Our Students and Their Research

The sediments of shallow coastal estuaries play important roles in storing, transforming, and removing nutrients from coastal ecosystems, and using quantitative models to accurately predict the response of sediment nutrient fluxes to environmental changes is crucial to environmental regulation. Microphytobenthos (MPB), photosynthetic microalgae inhabiting the sediment, are widespread in shallow areas and substantially alter the oxygen and nutrient dynamics of photic (illuminated) sediments, but past efforts to model ecosystem-scale sediment nutrient fluxes have largely neglected their effects. This study presents a sediment nitrogen model that computes seasonal cycles of diagenesis, nitrification, and denitrification, and includes a layer of MPB on the sediment surface that performs photosynthesis to characterize the effect of light exposure. Model outputs are validated against data from sediment core incubations at two sites in Chesapeake Bay. The model reproduces observed oxygen dynamics well; sediments exposed to light take up less oxygen during midsummer peaks in oxygen consumption than sediments in the dark, and become net sources of oxygen during spring and fall. Measured sediment nitrogen fluxes are more variable, and the model reproduces seasonal patterns less effectively, but overall both measured and modeled illuminated sediments supply less nitrogen to the water column than dark sediments. This model can be applied to water quality management to help develop recommendations for total maximum daily load of nitrogen to coastal systems. 

Cyanobacterial-derived dissolved organic matter (DOM) was analyzed using excitation emission matrix (EEM) fluorescence spectroscopy and the statistical Parallel Factor Analysis (PARAFAC) technique. The samples were exposed to simulated sunlight, which matches closely the solar spectrum, for a total of 20 hours while recording EEMs every twenty minutes to observe the photodegradation of the fluorescent DOM (FDOM). PARAFAC analysis was applied to deconvolute the EEM matrices into six separate components. The photodegradation kinetics was then calculated for each component and compared with previously obtained photodegradation data of marine and terrestrial FDOM. This 6 component PARAFAC model was similar to those generated from open ocean data and global DOM data sets. Regions previously considered “humic-like” FDOM showed similar fluorescence intensities and percent fluorescence loss when compared to marine DOM. However, the degradation kinetics of the “humic-like” and photo-labile component of bacterial-derived DOM was faster than that of terrestrial-derived DOM. This indicates significant differences in chemical composition. While it was previously believed the majority of marine FDOM was of terrestrial origin, marine FDOM samples showed degradation kinetics more similar to the bacterial derived FDOM.  This supported the hypothesis that a significant portion of marine FDOM is produced in situ. 

Coral Sr/Ca is often used to reconstruct past ocean temperatures in order to extend our knowledge of climate variability beyond the scope of instrumental records. Two sub-fossil corals with overlapping time record that grew in different reef settings on the island of Anegada, British Virgin Islands were used to test the replicability of a coral Sr/Ca record from different reef zones. Coral Sr/Ca was measured from coeval sections of core using ICP-OES and the geochemistry data were compared. Seasonal cycles of Sr/Ca in the corals were nearly identical, but the data differed in mean Sr/Ca. High levels of interannual variability in one of the corals contributed to low correlation between the two Sr/Ca records, likely as a result of the different environmental conditions in which the corals grew. Diagenesis is unlikely to have played a role in differing Sr/Ca data based on lack of evidence of diagenesis in observations of the surface of the corals and the coral x-radiographs. Data from this study suggests seasonal patterns of SST are the most robust feature of Sr/Ca-based SST records in Anegada from the two corals. 

This study assesses the use of OsmoSampler technology to monitor methylmercury (MeHg) production in a tidal marsh and examines temporal variability of MeHg in relation to controlling factors. We collected pore water samples in a Chesapeake Bay marsh using continuous pore-fluid sampling devices called OsmoSamplers. OsmoSampler technology has not previously been used in a coastal wetland or to investigate mercury cycling. We designed systems using OsmoSamplers to collect pore water samples for MeHg, methane, chloride, and sulfate analysis, sampling in a vegetated area and an area devegetated by clipping. Samples were collected over a 27 day period and stored in coils to create a temporal data set of in situ concentrations. This time series allows us to explore the methane-MeHg connection and the effects of vegetation removal on MeHg production. Some methanogens are known to methylate mercury, but the relative importance of the methane community in mercury methylation is not well understood. We hypothesized a positive correlation between methane and MeHg production, a decrease in MeHg production corresponding to vegetation removal, and that vegetation removal would inhibit sulfate reduction. We also sought to demonstrate the feasibility of using OsmoSamplers to look at MeHg flux in relation to these controls on mercury methylation. We conducted a lab experiment with a MeHg standard, comparative to the field experiment, to assess yield rate and resolution. This study is a preliminary exploration of OsmoSampler technology in a marsh environment. Using our system we have successfully collected samples from the vegetated area for methane, sulfate, and chloride analysis and collected and preserved samples from the devegetated area for MeHg and sulfate analysis. A number of issues arose when adapting the OsmoSampler technology to the collection of pore water in a terrestrial environment. We present the variability of measured concentrations with a discussion of adjustments for future long-term deployment.

There are ongoing oyster reef restoration projects in the Chesapeake Bay and these new ecosystems have yet to be fully explored. The carbonate system in an oyster reef is complex with contributions from respiration, calcium carbonate precipitation and dissolution, and other organic and inorganic processes. This experiment aimed to determine the overall effect of all of those processes by looking at the carbonate chemistry balance in a restored oyster reef. D filled with varying amounts of oyster reef material were left in the field for about a month before they were collected and an ex-situ incubation was performed over several hours. The trays with high densities of oysters had significantly higher rates of dissolved inorganic carbon increase and total alkalinity increase compared to trays with low densities or no oysters. High density trays had DIC fluxes of 29776.8±7593.2 µmol DIC m^-2 hr^-1 and TA fluxes of 20119.1±11107.6 µmol TA m^-2 hr^-1. High densities of oysters resulted in respiration controlling the carbonate system and forcing oyster shell dissolution. 

The Chesapeake Bay (CB) is the largest estuary within the United States. Sediment dynamics within this water body rely on comprehending the physical processes that determine the fate of fine sediments and nutrients. This study investigated seasonal and spatial changes in sediment erodibility at the Susquehanna Flats (SF) to show its interaction with the seasonal growth of Submerged Aquatic Vegetation (SAV). Positioned at the mouth of the Susquehanna River, the Flats receives approximately half of the total freshwater and suspended sediment flowing into the CB. From May 28-30, 2015, I assisted in collecting samples using sediment cores at six different sites around the Flats, accounting for an even grain size distribution. This ensured an unbiased data collection. Data were processed and analyzed using MATLAB scripts to process an implicit erosion equation, displaying results in the form of semi-logarithmic graphs. Our hypothesis stated that seasonal and spatial changes in SAV abundance were reflected in seasonal and spatial differences in sediment erodability. Analysis revealed that the presence of SAV biomass played a salient role in the erosion processes in the SF.

A rise in atmospheric CO₂ induces a greenhouse effect that also causes ocean temperatures and CO₂ levels to rise. These environmental changes may represent an additional energetic cost for blue crabs because they rely on the concentration of CO₂ in the water to deposit calcium carbonate in their shells. We conducted a respiration experiment to measure the effect of climate change on crab metabolism. Crabs were collected from the Chesapeake Bay and exposed to different heated and acidified conditions. After crabs had been exposed to the environmental conditions in the chambers for two molts, they were placed in respiration chambers to measure rates of oxygen consumption. Results indicated different trends in respiration rates between the treatments, although the patterns were not statistically significant. Crabs exposed to higher temperatures showed elevated respiration rates, while crabs exposed to high CO₂ demonstrated decreased respiration rates. The two factors of climate change (high temperature and high CO₂) did not demonstrate the highest respiration rate, but rather the crabs exposed to high temperatures and ambient CO₂ showed the highest mean respiration rate. These data suggest that crab metabolism may not change as much as expected due to climate changes. 

Salt and freshwater marshes exist at the interface of terrestrial and aquatic systems. Key differences exist between salt and freshwater marshes and even among marshes experiencing similar salinity; however, these differences are often not examined when drawing inferences about sustainability in the face of sea-level rise. The main purpose of this study was to examine five key parameters at play in both Monie Bay (salt water) and Jug Bay (freshwater): grain size, organic content, vegetative community, sedimentation rate, and turbidity over a period of two weeks. Previous studies have examined these parameters when studying marshes, although turbidity is often disregarded. In reality, the amount of sediment transported within the water column can have large implications on whether a marsh is likely to survive future sea-level rise or suffer heavy losses to erosion. Because of the complex nature of these systems, it is wise to be cautious of making long-term assumptions off of short-term data collection. 

The eastern oyster (Crassostrea virginica) is ecologically and economically important to the Chesapeake Bay, Maryland, USA. Its population, however, is currently estimated to be less than one percent of what it was historically. To restore oyster populations, techniques such as larval transport modeling are being implemented to aid the selection of sanctuary locations. These models can incorporate biological factors such as salinity-induced mortality, but no data from low-salinity areas such as the oligohaline Choptank River, a major focus of oyster restoration efforts, exist. The purpose of our study was to generate salinity-induced mortality data for oyster larvae from the Choptank River and compare their tolerances to those of oysters from different salinity regimes. We performed three experiments looking at the effect of salinities from 3 to 26 on the survival of larvae from ~4 to 48 hrs post-fertilization. While overall survival differed across experiments, we found a consistent minimum survival threshold between 5-7 and peak survival window between 9-16. These salinity values were about 7 lower than those of oysters from the polyhaline Long Island Sound (threshold: 12.5-15; peak: 17.5-27). This research has direct application to oyster restoration in the Choptank River and similar low-salinity areas by improving larval transport model predictions.

Nitrous Oxide, a potent greenhouse and ozone depleting gas, is produced in large quantities over oceanic oxygen minimum zones as a result of nitrification and denitrification. A majority of oceanic N₂O is produced at O₂ concentrations of less than 30 μmol/L, which are primarily located below the mixed layer. As a result, the time scales and pathways over which it effluxes into the atmosphere are poorly understood. Subsurface N₂O data collected at five stations by Dr. Alyson Santoro during a 2010 research cruise over the Eastern Tropical South Pacific low oxygen zone were analyzed in this study. In order to better understand the mid-depth ocean circulations that transport N₂O rich waters to the surface over this region, we used the Hybrid Coordinate Ocean Model (HYCOM) in conjunction with a Lagrangian float model. We found that the combination of coastal upwelling and horizontal advection of N₂O rich waters was responsible for both spatial distributions of atmospheric N₂O effluxes and their magnitudes. Station analyses also uncovered denitrification as the primary method for N₂O production in oxygen minimum zones.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.