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 2017

Project Title:

The Biogeography of Sulfide Oxidizing Bacteria in a Tidal Sub-Estuary of the Chesapeake Bay

Sulfide oxidizing bacteria hold ecological importance in their ability to convert naturally formed, yet toxic, hydrogen sulfide into the inert sulfate in marine sediment. Two sulfide oxidizing bacteria, Beggiatoa and cable bacteria, share common territory in marine sediments; however, not much is known about their interactions and natural distribution. The factors controlling the sulfide oxidizing bacterial distribution are not known, but it is hypothesized that salinity may be one of the determinants. To test this hypothesis, we sampled across a salinity gradient in the Choptank River and into the Chesapeake Bay, Maryland, collecting sediments from sites at conductivities from < 1 mS/cm up to 18 mS/cm. We found that Beggiatoa were at low abundance at all sites except at station on the west side of Chesapeake Bay, indicating that salinity alone does not define its geochemical niche. Enumeration of cable bacteria is still on-going, with preliminary results identifying their presence at the mid-Bay station in Chesapeake Bay. 

 

Project Title:

Effects of Inertia on Sargassum Distribution and Growth

The pelagic macroalgae Sargassum is a keystone species and is economically important. Previous models estimating its distribution and growth suggest that the Gulf of Mexico may be a seed region for Sargassum, where enough biomass is retained yearly to maintain a self-sustaining population in the Atlantic. We examined how adding inertia to a particle model could influence the retention rate and potential growth of modeled Sargassum within the Gulf of Mexico to determine its effectiveness as a seed region. We determined that adding inertia affected particles near regions of high divergence or high convergence more than the particles in more quiescent areas. We also concluded that there was a westward trend of particles when inertia was added which led to significantly fewer particles leaving the Gulf. Finally, we found that the potential for Sargassum growth is likely strongest near regions of modest convergence and divergence.

 

Class of 2016

Project Title:

Modeling the Response to Nutrient Load Reductions in Shallow Coastal Ecosystems

In an effort to reverse the effect of eutrophication, average nutrient inputs are being reduced in highly dynamic coastal shallow ecosystems; however, little is known about the response trajectories following these reductions. Long-term data (tide, nutrients, community respiration and hydrographic data) from the Chesapeake Biological Laboratory’s (CBL) monitoring program and a ~daily 10-day sampling, of the same measurements, were analyzed to understand the dynamics of the CBL pier ecosystem. A Water Column Model (WCM) was then constructed and coupled to an existing Sediment Flux Model (SFM) to simulate our ecosystem’s response to nutrient load reductions. A strong seasonal correlation (R2 = 0.7149) between community respiration and particulate nitrogen was observed as well as a dominance of nitrate in the early spring and a dominance of particulate nitrogen in the summer (as NO3 decreased). Daily variability for particulate nitrogen and algal N were generally low while high variability in community respiration seemed correlated with mean sea level (tide). Preliminary model runs showed significant differences in annual data and model dynamics. These results suggest that seasonal shift in the dominance of different nitrogen species reflect nitrogen inputs from the watersheds in winter-spring and accumulated algal material in the summer. Seasonal community respiration seems to be driven by the amount of organic material in the ecosystem while ~daily variability seems to be driven by the lability of the organic matter available as the tides changes. Finally, future improvements to the Shallow-Water Ecosystem Model are necessary for better simulations dynamics. 

Project Title:

Assessing the Fate of Organic Matter as it is Transformed Through Remineralization Processes in Surface Sediments

Organic matter in highly eutrophic estuaries goes through several degradation steps before being buried in the sediments. These steps are carried out by microbes that live in both the water column and the sediments. Understanding the breakdown of this organic matter is important, as it produces carbon dioxide and ultimately, methane, which are powerful greenhouse gases. The objective of this study was to characterize the organic matter that is available to the sulfate reducing and methane producing bacteria. Doing this would allow us to see how it changes in relation to the activity of sulfate reducers and methane producers and if there are differences in how it is transformed through different biogeochemical zones. This was done by first determining biogeochemical zonation in the Chesapeake Bay through down core changes in concentrations of sulfate, sulfide, methane, chloride, and dissolved organic carbon. Pore water was then analyzed for fluorescent dissolved organic matter components using excitation emission matrix analysis and statistical PARAFAC analysis. Three statistically different dissolved organic matter components were confirmed to be in the pore water. Changes in fluorescence of each component through the biogeochemical zones was observed, and thus it is possibility that it is utilized differently within these zones.

 
 
Project Title:

Ecosystem Changes in the Benthos: Length-Weight Measurements of Four Dominant Bivalve Species in the Northern Bering Sea from 2012 to 2015

Temporal and geographical trends in the sizes of members of the four dominant bivalve species (Nuculana radiata, Nuculana pernula, Macoma calcarea, and Ennucula tenuis) from in the northern Bering Sea in the St. Lawrence Island Polynya (SLIP) from 2012-2015 were studied. Clams were collected annually in July 2012-2015, and length and weight measurements were made on each individual clam collected. There was a significant decline in dominant bivalve length and weight over the 2012-2015 time period, and a significant replacement of the dominant species N. radiata by the dominant species N. pernula. In 2012, clam biomass was significantly higher in the northernmost stations (SLIP4 and SLIP4) than in the more southern stations (SLIP stations 1-3), but in 2015 clam biomass was more evenly distributed across the five stations. These changes in clam size and species composition over time may reflect population responses to ecosystem changes and food web re-structuring in the northern Bering Sea, particularly within the context of climate change. This length-weight dataset from 2012-2015 for dominant bivalves fills a gap in a larger dataset for benthic biomass which exists for the last 25 years in the SLIP.

 

 

Blog Posts:

See Sophie's posts to Fellowship Experiences, Maryland Sea Grant's blog written by and about fellows and their research.

"Exploring Climate Change in the Arctic Aboard an Icebreaker"

"Sea Ice, Sea Cucumbers, Sea Stars: My Summer Research off Alaska"

Project Title:

Oxygen consumption by Neomysis americana under realistic summer temperatures and salinity conditions

The mysid, Neomysis americana, inhabits shallow shelf waters and estuaries of the western Atlantic coast and plays a critical role in nutrient cycling and food web dynamics in these ecosystems. Despite their importance in these ecosystems, much of their spatial ecology and habitat requirements remain unknown. In Chesapeake Bay tributaries, previous research has shown that summer water temperatures can approach the lethal upper limit for N. americana. In this study, oxygen consumption rate (µg/min) of N. americana from the Patuxent River, a tributary of the Chesapeake Bay, USA, were measured experimentally as an indicator of metabolic stress under realistic temperature and salinity conditions. Mysids were incubated in a fully factorial design comprising three temperature (22, 26, 29°C) and salinity (2, 8, 16 ppt) treatments. The experimental design evaluated sex-specific responses as well as diel differences in oxygen consumption while accounting for mass-specific changes in oxygen consumption rate. Oxygen consumption rates were higher during the day for females and at low temperature (=22°C) and high salinity conditions (=16 ppt). There was no difference in oxygen consumption rate between males and females, but there was an interaction between temperature and salinity during both day and night incubations. Oxygen consumption was depressed at the lowest salinities in the 29°C treatment and in the highest salinities in the 22°C treatment (oxygen consumption rates peaked under intermediate conditions). This study indicates that the metabolic response of N. americana to temperature and salinity conditions is complex and that oxygen consumption rates can vary 3-4 fold within a realistic range of summer temperature and salinity conditions. These results also suggest that oxygen consumption rates should be only one of several indicators used to examine patterns of metabolic stress in N. americana in the wild.

Project Title:

Determination of stability constants for metal complexes with DTPA and salicylaldoxime at seawater ionic strength

The concentration of Gd-DTPA, an MRI contrasting agent, has been increasing in urban waterways over the past three decades. While Gd-DTPA is so stable in freshwater that toxic Gd3+ is unlikely to be released, little is known about the stability of this complex in marine environments. At high salinity, and thus high concentrations of Mg and Ca, Mg and Ca may compete with Gd sufficiently to release Gd3+ into the environment. In this study, the stability constants for complexes of DTPA with Ca and Mg were measured at seawater ionic strength by potentiometric titration to better understand the behavior of Gd-DTPA in marine environments. From these constants, the side-reaction coefficient for DTPA in seawater was calculated to understand the extent to which Mg and Ca compete for DTPA with trace metals and to determine conditional DTPA stability constants for seawater. This study concluded that Gd-DTPA largely dissociates in seawater, releasing toxic Gd3+.

Project Title:

Comparing Cable Bacteria Density Between Bulk and Root-Associated Sediments in Seagrass Beds

Cable bacteria, a newly discovered sulfide-oxidizing microbe, was recently identified with genetic techniques in the rhizosphere of seagrasses. These bacteria couple the reduction of oxygen in the water with the oxidation of sulfide in sediment and are believed to improve seagrass health by reducing the amount of sulfide intrusion that occurs in below-ground plant structures. Cable bacteria were extracted from sediment taken from three sites of varying salinity and dominant seagrass species within the Chesapeake Bay watershed. These bacteria were stained with a DNA probe using fluorescent in situ hybridization to determine the density of the filamentous bacteria present. Sediment was taken from three locations within each site: from root-associated sediment, bulk surface sediment within the seagrass bed, and bulk sediment collected away from the seagrasses. The bacteria were found at all sites and were present in high density within root-associated sediment, though the differences in density were not significant between sites or sediment type. The presence of these bacteria in sediment surrounding the roots of seagrasses could identify cable bacteria as an important mechanism controlling local geochemistry of seagrass beds and maintaining the health of the plants.

 

Project Title:

Sediment Small Core Incubation: An Analysis of Techniques for Measuring Nutrient Fluxes

In this study, two methods of sediment small core incubation, static and flow through, were compared to determine a relationship. To have results that were not site specific, sites of varying environmental conditions were used. A one-way ANOVA showed methods at the mid-bay sites to have a significant difference. While determined to be statistically insignificant, differences in shallow water sites could be seen when plotted. From the results it can be determined that there was a difference between methods although no constant trend was found.

Project Title:

Observing and Detecting Crassostrea virginica Filtration in Harris Creek, MD

Restoration of Crassostrea virginica, the eastern oyster, has been prioritized in large part due to the many ecosystem services that oysters provide including creating habitat and increasing water clarity due to their filtration. Yet, field observations of the ecosystem effects of C. virginica filtration on water clarity are limited. This study aimed to observe and detect C. virginica filtration over a subtidal, restored oyster reef in Harris Creek, MD. To evaluate the hypotheses that suspended solids would decrease in the direction of the tide due to oyster filtration, and that sediments would coarsen increasingly in the direction of the tide due to oyster bio-deposit resuspension and filtration, water properties, and particle size distribution were measured with an Acoustic Doppler Profiler (ADP), an optical backscatter sensor (OBS), and a Laser In-Situ Scattering Transmissometer (LISST). A clear signature of filtration was observed at maximum flood tide and maximum ebb tide: the amount of suspended solids in the water measured with the OBS and the particle volume concentrations measured with the LISST decreased over the reef in the direction of the tide. Floc particles greater than 66 um were not found to increase along the reef in the direction of the tide as was hypothesized. Instead, large patches of suspended floc particles were observed throughout the sampling period, and their source has yet to be determined. Results of this study provide evidence of the effect of C. virginica filtration in the field, and provide new evidence for patches of suspended flocs whose presence could have a significant impact on oyster filtration and associated processes.

Project Title:

Nitrogen Fixation among Aquatic Grasses of the Chesapeake Bay: a Comparison

To increase knowledge of the dynamic interactions between nitrogen fixing bacteria, submerged aquatic vegetation, marsh grass, and the surrounding environment of the Chesapeake Bay, samples of water, vegetation, and sediment were collected for analysis from eight diverse sites which varied in salinity, physical and chemical water quality and plant species. Nitrogen fixation rates were determined by performance of acetylene reduction assays using gas chromatography with a flame ionization detector and normalized to dry weight. A comparison of sample sites and microenvironments in relation to nitrogen fixation rates showed that nitrogen fixation was highest in the root zone and virtually absent from the water column during sampling dates. Anaerobic conditions increased N2 fixation rates in benthic samples. Positive correlations to fixation rates compared to sediment organic content and water column ammonia levels were also found. A persistent bloom of filamentous Lyngbya wollei dominated area N2 fixation rates in the Susquehanna Flats Vallisneria bed, causing trends divergent from other sites. Despite concerns of possible vegetation shading from floating Lyngbya mats and fixation driven eutrophication, the bed was dense and healthy with blades longer than those at any other site and encouragingly low nutrient concentrations. Overall, results point to relatively healthy ecosystems where nitrogen fixing bacteria exist in symbiosis with local vegetation.

Project Title:

Examining the Spatial and Temporal Dynamics of Forage Fish between the Patuxent River and Saint Leonard’s Creek using High Resolution Sonar Imaging

Forage fish are important planktivorous prey species within the Chesapeake Bay for many of the larger fish, birds, and mammals that inhabit the estuarine ecosystem. However, little is known about the distribution of these fish between the rivers and creeks that feed the Bay. A maximum of five transects were performed during each of two cruises, two in the Patuxent River and three in Saint Leonard Creek, where sonar data recorded fish abundance in the water column, which were then counted and paired with GPS data to provide a dataset with high spatial resolution, allowing fish abundances to be compared in and between transects, as well as between the creek and the river. Qualitatively, the forage fish prefer the creek to the river because of the abundant source of plankton as well as the lower risk of predation from larger piscivorous fish species. However, large schools of forage fish may prefer the river due to mobility. The sea nettle Chrysaora quinquecirrha may also impact forage fish spatial habits, but this data has not been analyzed yet. Future research includes this data, as well as examining the fish abundance at a higher temporal resolution to see more conclusive trends. 

Project Title:

Investigating How Oyster Filtration Model Simulations Vary with Different Filtration Rate Functions

This article outlines an investigation of oyster filtration formulations, which analyzed the changes in biogeochemical model simulations that resulted from different parameterizations of oyster filtration. Through controlled sensitivity analysis on the different aspects of modeled oyster filtration, we were able to identify and quantify variations in the model response that result directly from different expressions of the filtration rate. With numerical models being widely used to investigate the Chesapeake Bay region, this analysis yields further insight into their robustness and the necessity of accurate representations of the oysters’ physical processes.

Project Title:

Determining the Relationship of Land Use Practices to Sedimentation Rates in Adjacent Creeks

210Pb and 7Be radioisotopes can be used to determine historical sedimentation rates on decadal and seasonal scales. In the context of remediated agricultural lands, such sedimentation rates can allow for the observance of changes in land use practices over time. Over the past ~100 years Trippe Creek watershed saw multiple changes in land use with the most recent (~20 years ago) being the implementation of U.S. Department of Agriculture conservational programs. This study used 210Pb and 7Be analyses to examine changes in sedimentation rates in Trippe Creek. The purpose of the analysis was to determine if the implementation of conservational practices was able to lower the input of sediment into the creek. The results from the analysis were compared to210Pb and 7Be results from nearby Goldsborough Creek where the watershed is still dominated by active agriculture. Sedimentation rates in Trippe Creek decreased within the past 20 years while sedimentation rates in Goldsborough Creek increased. This comparison appeared to indicate that the implementation of conservational practices did relate to the decrease in sedimentation rates. 

Project Title:

Mass Balance Budget of Nitrogen in Tidal Creeks of the Patuxent River

Continuing poor water quality and high nutrient loading promoted the establishment of the Chesapeake Bay Total Maximum Daily Load (TMDL) and the development of Watershed Implementation Plans (WIPs). Calvert County, MD is expected to allocate $1.2 billion by 2025 in restoring the watershed. Awareness of the sources of excess nitrogen is a critical factor in establishing an appropriate management practice. We sampled water from Hellen Creek, St. Leonard Creek, and Island Creek of the lower Patuxent River and filtered for nutrients. Nitrogen box models were developed for the creeks to determine their greatest input of nitrogen. It was found that during the month of July the greatest input of nitrogen to these tidal creeks is from the Patuxent River. The nitrogen loading for Hellen Creek and Island Creek from the Patuxent were 10 kg/day and 6 kg/day, respectively. These data reflect the behavior of the system in the summer month however they may not correctly reflect the behavior during other months.

Project Title:

Assessment of fitness between cryptic species of Acartia tonsa in the Chesapeake Bay area in relation to hypoxia exposure

Acartia tonsa are copepods prevalent in the Chesapeake Bay and serve as a major prey component in estuarine food webs as well as aquaculture. Two genetically distinct cryptic species of A. tonsa have been identified in the Chesapeake Bay. These morphologically similar lineages, F and S lineage, appear to show preference to salinity levels with the F lineage preferring low salinities and the S lineage preferring a more saline environment. Research was conducted to determine a difference in lineage preference and tolerance to stressors of hypoxic conditions at different temperatures. Results showed A. tonsa of both lineages had a high tolerance to hypoxia at a temperature of 25°C and an increased mortality rate in a 30°C environment. Further tests and are needed to determine a significant difference in F and S lineage survivorship in the 30°C hypoxic conditions. Additionally, during the study population lineage proportions were determined from collection sites at varying salinities along the Choptank River. These F and S lineage proportions reiterated the salinity preferences of the lineages but also indicated a decrease in the prevalence of F lineage A. tonsa in the Choptank River since previous records in 2003. 

Project Title:

Assessing the Plant Community Controls on Carbon and How That Contributes to Methyl Mercury Production in the Pore Waters of a Chesapeake Bay Tidal Marsh

This study examines the role of vegetation composition on MeHg production in the Kirkpatrick Marsh, which is located at the head of the Rhode River. The Kirkpatrick Marsh has a diverse plant community but the community tends to be distributed in patchy assemblages. This study assesses whether or not the distinct plant community assemblages influence the biogeochemical environment around the roots and specifically the quality of dissolved organic matter (DOM), which has been shown to influence mercury cycling on larger scales. In order to accomplish this, we collected 33 pore water samples from 11 plots along two transect lines, covering the four plant communities that dominate in those specific areas in the marsh. Although vegetation removal and vegetation root experiments have been conducted, pore water species-specific vegetation studies have not been conducted in the Kirkpatrick Marsh. We collected pore water samples for MeHg, T-Hg, methane, chloride, sulfate, sulfide, CDOM (Colored Dissolved Organic Matter), and DOC (Dissolved Organic Carbon) on two dates: transect 1(TR1 6/23/2016), transect 2 (TR2 6/29/2016). Our results suggested that Phragmites australis was responsible for contributing higher quality CDOM, compared to the other plant species we examined in the marsh. Additionally, no evidence was discovered to suggest that any plant species was contributing a higher quantity of DOC to the marsh. Total mercury concentrations were significantly higher in plots containing Phragmites. As a result, we suggest that measures need to be instilled to monitor the spread of the invasive species, Phragmites australis, as it has the potential to take in inorganic and elemental mercury impacting the local pore water thereby maybe a growing source of mercury to the Chesapeake Bay. While MeHg concentrations were not statistically significantly higher than other vegetation types, on the date sampled, this was driven by the site closest to the land suggesting location was somehow altering MeHg accumulation in the pore water at this site. Further study is needed as the Phragmites community is the most biogeochemicaly favorable for methylation whose expansion could also greatly influence MeHg export.

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