Our Students and Their Research

The Chesapeake Bay has experienced extensive areas of hypoxia (< 2 mg O₂ L^-1) in the past half-century as a direct result of eutrophication. The copepod Acartia tonsa, serves as a valuable prey item to higher trophic levels in the Chesapeake Bay and past studies have detected negative hypoxic effects on reproductive success and egg development rates. Little is known, however, on the physiological mechanism causing increased egg hatching time by hypoxia in A. tonsa. The goal of this study was to examine if lowered A. tonsa respiration rates may be a potential physiological mechanism impacted by hypoxia in the Chesapeake Bay. Egg development rates of female A. tonsa and egg hatching success were measured in hypoxic (< 2 mg O₂ L^-1) and fully oxygenated (> 5.99 mg O₂ L^-1) waters. Egg respiration rate and development were measured in two-hour intervals for a total of eight hours using a Pro2030 Professional Series YSI probe. We expected lowered respiration rate in hypoxic conditions to contribute to decreases in A. tonsa egg production, sinking rates, and ultimately egg hatching success. Results of this study may inform how future A. tonsa populations will be impacted in a changing environment and how those changes impact the future health of their predators.

The Deepwater Horizon oil spill was a tragedy resulting in multiple people losing their lives and millions of gallons of oil being leaked into the Gulf of Mexico. This spilled oil contained compounds known as polycyclic aromatic hydrocarbons that are known for their toxicity, specifically, their carcinogenic properties. In this study, Orange Cup Coral and Crested Oysters were collected from oil rigs located around the oil spill on two different sampling dates after the oil well was capped. These samples were analyzed for their concentrations of polycyclic aromatic hydrocarbons. Results were then compared through temporal and spatial analyses to determine where the polycyclic aromatic hydrocarbons were originating.

The blue crab (Callinectes sapidus) is both ecologically and economically important in the Chesapeake Bay. It supports the most valuable fishery in the Chesapeake Bay and plays a key role in coupling energy flow between the benthic and pelagic ecosystems. As a result, future management is needed to maintain a sustainable habitat quality for the blue crab. To estimate the value of individual habitats, we must be able to estimate the production of blue crab. Yet, currently we lack knowledge on the growth of blue crab. The goal of this project is to evaluate the utility of a biochemical assay of nucleic acid concentrations as an index of growth. Although the amount of RNA and DNA in tissue varies between species, the quantity of DNA is constant in cells, but the level of RNA fluctuates with metabolic activity. This study explored the potential for RNA:DNA ratios to measure crab growth in laboratory settings, based on elevated water temperature and nutritional ration. Data indicated that concentrations of RNA, DNA, and RNA:DNA could be measured with precision. The experimental results showed a growth response to temperature and ration treatment. However, results indicated that the relationship between RNA:DNA and growth was equivocal. Based on these results RNA:DNA does not appear to be a valid index of growth in blue crab.

Our data have quantified and characterized the composition of sediment and water quality parameters at the Susquehanna Flats. These variables are important factors in determining the rate of growth of submersed aquatic vegetation (SAV) for a given year. This summer, the SAV has had a much weaker growth than previous years. An unusual year of weather, with heavy fall storms and warm winter temperature, appears to have contributed to lower SAV growth and abundance. During the summer of 2012, we found that there were higher concentrations of suspended solids and phytoplankton chlorophyll inside the bed and down-bay from the bed than would be expected (based on conditions measured in previous years). The suspended solids are mostly made up of inorganic materials, suggesting that input of these materials into the water column from the fall storms discharge was more important than the eutrophication effects from nutrient inputs. Elevated levels of sediment down-bay are attributed to continual resuspension and deposition of bottom sediments inside the bed. It is likely that this increase in suspended particles caused decreased water clarity which in turn caused reduced SAV growth. Biomass samples collected in this study revealed lower SAV growth associated with increased epiphyte levels. Understanding how environmental changes influence the growth and abundance of SAV can help to explain both disappearance and resurgence of SAV in the Susquehanna Flats.

Potential sperm limitation has become a concern for blue crabs Callinectes sapidus in Chesapeake Bay. However, methods to quantify sperm in blue crabs are time intensive, which limits the sample size that can be obtained for sperm limitation studies. Our goal was to develop an efficient method for rapidly preparing spermathecae samples that more quickly quantifies sperm in blue crabs. Ultimately such a method will help address questions of sperm limitation in the Chesapeake Bay. In this study, we tested two methods for grinding spermathecae, and we tested whether DNA fluorometry is an accurate method for determining sperm quantity in blue crabs. We processed spermathecae using a dounce homogenizer and a Polytron, and compared sperm counts using a paired t-test. We compared fluorometry with direct counting of sperm cells under a microscope, a more time consuming yet already tested process, to determine if the sperm quantities obtained from fluorometry correspond with sperm counts. Samples processed using the dounce homogenizer had significantly higher sperm counts (p=0.007). Fluorescence was negatively related to sperm count (p=0.003), which was opposite the expected pattern. Our results indicate that the dounce homogenizer should be used to process samples and that the fluorometry methods we tested should not be used to quantify sperm in blue crabs.

The Eastern North Pacific Stock of the blue whale (Balaenoptera musculus) which occurs along the Western Coast of the United States, has been considered endangered since 1966 and despite concerted efforts to protect this species through whaling restrictions, their population is currently not increasing. It has been suggested that ship strikes are potentially hindering the recovery of these animals, so it is important to study their migration patterns to identify where they are at risk in order to improve protection for this species. This study analyzes daily state-space modeled positions from satellite telemetry data for 104 whales tagged from 1994 to 2008 in order to determine the migration pathways and areas of high-use for these animals that might overlap with shipping traffic lanes. This analysis revealed that high-use areas of blue whales occurred off the coast of Santa Barbara and San Francisco at latitudes 34°N and 37-38°N and coincided with major shipping lanes. This overlap suggests that adjustments to the shipping lanes in the Santa Barbara Channel and San Francisco Bay region could reduce the risk of ship strikes for these whales and potentially help this population grow in numbers.

Tidal freshwater wetlands provide a variety of ecological and social benefits such as habitat and protection of shorelines from storm surges. Unfortunately, sea level rise and development pressures have greatly impacted these ecosystems. Widespread historical wetland losses and ongoing vulnerability underscore the importance of understanding tidal wetland responses and resilience to stressors in order to achieve successful management of these resources. Recently reported accelerated rates of sea level rise are of particular concern. The role of wetland macrophytes in responding to sea level rise by changing stem density and morphology to create feedbacks with sediment capture has been well documented in tidal salt marshes. In some cases, the plasticity of these morphological responses results in changed structural characteristics of the plants. However, these processes have been less well studied in freshwater tidal systems. This study carried out field manipulations of Zizania aquatica, a dominant species in tidal freshwater marshes of the Chesapeake Bay. Morphological and biomechanical measurements were documented in control and experimentally thinned, lower density plots. The plants growing at a lower density were structurally stronger as demonstrated by computing a "factor of safety" that relates the biomechanical load capability of the stems to ambient forces. This study concludes that plant density influences the morphological and biomechanical characteristics of Zizania aquatica.

Coastal intertidal estuaries in California and Maryland were analyzed for the presence of ammonia-oxidizing archaea and bacteria and correlation with dissolved inorganic nitrogen removal. DNA was extracted from sediment samples and real-time polymerase chain reaction was used to quantify the presence of the amoA gene subunit, a portion of the gene that encodes for the conversion of ammonia to hydroxylamine. Ammonia-oxidizing archaea and bacteria were found with relatively more archaea in the coastal sediments. The abundance of ammonia-oxidizing bacteria appeared to correlate with dissolved oxygen. Removal of dissolved inorganic nitrogen did not appear to be limited by the abundance of microbes found in the sediment.

Nitrogen burial through sediment accumulation is a major sink within the proposed nitrogen cycle for the Potomac River. This study explored the relationship between nitrogen concentration and deposition patterns. ²¹⁰Pb geochronology was used to calculate sedimentation rates for sediment cores collected in the Potomac River. There were examples of both steady-state accumulation and event-layer deposition at different points along the river. Upon further analysis, it was found that sediment deposition patterns have a direct relationship to the rates of nitrogen burial. It is postulated that either channel location or river position affects dominant deposition patterns, with differences also noted in profiles of median grain size profiles and nitrogen concentration profiles.

The paradox of plankton has remained an open question in oceanography since it was proposed by Hutchinson in 1961. This project explored the effects of two theories, contemporaneous disequilibrium and mixing of patchy plankton communities. A fully functioning, 3-dimensional physical model was paired with an 80 state model including 77 taxa of phytoplankton. Lagrangian particles, paired with a phytoplankton community, were run through the model under various levels of biological mixing to represent different levels of micro-scale patchiness. Surface plots and vertical profiles indicated that the model was effectively depicting real world conditions. Results showed that decreased levels of biological mixing restricted and reduced the biodiversity of a phytoplankton community. A phytoplankton community which was completely isolated and prevented from mixing yielded the lowest biodiversity among communities. This low biodiversity observed at reduced mixing rates indicates that greater biodiversity is not supported by community history during micro variations in physical and chemical properties. The increased biodiversity at moderate to high mixing indicates that mixing between patchy or semi-isolated communities increases the biodiversity of each community.

The seasonal occurrence of low dissolved oxygen (DO) in Chesapeake Bay, caused by excess of nutrients and vertical stratification, affects most of the main deep channel of the estuary. Gelatinous zooplankton, such as the ctenophore Mnemiopsis leidyi, can switch the energy transfer in the food web from fish to jellies because some species are more tolerant to low dissolved oxygen concentrations than their prey and competitors. However, few studies have observed predation rates under different dissolved oxygen concentrations. We examined the gut content of the lobate ctenophores, M. leidyi and Beroe ovata, collected at two sites in the Chesapeake Bay with distinct DO concentrations. In addition, we performed a clearance rate experiment on M. leidyi feeding on brine shrimp Artemia salina, under saturated oxygen concentration. Our results indicate that at moderate low DO levels ctenophores are preying more on copepods than higher and anoxic DO levels. Early feeding experiments with A. salina suggest high ingestion and clearance rates of ctenophores. Understanding how gelatinous zooplankton change the energy transfer under low dissolved oxygen levels will enhance management tools could allow for better management of fisheries in the bay.

River ecosystems are influenced not only through environmental parameters but also through the mixing of different water currents. Mixing of water can alter zooplankton population dynamics through the change in environmental parameters; alternatively, movement of water can alter zooplankton populations through the removal or addition of new individuals. In addition to measuring the impact mixing has on zooplankton population, the study is also examining how different tributaries might have different mixing coefficients. Within the Little Choptank River, there are numerous tributaries that run in several axes. One hypothesis is that tributaries that run north to south will mix differently than tributaries that run from east to west. The different hypotheses were tested using thermistors and weather stations to calculate the heat budget; a zooplankton net was used to survey the zooplankton community. Results showed that while mixing did occur, they did not differ at various tributaries; furthermore, mixing is positively correlated with zooplankton, but zooplankton did not correlate with any other environmental parameters except for suspended solids. This would indicate that mixing is moving zooplankton instead of altering their environment.

In oceanic systems, microorganisms secrete siderophores to facilitate the uptake of Fe(III) which is a scarce and essential nutrient. Siderophores are capable of binding to metal ions other than Fe³⁺. In this research, desferrioxamine B was used as a representative siderophore and the stability constants for its complexation with Cu²⁺ and Ni²⁺ were determined at seawater ionic strength (0.7 M). Stability constants, log β₁, log β₂, and log β₃ for the Ni-DFOB complex were found to be 4.65±0.04, 7.72±0.03, and 9.76±0.07, respectively. Stability constants for Cu(II), log β₁ and log β₂, were determined to be 7.86±0.06 and 13.09±0.04, respectively, but log β₃ could not be resolved from titration data in the presence of excess DFOB, indicating that Cu²⁺ does not form a bond with the third hydroxamate group of the siderophore.

Water quality in Maryland's Coastal Bays has been declining dramatically due to land runoff. In Johnson Bay, nutrient levels are quite high considering low-density land usage. We hypothesize that nutrients and microbial contaminants may be from runoff of poultry manure from feeding operations and agricultural fields. We also hypothesize that microbes from these sources will exhibit resistance to common antibiotics used in nearby feeding operations. This study seeks to measure water quality parameters to identify nitrogen sources and assess antibiotic resistance and abundance of isolated indicator species of bacteria (e.g. Escherichia coli and Enterococcus spp.). Surface water samples were collected at three sites along three creeks that flow into Johnson Bay in order to measure water quality parameters and determine bacterial abundance. Specific agar plates were used to assess bacterial contaminant abundance and a modified chromogenic agar impregnated with four different antibiotics (tetracycline, ciprofloxacin, gentamicin, and oxacillin) were used to assess E. coli antibiotic resistance. Nutrient levels were very high, particularly nitrogen at the head of Powell, which was in excess of 600 M and fecal bacterial abundances were extremely high at the majority of sites. High nutrient levels and contaminant bacterial abundances measured in Powell Creek indicate it is the main source of pollutants. Finally, bacteria demonstrated the highest resistance to oxacillin, but overall the data was inconclusive because it did not decisively identify the sources of nutrient pollutants. These findings will help to better understand how land use patterns affect water quality and sources of contamination from runoff.

Nitrous oxide (N₂O) is a greenhouse gas and a stratospheric ozone depletor produced by denitrification and nitrification. Agriculture is an important source of N₂O. For my project, I looked at changes in N₂O fluxes over the course of the day to evaluate whether time of day matters to sampling. Static soil chambers were used to measure the fluxes. Low positive and negative fluxes were measured during a 10 hour sampling in a corn field, but fluxes were only significantly above or below zero for 50 percent of the measured fluxes. In a longer 24 hour sampling N₂O fluxes were below detection for the entire 24 hour period. I was unable to determine if a diurnal flux cycle existed because fluxes were typically undetectable. The low to non-existent fluxes are likely due to extremely low soil moisture and high soil temperatures.

Methane production is the last stage of organic matter decay. Several microorganisms take part in organic matter decomposition, including aerobic bacteria, sulfate-reducing bacteria, and the methane-producing methanogens themselves. Another group of microorganisms, the methanotrophs, reverses this process of methanogenesis with methanotrophy, where sulfate is reduced and the methane is oxidized back into carbon dioxide. Methanotrophic activity determines methane flux, and sulfate availability determines methanotrophic activity. It was originally hypothesized that the Chesapeake Bay's salinity gradient should therefore control methane flux. The flux-salinity data did not support this idea, however; it actually showed flux increasing as salinity increased. Another possible hypothesis was developed: dissolved oxygen content may serve as a control over methane flux, rather than salinity. While methanotrophs oxidize methane anaerobically in the sediment, methane can also be oxidized aerobically by bacteria both in the top layer of the sediment or in the water column, provided there are oxic conditions. The Chesapeake Bay undergoes hypoxic conditions seasonally, during which times there should be increased methane flux. We did not acquire sufficient data to explore this possibility, so further study must be devoted to tracing methane's movement after it passes from the sediment into the water column.

Eutrophication of the Chesapeake Bay has been a concern of environmentalists for several decades. A major source of nutrients that lead to eutrophication is from small streams and tributaries running throughout the Chesapeake Bay watershed. Studies show that degradation of water quality is prevalent in urbanized areas, and efforts are being made to help prevent pollution caused by decreased water quality from reaching the Bay. The purpose of this study was to examine the nutrient retention effectiveness of a stream restoration site in Anne Arundel County, Maryland during base- and storm-flow conditions. Concentrations of major anions (chloride (Cl^-) and sulfate (SO₄^2-)) and nutrients (nitrate (NO₃^-), total dissolved nitrogen (TDN), and total dissolved phosphorus (TDP)) in precipitation and the streamwater of both a partially suburban and a predominately forested catchment were determined. We found that nitrogen (N) and phosphorus (P) are being processed (uptake or loss) during base-flow periods but not during storm-flow periods. Additionally, anion and nutrient concentrations were generally higher in the urban catchment stream than in the forested catchment stream. The results of this study indicate that restoration sites can be effective at removing some N and P in streamwater thereby reducing nutrient loads to tributaries of the Chesapeake Bay.

The purpose of this research experiment was to analyze and determine the steady-state growth and elemental ratios of a marine diatom, Thalassiosira pseudonana, grown in a turbidostat when both nitrogen and phosphorus nutrients were varied in solution at non-limiting levels. Nitrogen as nitrate and phosphorus as phosphate concentrations were provided to the turbidostat cultures in different ratios. Direct measurements of nitrogen and phosphorus in the cells were not available as of this report, but indirect measurements based on the difference in the nitrogen and phosphorus concentrations in the media and culture vessels indicated an increase in nitrogen and phosphorus of the biomass with an increase in the nitrogen and phosphorus of the media. Variation in phytoplankton nutrient concentrations may play a role in nutrient transfer and the nutrition of higher trophic levels.

Microzooplankton are grazers that are less than 200 μm. They are important in regulating population levels of bacteria and other microorganisms in the ocean. Also they have an important role in the microbial loop transferring dissolved organic carbon and they are an important food source. Violacein pigment is a toxic pigment produce by bacteria as a chemical defense and it has shown to kill freshwater microzooplankton. We experimentally investigated study how the violacein-producing bacteria affected microzooplankton grazing and phytoplankton growth in estuarine plankton communities. Our results show that the addition of VPB has a negative effect on growth of some phytoplankton but little effect on growth than microzooplankton grazing.

A tidally and width averaged two dimensional numerical model is used compare salinity and velocity trends in the Chesapeake and Delaware Bays. Extensive data was collected on both bays for 2009 to increase the accuracy of the model and allow for future hindcasting. The salt intrusion length, stratification, longitudinal salinity gradient, and residual velocity were determined by varying the river discharge rate and the rise in sea level. Results were compared to theoretical expectations for both bays while new salt intrusion length data was acquired for comparison in the Delaware Bay. In general, the salinity field responded less sensitively than expected while the velocity behaved greater than expected to rises in sea level but as expected to changes in river discharge rate. A comparison of time-dependent models to quasi-steady models gave corrections to stratification..