Our Students and Their Research

Emitted through natural and anthropogenic sources, the impact of mercury on human health has drawn increased awareness in recent years. Methylmercury, one of three mercury species, acts as a neurotoxin that is able to bioaccumulate through all levels of the food chain and may lead to neurological damage, fatalities, or brain damage in humans. Sulfate reducing bacteria (SRB) are considered the organisms mainly responsible for mercury methylation. However, factors such as dissolved organic carbon (DOC), pH, total mercury concentration, mercury complexation and sulfate and chloride anions are thought to control the availability of mercury to SRBs. This study will focus on answering the following questions: Are there significant differences in pH, DOC, total mercury, sulfate or chloride anion concentration between Lake Lariat and Liberty, or Clopper lakes, and can these explain the differences in fish concentration? and Do the measured parameters help explain the concentration of methylmercury in Lake Lariat at two different snapshots in time? Water and sediment samples were collected and analyzed for the presence of methyl and total mercury. The role of anions and DOC were also determined for each reservoir.

P. minimum, a toxic dinoflagellate found in the Chesapeake Bay, was investigated to determine the degree that lipid composition could be changed with environmental conditions. P. minimum was cultured in CO₂-limited conditions, CO₂-replete conditions, high and low light, and in different concentrations of selenium, in two forms, Na₂SeO₃ and Na₂SeO₄. The CO₂ cultures were obtained from Gary Wickfor's lab. He found that the CO₂-limited culture caused mortality of scallops in 24 hours while the CO₂-replete culture caused mortality in 4 days. Lipid composition in the CO₂-limited cultures indicates no change in sterols other than the loss of phytol but does indicate a change in the fatty acid composition. In the selenium experiment, only the high light cultures were filtered for analysis as the low lights had not reached a high enough concentration. Preliminary results indicate that Se has an effect on growth rates of P. minimum. Lipid composition, including 18:5n3 levels, did change with Se concentration.

Emergent aquatic plants that can readily take up nitrates and phosphates are often used in stormwater ponds to reduce the amount of nutrients that enter ground and surface waters. Two ten-day studies were conducted to determine TN and TP uptake rates for four freshwater plants: variegated water celery (Oenanthe javanica), canna lily (Canna glauca), bogbean (Menyanthes trifolia) and alligator-gat or water canna (Thalia dealbata). Plants were kept in solution with a target concentration of 3.0 mgTN and 0.5 mgTP. Relative growth rate and TN and TP uptake rates were measured and recorded. Average relative growth rates (% increase per day) for the first replication were as follows: C. glauca (3.18 %), O. javanica (2.72 %), T. dealbata (1.45 %) and M. trifolia (1.17 %). TN and TP uptake rates were calculated from initial and final nutrient composition and plant weights. Uptake rates for the first replication were C. glauca (179 mgN/g day^-1, 18.6 mgP/g day^-1), O. javanica (112 mgN/g day^-1, 35 mgP/g day^-1), T. dealbata (36 mgN/g day^-1, 1 mgP/g day^-1) and M. trifolia (25 mgN/g day^-1, 5 mgP/g day^-1). Using the same techniques, average relative growth rates for the second replication were as follows: M. trifolia (1.22 %), C. glauca (0.56 %), O. javanica (0.08 %) and T. dealbata (0.04 %). There were no significant differences among these relative growth rates. Two uptake rates were determined for each nutrient in the second replication. These uptake rates were: C. glauca (194 mgN/g day^-1 and 209 mgN/g day^-1, -17 mgP/g day^-1 and -20 mgP/g day^-1), O. javanica (26 mgN/g day^-1 and 14 mgN/g day^-1, -7 mgP/g day^-1 and  -12 mgP/g day^-1), T. dealbata (53 mgN/g day^-1 and 39 mg/g day^-1, 103 mgP/g day^-1 and -112 mgP/g day^-1), and M. trifolia (20 mgN/g day^-1 and 17 mgN/g day^-1, 1 mg/g day^-1 and 1 mg/g day^-1). C. glauca's TN uptake rates were significantly higher than all other TN uptake rates. There were no statistical differences among TP uptake rates.

Atmospheric deposition represents approximately 25% of the total nitrogen loading to the Chesapeake Bay, and ammonia(um) represents about 1/3 of this flux. In this study, ammonia(um) fluxes across the air-water interface were calculated using hourly atmospheric and surface-water measurements to investigate the temporal variability of ammonia(um) deposition directly to the Chesapeake Bay. A two-channel mist chamber collection system was used to collect hourly gas-phase ammonia and particulate ammonium concentrations at an over-water site on the Patuxent River with concurrent water measurements to calculate hourly air-water fluxes of ammonia and ammonium. Measurements were made on two separate dates in the summer of 2004. On July 29th, the net flux into the bay ranged from 74 to 440 ug NH₃-N m^-2 day ^-1 over an 8 hour time period, and on August 4th the net flux into the bay ranged from 119 to 1210ug NH₃-N m^-2 day ^-1 over a 6 hour time period. Gas-phase deposition dominated the overall deposition although particulate ammonium concentrations were usually larger by an average of 89%. The dominant factors for the large variability in the fluxes were the variation in gas-phase NH₃ and its larger gas-phase exchange coefficient.

The results of this work were presented at the American Society of Limnology and Oceanography (ASLO) 2005 Aquatic Sciences meeting. Download the poster (pdf, ~500kb).

Water quality over and downstream of a restored eastern oyster reef (Crassostrea virginica) was compared to upstream of the reef. Total suspended solids, chlorophyll a, urea, and ammonia, as well as the abundance of different size classes of phytoplankton were measured from surface and bottom waters along a transect upstream, over, and downstream of the reef. There was no significant difference in any of these parameters over the reef as compared to upstream of the reef or upstream versus downstream of the reef. This is likely due to the heavy sediment and nutrient loads that enter the estuary via farm runoff, thereby masking any effect the oysters have on water quality. These results suggest that other means of improving water quality in the estuary, such as introduction of a nonnative oyster or policies controlling runoff and sediment loads, should be investigated further.

The Chesapeake Bay contains chemical and biological gradients that influence bacterioplankton communities. Heterotrophic bacteria in the spring create anoxia and establish a seasonal oxygen gradient lasting until fall. Bacterioplankton communities have not yet been studied across a seasonal oxygen gradient. Community composition and cell abundance was examined in June and July, production in July and August, and respiration in August. The composition of bacterioplankton communities was characterized with denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S ribosomal DNA. As anoxia set in, community composition shifted only slightly in both surface and bottom waters. Similarly, community composition varied only slightly between oxic and anoxic waters. Bacterial abundance revealed similar trends in hypoxic and anoxic waters, but as anoxia set in an unusual sheathed cell became numerically dominant in anoxic and hypoxic waters. Bacterial production also exhibited similar trends across the oxygen gradient. Respiration was highest at the top of the gradient and decreased in the middle, but then increased near the bottom of the oxycline in extremely hypoxic waters. This implicates that the seasonal oxygen gradient influences the composition and activity of bacterioplankton communities.

The results of this work were presented at the American Society of Limnology and Oceanography (ASLO) 2005 Aquatic Sciences meeting. Download the poster (pdf, ~230kb).

Sediment biogeochemistry and nutrient cycling was investigated in estuarine beds of two species of submersed plants, Potamogeton perfoliatus and Ruppia maritime. Comparing beds of these two species, overall plant biomass was 1.5 to 2 times higher and ratios of above-ground to below-ground biomass were 2 to 3 times higher for P. perfoliatus. Rates of photosynthetic O₂ production per unit plant biomass were similar between the two plant species; however, biomass-specific respiration rates were significantly higher for P. perfoliatus. Fluxes of key solutes, including O₂, N₂, DOC, DON, NH₄⁺, and NO₃^- were measured using triplicate intact sediment cores collected from within each plant bed (containing plants, sediment and water) and from adjacent areas without vascular plants (containing only sediment and water). Sediment at each sampling site was also characterized by measurements of benthic chlorophyll-a content and solid-phase nutrient concentrations. Higher rates of total inorganic nitrogen recycling in P. perfoliatus sediments suggest enhanced trapping of particulate organic matter. Rates of coupled nitrification-denitrification (dark N₂ flux) were significantly higher in sediments containing both submersed plants than in adjacent 'bare' sediments, and rates were four-fold higher in P. perfoliatus sediments in comparison to R. maritime beds. These results suggest that the relatively widespread distribution of the opportunistic plant species, R. maritime, can initiate changes in sediment biogeochemistry that could promote further growth of submersed plants in otherwise marginal habitats. On the other hand, more stable plant species such as P. perfoliatus appear to have even greater ability to enhance sediment conditions by increasing nutrient recycling rates while maintaining relatively low rates of nutrient release to overlying water and associated stimulation of algal growth. These results, thus, provide valuable information to help guide submersed plant restoration in these estuarine regions.

Vertical and horizontal plankton tows were performed at two sites in the Choptank River to collect the copepods Eurytemora affinis and Acartia tonsa. E. affinis was collected from the Choptank estuarine turbidity maximum (ETM) and A. tonsa from the Horn Point pier seaward of the Choptank ETM. Adult copepods were selected for a 24 h grazing experiment where they were subjected to feeding in water from each collection site (ETM water and non-ETM water). Feeding was evaluated by gut fluorescence and microscope enumeration of prey before and after the grazing period. Ingestion rates (Is) and gut chl-a content were calculated from gut fluorescence. Ingestion rates (I) and clearance rates (F) for specific prey types were calculated from microscope counts. E. affinis had higher ingestion rates and gut chl-a content than A. tonsa in ETM water. A. tonsa had higher ingestion rates and gut chl-a content than E. affinis in non-ETM water. E. affinis' ingestions rates and gut chl-a content did not change from ETM to non-ETM water. Both copepod species showed a preference for prey >20 μm revealing a trophic cascade.

Arenicola cristata larvae were exposed to treatments of sediment with high levels of ammonium. These treatments were placed within dark and light areas. We measured growth of A. cristata under the different treatments and compared them to a control of natural sediment under a light area. We also did HPLC analysis on the sediments to look for phyto-pigments. We found that the light treatment had significantly higher growth compared to the other treatments. Not surprising the light treatment also had significantly higher concentrations of phyto-pigments. This leads us to believe that A. cristata larvae grow better in areas with high concentrations of ammonium. A. cristata larvae have been observed avoiding recruitment in such environments, despite the benefits for growth.

The effect of breakwaters on adjacent seagrass habitats is currently unknown. The goal of this study was to quantify the effect of a breakwater on a seagrass habitat. Two deployments were performed at Bishops Head Point, MD in which significant wave height, total suspended solids (TSS), chlorophyll a, light attenuation and intensity at sediment surface, temperature, epiphytic loading, sediment organic matter and grain size, and nutrients were measured in a site protected by the breakwater and a nearby exposed. No significant difference was found between the sites for wave height, TSS (total and organic), light, chlorophyll a, nitrogen concentration, or phosphorus concentration. The data from the first deployment showed that the sheltered site was significantly warmer than the exposed site. During the first deployment more epiphytes were observed at the sheltered site than the exposed site. The same pattern was evident during the second deployment, but the difference was not longer significant. The sediments in the protected area had significantly higher percent organic matter than sediments found immediately outside the breakwater. Most of the sediment behind the breakwater was composed of small (<125 μm) grains. We concluded that the breakwater studied may have a negative effect on seagrass habitat.

Nutrient cycling, with a focus on the nitrogen cycle, was observed in two beds of the seagrass Ruppia maritima. These two plant communities differed markedly in their contrasting ratios of biomass in vegetative versus reproductive shoots. Net fluxes of N₂, O₂, and NH₄⁺ across the plant-sediment interface were calculated for light and dark periods. Respiration rates per unit plant biomass were virtually identical for both sites, suggesting that reproductive and vegetative shoots have the same respiratory demands. In contrast, reproductive shoots had lower rates of oxygen production per unit biomass. The site with a higher proportion of vegetative shoots had high rates of denitrification (N₂ production), likely due to the higher rates of sediment nitrification stimulated by O₂ release from plant roots. On the other hand, the site with plant biomass dominated by tall reproductive shoots had higher rates of NH₄⁺ recycling, indicating that the presence of reproductive shoots enhances the nutrient quality of surrounding sediment. These findings suggest that success of seagrass transplantations would be enhanced in R. maritima beds with a balance of vegetative and reproductive shoots.

The ecological role of the eastern oyster, Crassostrea viriginica, in nitrogen cycling has provided a new outlook on the "value" of this organism within the estuarine system. It has been suggested that the oyster may play a crucial role in the removal of nitrogen by denitrification. To test this hypothesis an experiment is being conducted in La Trappe Creek, a tributary to the Choptank River, to study how oysters influence nitrogen fluxes between the sediment surface and the overlying water column. In order to make the proper attribution of fluxes to the oysters, the amount of biodeposits that accumulates at the sediment surface needs to be quantified. Using fluorescent particles as a tracer for the biodeposits, I was able to estimate this number from a percent recovery. Variation existed between the number of particles released by the individual oysters for each of the three trials and in the number of particles recovered beneath the oysters. Ultimately, there was little difference in the percent recovery of particles from the three trials; an average of 3.2% recovery was calculated.

Epifaunal prey items in the diet of the Hippocampus erectus were categorized and quantified for four locations in the Chesapeake Bay. Frequency of Occurrence and Mean % Number showed gammarid amphipods to be the primary diet item and the two indices were well correlated. Availability of epifaunal prey in the mouths of the Patuxent and York Rivers was sampled and selectivity indices were calculated for the two locations. The Strauss Index was found to be most useful for the current research, while the Ivlev Index and Forage Ratio are better descriptors of prey selectivity with a more complete sample. Due to its symmetrical values, the Ivlev Index is preferable over the Forage Ratio.

We investigated the effectiveness of using flow cytometry parameters of light scatter and fluorescence as a means of measuring phytoplankton carbon per cell, chlorophyll per cell, cell volume, and cell abundance to monitor growth. As primary producers, phytoplankton are an important indicator of Chesapeake Bay ecosystem health. Common methods of monitoring phytoplankton do not take into account chlorophyll-to-carbon (ChlC) ratios or phytoplankton size spectra. We first considered the effects of mismatched sheath fluid and sample salinity on light scatter. We determined that matching sheath fluid to sample salinity gives the same forward light scatter (FLS) and side light scatter (SLS) values as using 0 salinity sample and sheath. Using 13 cultures, ranging in size from 1-30 m, we were able to produce a calibration curve for SLS to C/cell. Among our cultures, C/cell varied from 0.1 to 3600 pg/cell. The difference between the predicted and observed C/cell varied from 4 to 116% with an average of 41% and a median of 35%. This allowed us to create size spectra for our samples from the ACE-InC cruises of the Choptank and Patuxent Rivers. Size spectra data revealed the trend of larger cells tending towards the mid-river station and smaller cells tending towards the upper and lower stations in the river. This case study demonstrates that FCM parameters can provide complimentary information to chl a-biomass measurements and improve our ability to monitor phytoplankton as an indicator of Chesapeake Bay health.

In this study, a comparison was done between the Florida and Chesapeake Bays to better understand the biological and chemical influences of MCB on resuspension. Using a new erosion device, we were able to analyze eroded materials at different levels of shear stress, and better quantify the relationship of MPB, biomass and EPS concentration on resuspension. Our results showed that while EPS and chla are good determinants of erodability, other factors may be present which can override the influence of MPB.

PCBs are toxic, organic compounds found in the environment. They pose numerous threats to fish, wildlife and humans. Their hydrophobic nature makes them readily bind to organisms, soils and sediments. Determining binding constants, or how well PCBs bind to organisms or sediments, is integral in defining the extent of their potential harm on the environment. Therefore, binding constants for 18 PCB congeners from two samples, Cypress Swamp DOC water and a particle water sample from Fishing Bay, were found. Prior to finding the binding constants, however, several preliminary experiments were performed including a technique verification, PUF sample, and an exposure series using SPME fibers. Since the SPME technique is till relatively new, a new SPME verification experiment was also performed to ensure that SPME fibers only measure the dissolved, as opposed to the total concentration of PCBs in a given sample. Results show that the binding constants obtained generally followed expected trends and that they are comparable to other studies done in the Upper Hudson River. Results from the SPME verification reveal that indeed SMPE fibers only measure the dissolved phase concentration of PCBs in a given water sample.