List

Class of 2014

Project Title:

Understanding the Toxic Effects of Oil Exposure by Measuring DNA Damage in the Blue Crab Callinectes sapidus

Increased use and transport of potentially harmful resources demands a necessary understanding of the toxic effects of the compounds to the surrounding environment. The present study describes the observed toxicity of offshore oil exposure to freshwater invertebrates, likely due to the presence of polycyclic aromatic hydrocarbons (PAHs) present in crude oil. The study was conducted using the basic comet assay protocol to measure the degree of DNA damage and DNA repair capacity of cells exposed to environmentally relevant concentrations of water accommodated fraction (WAF) crude oil ranging from 10 μg/l to 1,000 μg/l. Blue crabs (Callinectes sapidus) were exposed to oil samples taken off the shore of Southern Louisiana, which were then manually weathered. The comet assay technique allowed for rapid analysis of 400 assay samples measuring strand breakage by evaluating tail DNA, tail length and the olive tail moment, a ratio of the two values. Over 130 blue crab larvae were sampled assessing for damage in both the hepatocytes and blood cells. A linear relationship between dosage and DNA damage was expected but minimal significant damage was observed. DNA from blue crab larvae showed little significant impact from acute exposures to WAF crude oil.

Project Title:

Algorithms to Adaptively Track Salinity Gradients with an Autonomous Underwater Vehicle

A REMUS-600 autonomous underwater vehicle (AUV) was used to test algorithms designed to adaptively track hydrographic features in estuaries and the coastal ocean. These algorithms are derived from animal-inspired models for chemical plume source detection; the current study extends this work to achieve ecosystem-scale measurements (over tens of kilometers) to autonomously follow scalar fields in a quasi-Lagrangian framework. The algorithms were implemented using Mission-Oriented Operating Suite-Interval Programming (MOOS-IvP). We conducted repeated surveys to track salinity in the Choptank River, Maryland to develop and test these algorithms. The key features of the algorithms are the ability to set multiple target thresholds in both the scalar field and the depth range over which thresholds must be met. These algorithms will allow AUVs to track estuarine features of interest such as temperature, salinity, and dissolved oxygen in order to advance basic research on horizontal mixing and circulation in estuaries.

Project Title:

Effects of Environmental Oxygen Partial Pressure on Nauplii Respiration Rates of the Planktonic Copepod, Acartia tonsa

Respiration, along with photosynthesis, are the driving processes of life. Copepods, in addition to many other marine organisms, rely on diffusion across their cell membrane to obtain the required oxygen requirements of respiration to support growth and regulate metabolic functions. This diffusion rate is determined by the pressure gradient of oxygen between the organism and the dissolved oxygen in the water. Hypoxia has become increasingly prevalent in coastal waters including the Chesapeake Bay, threatening populations of the common planktonic copepod, Acartia tonsa. Although literature suggests a behavioral adaptation of adult vertical migration out of these areas, eggs commonly sink into hypoxic or anoxic zones. Therefore, it is possible that eggs and early stage nauplii are forced to maintain a minimal respiration rate in order to survive. In order to predict the population success of A. tonsa species in coastal areas, we studied the effect of environmental oxygen partial pressure on stage I-II nauplii respiration rates. Although our data does not show a significant decrease in respiration with decreasing oxygen partial pressure, it acts as a promising guideline for future studies. Using the methods discovered in this study, along with more water sterilization techniques, more reliable data can be obtained for future nauplii respiration studies.

Project Title:

Geochronological Analysis of Sediments in Tidal Freshwater Marshes in the Patuxent River

Freshwater marshes are very important because of their diversity and productivity. Also, they help maintain water quality for surrounding ecosystems. Freshwater marshes have become threatened by multiple factors like anthropogenic activities and climate change. Sea-level rise is considered the strongest threat for the marshes. The rising of the water may cause degradation and even elimination of freshwater marshes and other coastal environments. In order to determine if freshwater marshes are able to keep up with the sea level rise, low and high-marsh peat cores were taken from the Patuxent River and were analyzed to determine their grain size, organic content, and 210Pb activities to determine the accumulation of sediments in this marsh. The organic content, as well as grain size, increased with distance from the river edge because the low marsh is more influenced by the river. Sand and organic content of the high marsh suggests that sediment is derived from organic decomposition of plant material. Accumulation rate of low marsh appeared to be greater and faster than the high marsh. The accumulation rates of sediment at this site were found to be greater than the sea level rise, but have been decreasing in both the high and low marsh over the past decades, without showing trends of increase or stabilization. This relationship between sea level rise and accumulation rate is crucial to understand to help maintain freshwater marshes.

Project Title:

Investigation of Interspecies Differences in Sr/Ca SST Reconstructions for Diploria and Orbicella Corals

Coral strontium to calcium ratio (Sr/Ca) measurements can be used to generate reconstructions of past climate, providing information that allows us to not only place current global conditions within a historical context but also improve predictive models. Most field studies use only one species for a coral-based climate reconstruction, but there is a growing recognition of the need for replication in these studies to explore potential species and growth effects and increase confidence in the models. We sampled two species, Diploria strigosa and Orbicella annularis, for two different time periods, 1355-1360 and modern (>1930s), to examine potential interspecies differences in how they record climate. After sampling with a microdrill at an average rate of 12 samples a year, the samples were diluted with 2% nitric acid to create ~20 ppm Ca solutions, which were then analyzed with an Inductively-Coupled Plasma Optical Emissions Spectrometer. The data did not contain regular seasonal cycles as expected for Sr/Ca in pristine coral material, most likely as a result of diagenesis and sampling of non-thecal material. A large Sr/Ca spike in January-February, 1360 found in both species may be a result of an environmental perturbation affecting both corals, such as an upwelling of Sr-rich waters.

Project Title:

Trace Metal Contaminant Analysis in an Estuarine Sediments and Water, Baltimore Harbor, Maryland

To understand the degree of contamination of Baltimore Harbor and the adjacent estuary, we were requested to quantify trace metals in the sediment and surface water. Trace element concentrations in estuarine waters are extremely low and analysis is complicated by matrix interferences from major elements such as sodium and chloride. For the water samples, we intended to compare a solvent extraction method to separate the metals from the salt matrix with collision cell techniques designed to minimize such interferences. We were not able to finish the solvent extraction method, thus we will focus on the collision cell results. Metals of interest were extracted from sediment samples using microwave-assisted acid digestion. Diluted aliquots were analyzed by ICP-MS. From the resulting data, we looked at total concentrations of these metals in both the sediments and the corresponding water samples and compared the two to find the associated partition coefficients. Certain locations exhibited toxic levels of trace metals in the sediments, but the water itself showed fairly low concentrations across all locations. This research is part of a broader study by the state of Maryland to determine the influence of sedimentation rates on trace metal concentrations for these locations and how future dredging might affect these areas.

Project Title:

A Potent Greenhouse Gas Source: Nitrous Oxide Emission during Base Flow and Storm Flow in the Choptank River Basin

Nitrous Oxide (N2O) is a potent greenhouse gas, and streams, rivers, and groundwater act as sources of N2O to the atmosphere. Four streams were sampled in four agriculturally dominated watersheds in the Choptank River Basin. N2O gas concentrations and stream parameters were measured to estimate N2O flux to the atmosphere for comparison with N2O fluxes estimated with the IPCC protocols. N2O fluxes (F, μmol N2O - N m-2 h-1) were calculated using measured N2O-N concentrations (Cm, μmol N2O-N L-1) in excess of atmospheric equilibrium (Ceq, μmol N2O-N L-1) and a gas exchange coefficient k (m h-1) (F = (Cm - Ceq) * k). Fluxes of N2O ranged over 0.49-190.69 N2O-N m-2 h-1 in the four watersheds, and were similar to those calculated using the IPCC protocols and also to N2O fluxes from nearby agricultural fields. The IPCC flux estimates were not statistically different from the empirically derived fluxes (p > 0.05). This approach can potentially provide a quick method to estimate the role of gas fluxes from streams in watersheds. Also, sampling occurred during a storm event—N2O levels decreased during the storm event, while DO levels increased—which suggests that the rainfall itself caused an increase in gas exchange.

Project Title:

Examination of the Effect of Sex, Age, and Maturity on Winter Blue Crab Distributions in the Chesapeake Bay

The blue crab Callinectes sapidus is an economically and ecologically important species in Chesapeake Bay. While past studies have examined blue crab spatial distributions of density and habitat partitioning in small-scale environments, none have examined con-specific factors affecting large-scale winter blue crab distributions throughout Chesapeake Bay. My study was conducted to describe large-scale blue crab spatial distributions in Chesapeake Bay that may be useful in future management efforts of blue crabs in Chesapeake Bay and Chesapeake Bay as a whole. Using data obtained from the annual winter dredge surveys in Chesapeake Bay, I created maps to examine spatial distributions of blue crab catch per unit effort (CPUE) and differences in habitat use between male, female, mature female, immature female, age-0, age-1+, age-1+ male, and age-1+ female blue crabs. Regression analyses were performed to determine the effect of the presence of one category of blue crabs on the presence of another. Winter spatial distribution and habitat partitioning of blue crabs in Chesapeake Bay were consistent with studies of blue crab habitat use conducted by Ramach et al. (2009) in the Rachel Carson National Estuarine Research Reserve and Hines et al. (1987) in the Rhode River subestuary. Male and juvenile female blue crabs have similar winter spatial distributions in Chesapeake Bay with a strong positive correlation between their CPUE values, indicating these classes of blue crab have a high degree of spatial overlap. Several hypotheses could explain this pattern including attraction between the sexes or similar habitat or environmental preferences. Age-0 and age-1+ blue crabs overlap substantially in their winter habitat use in Chesapeake Bay and their CPUEs were positively correlated. Immature female and mature female blue crabs were negatively correlated and occupied different areas of Chesapeake Bay; this difference was likely due to the migration of mature females to the mouth of Chesapeake Bay for spawning. The presence of different categories of blue crabs do not explain the winter spatial distributions observed. Further studies may be necessary to determine environmental factors driving winter blue crab distributions in Chesapeake Bay.

Project Title:

The Behavior of Mercury in Coal Combustion Residue-Contaminated Sediments

Coal combustion residues (CCRs), the products of coal combustion, contain high concentrations of heavy metals such as mercury. Recent structural failures of on-site containment ponds and leaching of CCRs has potentially endangered the health of adjacent water bodies. This study examines the influence of CCR enrichment of river sediments through the study of mercury, an abundant constituent of CCRs, and the concomitant production of methylmercury. We hypothesized that CCR contamination increases the exposure to mercury for aquatic life through leaching and resuspension mechanisms. Resuspension experiments were conducted using CCR-contaminated sediments from the Dan River and uncontaminated sediments enriched with 0%, 10%, and 30% CCRs in the laboratory. Sediments were sieved to obtain the silt-clay fraction, which was then resuspended in solution with a dispersant to obtain the separate silt and clay fractions and then analyzed for total mercury concentrations. We found that CCR particles and the mercury they contain are present primarily in the silt and clay fractions of sediment and there is a direct relationship between CCR concentrations and total mercury concentrations. These findings have implications for both the bioavailability of mercury to methylating bacteria, higher organisms prone to direct ingestion of fine particles, CCR spill event remediation, current industrial waste disposal practices, and further research required in this field. Our seven day incubations of river sediment cores enriched with CCRs did not increase methylmercury in porewater above controls, suggesting that there is no immediate risk of increased methylmercury bioaccumulation, however this does not necessarily reflect the long-term effects of CCRs on river ecology, which requires further research.

Project Title:

Predator-Prey Interactions of Copepod Eurytemora carolleeae Nauplii and Dinoflagellate Heterocapsa rotundata in Winter Conditions

We quantified the potential impacts Eurytemora carolleeae nauplii have on winter Heterocapsa rotundata abundances and added to our understanding of how different concentrations of H. rotundata impact the survival of E. carolleeae nauplii. Both H. rotundata winter blooms and E. carolleeae nauplii are poorly understood due to being understudied. We conducted grazing and survival experiments to determine ingestion, clearance, and mortality rates of nauplii according to varying concentrations of H. rotundata. We found the functional response of the nauplii to be Holling type II response with a maximum ingestion (Imax) of 0.2 μg copepod-1 day-1 and half saturation constant (K) of 245.2 μg C L-1. Any increase in H. rotundata concentration resulted in increased survivability of nauplii compared to when no food was present. As H. rotundata concentrations increased, the number of nauplii supported and the total amount of carbon removed increased but the percent of the total carbon concentration removed decreased. Nauplii in a supported population can theoretically remove on average 11% of the H. rotundata standing stock. Nauplii in a stable population do not seem capable of depleting their food supply. This has important implications for striped bass larvae that feed on E. carolleeae copepodites in the spring.

Project Title:

Microbial Activity and Abundance Along Land Use Gradients in Chesapeake Bay Watersheds

The importance of microbes to daily human life is becoming ever more apparent as new discoveries are made about the processes which govern how nutrients are cycled on Earth and how humans have and continue to alter those processes. One such set of processes, the nitrogen cycle, is particularly related to the metabolic functions of microbes. Stream and riparian systems have been found to harbor a variety of microbial species which provide a number of ecosystem services, many of which are intimately connected to nitrogen cycling. In this study, some of the metabolic processes of stream microbes were tracked and assessed along physical and chemical environmental gradients, which were ultimately compared to an assessment of human land usage. The results of this study show intimate connections between the ways in which land is used and the activity of microbes in nearby streams via a chain of interactions between changes and differences in the physical structure of microbial micro-environments and shifts in chemical concentrations. Future study could further aid in understanding how these micro-communities interact on a large scale such that management and educational practices can be devised.

Project Title:

Time-Resolved Photochemistry of Marine Fluorescent Dissolved Organic Matter

Marine dissolved organic matter (DOM) is one of the largest carbon reservoirs on Earth, but its composition, sources, and reactivity, all of which are essential in understanding the global carbon cycle, remain vague. The fraction of DOM that absorbs light in the ultraviolet (UV) and visible light spectrum is referred to as chromophoric DOM, or CDOM and is believed to be associated with the biorefractory component of DOM. In this study, fluorescence spectroscopy, specifically the excitation-emission matrix (EEM) technique, was used to characterize CDOM during prolonged photochemical degradation experiments. Solid-phase extraction was used to concentrate the DOM from six Sargasso Sea water samples collected from surface and various depths down to 4,537 m. Each sample was irradiated for 24 hours using a custom-built solar simulator system where absorbance and EEM spectra were recorded every 20 minutes using a flow through cell within the cuvette holder of an Aqualog fluorometer. Four distinct fluorescent peaks decreased while one increased during irradiation. This general trend was observed in samples from all depths. However, distinct differences of how quickly the fluorescence intensity changed for each peak area were obvious between depths, indicating distinct differences in the CDOM between depths. A parallel factor analysis (PARAFAC) model was also used to develop three PARAFAC components that reflect the variability of the fluorescence pattern during irradiation experiments and between samples. Observations of time-resolved changes in fluorescence and absorbance during solar simulated irradiation produced a more complete picture of the photo-reactivity of DOM with pronounced differences between the mixed layer and samples from greater depth.

Project Title:

Locating the Source of Denitrification in a Restored Oyster Reef

Reefs created by the eastern oyster (Crassostrea virginica) provide a number of ecosystem services, and their re-establishment is a priority in Chesapeake Bay restoration strategies. While nitrogen removal via denitrification has been shown to be a valuable ecosystem service of oyster reefs, mechanisms by which remineralized nitrogen is transformed have been incompletely identified. Oyster reef community components such as intact oysters, oyster shell and sediment were collected at restoration sites and denitrification was measured using the N2:Ar approach under ambient conditions and after experimental manipulations. Denitrification was observed in all reef segments, perhaps most consistently in oyster shells that supported a polychaete worm community that may create microenvironments conducive to coupled nitrification-denitrification. These results suggest oyster shell and associated microbial and metazoan communities may be important for oyster reef denitrification. Ambient sediment denitrification rates were low, but increased greatly with nitrate amendment, suggesting sediment nitrification rates were low. Understanding how the components of oyster reefs transform nitrogen is a first step in the development of models to predict denitrification rates in reefs with different community structure and physical-chemical conditions.

Project Title:

Consequences of Diversification Among Acartia tonsain the Chesapeake Bay

Acartia tonsa is an ecologically important copepod in many of the world's coastal bays, and is a common prey item for many commercially important fish in Chesapeake Bay. Despite its significance, little is known about A. tonsa genetic diversity within and between ocean ecosystems, and the possible ecological consequences of cryptic genetic diversity in this species. Previous studies have revealed two divergent, cryptic lineages of A. tonsa in Chesapeake Bay that sort spatially by salinity preference, but are also found to be sympatric at moderate salinities (6-12 ppt). The main objective of this study was to find out if these two clades (Fresh-F, and Salt-S) could mate and successfully produce hybrid offspring and any possible fitness consequences to these hybrid matings (hybrid breakdown, hybrid inviability, or hybrid sterility). A protocol was developed to isolate copepods early in development, identify their lineage, and make pair crosses with adults. Strict experimental controls were needed to ensure matings were only between the chosen paired individuals, and offspring fitness differences between within-lineage and hybrid crosses were recorded to measure consequences of hybridization. We found that hybrid (FxS) crosses produced the most eggs, followed by F lineage crosses, while S lineage crosses had the fewest, though there was no significant difference among the crosses (ANOVA P-value = 0.27). However, hatching success differed significantly between hybrid and parental crosses, with an average of just 3.75% for hybrids versus more than 60% in the parental crosses, and those hybrid individuals that did hatch, died within two days as nauplii larvae. Our results show that the two lineages of A. tonsain the Chesapeake Bay show hybrid inviability as they were able to produce hybrid offspring, but these offspring never made it beyond early development. Overall, our results confirm previous genetic data that suggested long term reproductive isolation between the clades, and suggest that where these lineages co-occur, there may be limitations to population productivity due to poor hybrid fitness, though this will need to be explored further. This experiment marks the first successful, directed crosses in Acartia tonsa in the literature, and the developed crossing protocol will be useful for future breeding studies in this species.

Project Title:

Determination of the Mineral Composition of Blue Crab (Callinectes sapidus) Carapace: Assessment of Spatial Variability and Response to Acidification in the Patuxent River, Chesapeake Bay

The goal of the experiment was to determine the mineral composition of the blue crab carapace, specifically calcium, magnesium, and strontium. These concentrations were determined spatially, after acidification, and ontogenetically. Calcium carbonate (CaCO3) percentage was also determined as it is the main constituent of the blue crab carapace. Results showed that the concentration of CaCO3 was 30% less than previous values reported in the literature. It was also determined that there is a significant difference in calcium concentrations in the carapace between juveniles and adults. No significant difference was observed in the concentrations of calcium and strontium in the carapace of crabs between different rivers in the Chesapeake Bay. As of now the acidification data set is incomplete but early numbers show no significant difference between calcium and strontium in the treatments.

Project Title:

Response of Phytoplankton Community to Nutrient Quality and Quantity Change in Two Coastal Lagoons, Florida Bay and Maryland/Virginia Coastal Bays

Coastal lagoons are characterized as shallow water bodies that are separated from the ocean by barriers. Differences in the quality of the nutrient pool, and the resident phytoplankton community can lead to different types of algal blooms in coastal lagoons. An experiment was conducted to quantify the effect of different types of nutrient enrichments on the phytoplankton community in temperate lagoons. Ten liters cubitainers containing water from Maryland/Virginia Costal Bays were exposed to six nutrient treatments and a control. Experimental cubitainers were enriched with different nitrogen forms and N:P ratios, as follows: ammonium (NH4+) 16µM; nitrate (NO3-) 16µM; dissolved organic nitrogen (DON) 16µM; ammonium 16µM + phosphate (PO43-) 1µM; nitrate 16µM + phosphate 1µM; and dissolved organic nitrogen 16µM + phosphate 1µM. All cubitainers were incubated in the field for 48 hours. This experiment was conducted twice during the summer of 2014. Results demonstrated there were no significant increases in any of the phytoplanktonic communities, however minor increases in these communities were possible to observe. The nano-plankton community dominated when exposed to ammonium and ammonium + phosphate, while prokaryotes only increased when exposed to ammonium. Pico-eukaryotes did not show any dominance in any treatments. Results suggested that responses of phytoplankton community from enrichment in lagoon ecosystems are diverse. 

Project Title:

Method Development for Field Measurements of Water-Air Methane Flux in Relation to Invasive Species Phragmites australis

Methane is sequestered in coastal wetlands and estimates suggest that 16% of total methane emissions each year come from these ecosystems. While methane is formed under anoxic conditions in the sediment, it can be released into the atmosphere by traveling through plant stalks. Mozdzer and Megonigal (2013) found in laboratory mesocosms that invasive Phragmites australis has a higher potential to release methane into the atmosphere compared to native Phragmites. As the biomass of invasive Phragmites increases, it is hypothesized that the methane flux would be higher where these plants are found. A method was developed to measure water-air methane flux in an invasive population of Phragmites and a population of Typha latifolia at Cove Point, a coastal wetland in Maryland. During flux measurements, an area of water was covered with a flux chamber, and air samples were taken at discreet times over a fifteen or thirty minute period. Results showed that invasive Phragmites (94.5±50.2 ppm/m2/min) had a higher flux than Typha (3.8±2.75 ppm/m2/min). Including all sites, it was observed that methane flux was variable (31.3±47.6 ppm/m2/min). Due to this high variability, it was concluded that the method employed was too short to capture the full flux rate. Therefore, it is recommended that the method is tested during initial field campaigns to verify that the time series is long enough.

The Blue Crab: Callinectes Sapidus

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