Our Students and Their Research

Intact sediment cores with different densities (0, 3100, and 18,640 ind.m^-2) of the amphipod Leptocherius plumulosus, were incubated in laboratory microcosms. Denitrification and oxygen concentrations were determined from sediment-water fluxes using a Dissolved Gas Analyzer, an instrument that directly measures N₂ (and O₂ rates). Average rates of coupled nitrification/denitrification in bioturbated cores increased by 50 and 100%, respectively, while O₂ consumption rates increased by 38 and 100%.

The effects of nutrients on the higher trophic levels within estuarine systems is not completely known. This study will determine the effects of zooplankton densities on the growth rates of ctenophores. It was found that smaller ctenophores were not affected by increased zooplankton abundances. However zooplankton density did have an effect on larger ctenophores studied. It was found that small ctenophores grow more than large ones, therefore there is a size effect on growth rates.

In this study, embryo-larval assays were carried out with two species of fish; the mummichog, Fundulus heteroclitus, and the Japanese Medaka, Oryzias latipes. Fertilized eggs were collected, and embryos were exposed to two peroxisome proliferating chemicals; Clofibrate and Dibutyl Phtalate. The embryos were examined under a microscope through their development, and deformities were noted. In general, results were inconclusive. A slight trend was seen in the Diburyl Phthalate study towards higher instance of deformities in medaka embyros exposed to DBP. Both species had high mortality at the 10ppm treatment of each chemical, and Dibutyl Phthalate seemed to be a stronger PPA than Clofibrate.

The ctenophore, Mnemiopsis teidyi, was collected from two sites on the Patuxent River. Ctenophores were placed in three experimental treatments: (1) unstratified water column, (2) temperature stratified water column, and (3) dissolved oxygen and temperature stratified water column. Experiments to determine oxygen and temperature stratified water column. Experiments to determine ctenophore sensitivity to low dissolved oxygen levels were also conducted. Stratification sensitivity to low dissolved oxygen levels were also conducted. Stratification experiment results indicate ctenophores avoid both high surface temperatures and low dissolved oxygen levels (≤ 1.4 mg/L). Results of the sensitivity experiments show that 14 M. leidyi can withstand very low dissolved oxygen levels for at least a period of 96 hours.

The egg production rates of Acartia tonsa were studied using two main experiments. In the first, two different species of diatoms, Rhizosolenia fragilissima and Thalassiosira weissflogii, both differing in size, were fed to A. tonsa adults. Egg production rates decreased significantly in the R. fragilissima treatment compared to the T. weissflogii treatment. In the second set of experiments, increased A. tonsa adults grown in increased nutrient and metal treatments were compared to those growing in control treatments. Egg production rates decreased in the nutrient plus metal treatments.

In this study, dissolved lipids in the Delaware Bay were classified using the Iatroscan Thin Layer Chromatography Flame Ionization Detector. Samples were collected from 7 stations along the Delaware in June 1996. Water was filtered and DOM was separated into High Molecular Weight (1-30 KDa) and Very High Molecular Weight (>30 KDa) fractions for analysis. One important result of this study was the development of a workable method for analyzing dissolved lipids, especially phospholipids, using TLC-FID. From the data obtained, we can make several generalizations about the dissolved lipid content in the Delaware. We found that lipids occur at high and very high molecular weights and that these two fractions are usually different in their distribution. Mostly phospholipids, indicative of plant rather than animal sources, are present in the DOM. We also analyzed the lipid content of the diatom Skeletonema for comparison, and found that it may be a significant source of dissolved lipids, especially at the chlorophyll max.

Bluefish (Pomatomus saltatrix) is an important commercial and recreational fish that utilizes Chesapeake Bay along the U.S. Atlantic coast during its annual migrations. Little research has been done on feeding ecology, growth, and habitat utilization of bluefish in Chesapeake Bay. Thirty young-of-the-year bluefish collected from previous research programs, along with l0 bluefish recently captured by Maryland DNR and, 7 bluefish from personal seining efforts were included in analyses. Diet analyses of all 47 YOY bluefish from the Upper, Middle, and Lower Chesapeake Bay regions were used to estimate regional patterns and changes in diet with age/growth. Bluefish <75mm had higher gut fullness values and a more varied diet that included primarily fish, but also crustaceans and polychaetes. Bluefish >75mm preyed almost exclusively on fish, mainly bay anchovy, with an occasional crustacean or polychaete. Otolith daily increments of 20 YOY bluefish were counted to determine age, hatch dates, and growth rates. Hatch dates were predominantly in two distinct periods, spring and summer, possibly indicating two distinct cohorts. The hatch dates suggested a spring-spawned cohort (March 3l-May 15 hatch dates), and a summer-spawned cohort (July l3-August 25 hatch dates). The allometric equation describing the weight-length relationship of YOY bluefish is W=(1 x 10^-6) (L^3.337). An average growth rate of l.52mm/day was estimated from the slope of the linear regression of length on otolith increments (=age).

This experiment studied the mechanism by which diatoms reduce NO₃^- to NO₂^- and NH₄⁺ to dissipate electron energy acquired from the sunlight due to turbulent mixing. This mechanism allows the diatoms to not only survive conditions of high nitrate concentration and excessive irradiance, but to actually thrive and benefit from it. To demonstrate that this mechanism occurs, three diatoms and three flagellates were placed in an irradiance which allowed phytoplankton to grow at a normal rate as well as conditions of irradiance ten times that of the normal growth rate. This was repeated at temperatures of 20°​C, 15°​C, and 10°C. Concentrations of NO₃^-, NO₂^-, and NH₄⁺ were measured over a span of three hours for cultures at normal irradiance and at high irradiance. As a result, the diatoms, at all temperatures, produced more NO₂^- and NH₄⁺ at high irradiance. This remained relatively true for all species. The flagellates, while they may have produced NO₂^- and NH₄⁺, did so at the same rate as the normal irradiance cultures or at a rate even less than the normal light treatment. This remained relatively true for all species at all temperatures. This evidence suggests that diatoms have an additional mechanism to dissipate electron energy that flagellates do not appear to have.

Two experiments were run to determine the effects of food type, food regime, and water temperature on the consumption and growth rates of young of the year Acipenser brevirostrum (shortnose sturgeon).  Food regime and water temperature alone did not have significant effects on growth or consumption rates.  Food type was highly significant for growth rates.  Sturgeon fed bloodworms grew significantly less than those fed protein pellets.  Water temperature and food type combined had a significant effect on food type.  As temperature increased, sturgeon consuming bloodworms grew less and those consuming pellets grew more.

 A protocol for the live-staining of cryptophyte algae was developed to trace the uptake and retention of cryptophyte nuclei and plastids by Mesodinium rubrum, a photosynthetic species. The algae were able to be stained with Syto^TM under certain conditions, however, prolonged exposure to the stain caused cells to die and lyse. Occasional unexplained loss of staining was seen on some slides prepared. The cryptophytes were not able to survive the washing procedure necessary for experimentation. Experiments performed under these conditions showed a direct effect of motility and perhaps viability on feeding of Mesodinium rubrum. Live cryptophytes were fed to Mesodinium, fixed and stained with DAPI and observed. This method was successful in showing the uptake of nuclei at different time increments. Uptake was seen within minutes and retention appeared to last for up to 72 hours after introduction of prey. Mesodinium is able to take in and retain more than seven cryptophyte nuclei at once. Nuclear staining is useful in feeding experiments to quantify the uptake of cryptophyte organelles by Mesodinium rubrum.

Water samples from a fish farm in Princess Anne, MD were used as inoculum to filtered Choptank River water, to which were added five different treatments of nutrients Twice a week from June 20 to July 24, 1998, samples of the mixtures were 8 pm filtered to eliminate all heterotrophic organisms except planktonic bacteria. All the water samples were incubated for 24h in gas tight glass flasks. Aliquots were taken at intervals and analyzed for oxygen and carbon dioxide concentrations, bacterial production, and nutrient content in order to calculate values for respiratory quotients (RQ) and bacterial growth efficiencies (BGE). Variations in both RQ and BGE were seen over the course of the experiments as well as by treatment. The empirically based values of BGE were compared to BGE calculated with an assumed constant RQ of one. The comparison indicates a similar overall pattern by day and by treatment. However, variations between the two techniques show that using a constant RQ can lead to either an over- or under-estimation of the amount of bacterial production that indicates the importance planktonic bacteria in an aquatic ecosystem and in turn, skew the estimate of total carbon production in that system.

Microphytobenthos may contribute significantly to the primary productivity of an estuarine system and may reduce sediment resuspension. Resuspension of microphytobenthos was measured under controlled frictions velocities (u.). Additionally, a two week observational study of the microphytobenthos in the Horn Point Cove was conducted to determine whether resuspension due to friction velocity can account for the concentration variability of chl found in the water column. In the microcosm, we found a logarithmic relationship between u* and resuspension. However, there was a linear relationship between sediment chl concentration and resuspended chl concentration. These data suggest a positive feedback loop. As microphytobenthos grow near the sediment surface, they stabilizethe sediment. As the sediment becomes more stable, the water column becomes clearer, contributing to higher photosynthesis rates. In Horn Point Cove, there was a strong correlation between total suspended solids and turbidity in the water column. However, there was very little relationship between either of these parameters and chl concentration in the water column. This suggests that the resuspension of sediment and the concentration of phytoplankton in the water column are not controlled by the same forces. Although the relationship between u* and chl resuspension in the laboratory is strong, this relationship is weak in a natural system. This means that chlorophyll variation cannot be calculated from the measurement of other parameters in the system.

Growth rate was determined for the calanoid copepod, Acartia tonsa, using experimental ecosystems filled with water from the Choptank River, a sub-estuary of the Chesapeake Bay. Sampling occurred daily over a 15 day period in 3 replicate 1000 L mesocosms. Copepods sampled from one tank were sorted into size categories and measured for length and weight to determine a mean weight per copepod for each size category. In the remaining two tanks, growth rates were determined for the first 9 days of the experiment from the increase in copepod biomass (0.521 and 0.552d-t). These growth rates were compared to growth estimates from two commonly used models, the Huntley-Lopez and Hirst-Sheader models. Model results were similar to the direct growth rate estimates. Because copepod populations in this experiment experienced distinct conditions (i.e. food abundance and a majority of juvenile copepods), the Huntley-Lopez estimate closely resembled the direct growth estimate. The Hirst-Sheader model, however, is more likely to produce accurate estimations under all conditions, regardless of food availability or population dynamics.

This experiment explores the growth of the Chesapeake Bay blue crab, Callinectes sapidus Rathburn, through an in situ study. This experiment was performed to asses the adequacy of the current growth model used for stock assessment, the Von Bertalanfy model; along with collecting data for the creation of an incremental stage based model more representative of the molt cycle growth method of the crabs. Thirty crabs of various sexes and sizes were randomly selected and placed in hardware cloth and PVC cages and allowed to rest insitu. Weekly monitoring yielded carapace width growth data that as of now does not provided a distinct enough picture to test the proposed hypotheses. However movement toward a new model has been bade since a growth percentage per mol coefficient appears through regression analysis of the data. The coefficient is 19.02 initial size.

Increasing amounts of studies are focusing on the role of denitrification in the determination of the total amount of N in an ecosystem, particularly in estuarine environments. Consequently, the methods of measurement and analysis have recently come under close analysis. There is debate over the "best" method for measurement - three of the major methods being acetylene inhibition, r5N isotope tracer, and N₂ flux. Everyone's main goal is to find the most accurate method using the simplest procedure. This experiment proposes that it is possible to make sampling for denitrification easier by using the smallest size core that will give accurate results. This was tested using the N₂ flux method and sediment collected from a MEERC research tank. The sediment was collected and incubated with stirring overnight. The following day some of the cores were subcored and all cores were sampled along a time course to allow the measurement of rate of denitrification. The 3" and 4" continued to be stirred but the 1" was no longer stirred. It was found that although the 3" and 4" showed comparable results, the 1" cores were significantly different. This was not our hypothesized result, but it does lead us to believe that smaller cores that are not stirred are not suitable at this time for denitrification measurement using the N₂ flux method. Because of some experimental variability (especially the fact that the 1" cores were not stirred) additional studies are recommended.