Our Students and Their Research

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.

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.

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.

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.

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.

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.

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 (PO₄^3-) 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. 

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.

A relatively new ecological engineering design developed to reduce stormwater runoff and pollutant loads is called a Regenerative Stormwater Conveyance (RSC). RSCs are implemented in degraded headwater streams using sand, woodchips, cobble, and ironstone boulders to create a series of stepped pool conveyance structures. Concern has been raised that RSCs exhibit excessive accumulations of flocculate mats created by iron-(Fe)-oxidizing bacteria. Iron sources and other factors responsible for mat formation are poorly understood, as well as how RSCs affect stream habitat and water chemistry. Leaching experiments were done on riparian soils and substrates used in RSC construction with and without added dissolved organic carbon (DOC) to identify Fe sources and the factors driving its release. Riparian and hyporheic ground- and surface-water chemistry, and physical habitat parameters, from six RSCs and three reference (control) streams in urban catchments of the Coastal Plain and Piedmont physiographic provinces of Maryland were compared. A longitudinal transect of one RSC was done to evaluate potential density changes of Fe-oxidizing bacterial mats and the solute composition of water in successive pools/riffles of the structure. This study suggests that both ironstone and riparian soils contribute to Fe bacterial mat formation, a process accelerated by the presence of DOC.

Selenium (Se) is a contaminant of concern in areas affected by coal ash disposal and runoff from seleniferous soils, and may occur in inorganic or organic forms. To quantify the different effects of organic and inorganic selenium, we dosed the food of gray tree frog (Hyla versicolor) tadpoles with selenium dioxide at concentrations of 16.27 and 34.2 μg g^-1 Se dw and selenomethionine (SeMet) concentrations of 14.23 and 32.8 ug g^-1 Se dw. As tadpoles accumulated Se throughout metamorphosis, we measured biological endpoints including metabolic rate, survival, and growth. A behavioral test was also administered. Chemical analyses on total accumulated Se and mercury (Hg) have yet to be completed. Analysis of food was done via chemical digestion and mass spectrometry. Midpoint respirometry analysis showed that SeMet high tadpoles had significantly higher metabolic rates in μl O₂ g^-1 min^-1, with a mean of 3.26, than control (mean=2.20, p=0.0024), SeO₂ low (mean=2.52, p=0.0337), and SeMet low (mean=2.66, p=0.0176) tadpoles. No further significance was found regarding respirometry, growth, survival, or behavior. However, survival to the end of the study was lowest for SeMet high and SeMet low tadpoles, suggesting organic Se had more toxic effects. SeO₂ did not appear to be toxic, but because of the possibility of conversion to an organic form in a natural ecosystem, efforts to restrict release of Se rich wastes are warranted.

A wetland habitat at the Poplar Island ecosystem restoration project (Cell 6) has recently become home to a variety of birds, which use the pond for its nearby food and protection from predators. Additionally, a variety of cyanobacteria inhabit the site, including Anabaena, Oscillatoria, and Microcystis. Toxins produced by some of these cyanobacteria resulted in bird fatalities after a Microcystis bloom in summer 2012. Here, we examine the role that temperature, nutrients, and species composition plays in predicting how blooms might occur, whether toxins are produced, and how nitrogen fixation is performed. We found that cultures of cyanobacteria (primarily Anabaena) grow well in temperatures as high as 30°C, and that Anabaena from Cell 6 produces low levels of microcystin. Nitrogen fixation levels in cultures tended to spike periodically, possibly in response to fluctuations in limiting nutrients. Finally, nutrient concentrations played an important role in cyanobacterial growth and nitrogen fixation.

Due to high nutrient content in dredge material encompassing Poplar Island significant amounts of lodging has resulted. Additions of silicon are used in order to determine their effect on Spartina alterniflora on Poplar Island. Biomechanical tests were performed in order to determine the elastic modulus and measurements were taken to determine the second moment of area. ANOVA statistical analysis was performed and determined there was no statistical significance in any mean values depicted in the bar graphs in average stem length.

The Chesapeake Bay (CB) is the largest estuary in the United States. Water quality models in the CB depend on a predictive understanding of the physical, chemical, and biological processes that influence the transport and fate of sediments and nutrients. This study characterized flow at the Susquehanna Flats (SF) in the upper CB to show how submerged aquatic vegetation (SAV) influences hydrodynamics. The SF lie at the mouth of the Susquehanna River, which is the primary source of fresh water for the CB. From July 7-12, 2013, we deployed tide, wave, current velocity, and pressure gauges at seven sites around the SF to gather time series data on flow and its attenuation in and around the SAV bed at the SF. The design shows how spatial features including bathymetry, delta geometry, and grass bed geometry affect patterns of flow and transport. Data were processed and analyzed using standard MATLAB time series analysis techniques. Our hypothesis was that the flow would predominately be diverted around the grass bed into adjacent deep, unvegetated channels. Seagrass was minimally present during the deployment. Analysis shows that tide, river flow, and wind all play key roles in the physical processes at the SF.

The lateral salinity gradient responds to wind and tidal forcing in the Chesapeake Bay with spatial variation in salinity. The layers of isohalines tilt to the western shore during flood tide and slack before ebb tide, while the isohalines flatten during ebb and slack before flood tide, creating a semi-diurnal seesaw motion of the salinity gradient. The degree of the tilt correlates to the sensitivity of the tide to the Coriolis Effect, a phenomenon previously not well understood spatially. Sensitivity to wind forcing occurs on a more drastic scale and results in tilting to the Eastern shore. Up-welling and down-welling zones appear on the shallow shoals during wind events. The shallow shoals also experienced regularly changing salinity from 11-15 PSU within the interval of six hours. Both up-welling and down-welling zones and the seesaw motion of the salinity gradient may indicate lateral mixing.

The Macando oil spill three years ago released an unprecedented amount of oil into the Gulf of Mexico. The environmental impacts of this massive influx oil are still being studied today. This project focuses on the effects oil has on microbes in deep-sea anoxic sediments. In the anoxic sediments, three major microbial zones exist due to competitive-substrates. The primary microbes studied within each of the three zones respectively are the sulfate reducing bacteria, the methanotrophs, and the methanogens. In this experiment, incubations were done to produce a time series for each zone tracking the changes in concentration of sulfate and methane. It was hypothesized that the addition of oil will cause sulfate to be depleted faster in the sulfate reduction zone and methane to be produced sooner in the methane production zone. The results showed that sulfate remained constant throughout the time series despite oil being added, and that methane was produced sooner for the oil groups than the control groups in two of the three zones. These results contradicted the idea of competitive substrates. In conclusion, a new hypothesis was formed stating that the addition of oil has a greater effect on methane production via non-competitive substrates.