Our Students and Their Research

Membrane inlet mass spectrometry has been employed in the study of a number of microbial processes in natural environments and proposed for other including denitrification. This paper proposes a methodology for studying metabolic activity in estuarine sediments and describes preliminary experiments designed to measure denitrification activity. N₂:Ar rations were used to detect changes over time in N₂ concentration in intact core samples and used to detect N₂ disequilibria with depth in sediment pore waters. Denitrification was measured in a NO₃^- amended core in the first of three experiments but no denitrification was detected in subsequent experiments. These inconsistent results are attributed to probable variations in the microbial communities of the individual cores. Pore water samples showed a subsurface N₂ peak, possibly a result of denitrification. There were also indications that physical processes may be contributing to disequilibria in sediment gas concentrations, making it difficult to draw conclusions about microbial activity.

Due to overharvesting and parasitic diseased, the Chesapeake Bay population of oysters has declined dramatically. Aquaculture of oysters is presently impeded in part by the high costs (electrical and labor) of producing large quantities of the microalgal food required by the larval and juvenile life stages. Development of a diet consisting of commercially produced, inexpensive heterotrophic algae would reduce hatchery energy and overhead costs required to produce the phototrophic algae now used.
The effectiveness of algal diets was determined by comparing growth and mortality in larvae fed combinations of commercially produced heterotrophic algae, phototrophically grown algae, and starved controls. The heterotrophic algal diets were successful in raising larvae through metamorphosis, although with a lower growth and survival rate than the phototrophic algal diets.
The effects of resuspension by sonication on the heterotrophic algae was determined using a Coulter Multisizer and by DOC analysis. Sonication increase particle numbers and decreases their size. Resuspended heterotrophic algae solution contains approximately lOx more DOC than phototrophic algal cultures and the heterotrophic algal particles are considerably more dense than the phototrophic algae.
Determination of clearance rates using the Coulter Multisizer proved unsuccessful due to bacterial growth and feces production in the cultures. The use of a Flow Cytometer to determine clearance rates appears possible, but not completed at the end of this study.

I performed predation experiments to examine how summer oxygen depletion in the Chesapeake Bay affects the trophic interactions of fish larvae. The larvae of the naked goby Gobiosoma bosci were presented to adult gobies at varying dissolved oxygen levels. The feeding rate of the adults was found to be significantly lower at 1 mg/1 oxygen than at saturation levels and showed a decreasing trend at 3.0 mg/1 and 2.0 mg/1. This was due to the fact that the larvae tended to rise in the tank as DO decreased; at levels of 1 mg/1 larvae were almost completely spatially separated from the adults. When amphipods were used as the prey item, the adult gobies ate significantly less when the DO level was 1.0 mg/I than at saturation levels. Amphipods remain on the bottom of the tank at all DO levels. Therefore this decrease in feeding was due to the adult's decreased capability. When both larvae and amphipods were used as prey, it seems that fewer larvae are eaten. However, feeding rates on both still decrease when DO level was sufficiently low. My results suggest that low dissolved oxygen has complex effects on predator prey interactions and can possibly have important effects on trophic systems.

The abundance and distribution of eelgrass (Zostera marina L.) in Chesapeake Bay have declined in recent decades. The decline has been attributed to changes in water quality, such as increased turbidity and eutrophication. Plant survival is linked to maintenance of a positive carbon balance, and because shading and anoxic conditions probably reduce the carbon balance in eelgrass, low energy reserves could result in mortality. The Chincoteague Seagrass Study conducted in Chincoteague Bay, Virginia, provided the opportunity to quantify the non-structural carbohydrate reserves along the rhizomes of eelgrass to determine what effects shading and increased sediment oxygen demand have on the carbohydrate reserves of this species. It was determined that starch is conservative in eelgrass, and that shading has the most detrimental effect on the sugar reserves. Further, the interaction of shading and increased sediment oxygen demand reduced sugar reserves to the lowest levels recorded, and also decreased the amount of relative growth observed for the youngest internode tissue.

Contact rates between planktonic predator and prey are a function of the relative densities and velocities of predator and prey. Recent theories have postulated that small scale turbulence increases the contact rate between the animals by increasing their relative velocities. We explored this theory using the Sea Nettle, Chrysaora quinquecirrha, as the predator and the Brine Shrimp, Artemia salina, as the prey. Sea nettles were exposed to three turbulence levels in a constant density of brine shrimp. Although the large and medium animals did not show a significant decrease, the smaller (1.5-2.7 cm bell diameter) sea nettles captured more prey in the turbulent environment.

I have constructed a simple generic model of a marine ecosystem. The model is designed, by the simulation of the phytoplankton population, dissolved inorganic nitrogen, total organic carbon and oxygen, and the interactions and flows between them in the water column and sediments, to characterize community production, respiration and nitrogen cycling in a generic coastal marine ecosystem. In constructing the model, conceptual considerations and quantitative relationships were compiled from the relevant literature or were derived from empirical observations. As a test of its merits, the model has been calibrated so as to simulate the behavior of the mesohaline reach of the Chesapeake Bay. This was done using input data for this region of the Bay. Preliminary results of the calibration attempt indicate that the model is capable of replicating seasonal patterns from the Chesapeake. Perturbation experiments were carried out, and the results of these indicate that the model will be useful in assisting our understanding of how the various components of the ecosystem work together to determine overall behavior and individual phenomena.

Zoestera marina is a submerged saline aquatic angiosperm that densely inhabits Chincoteague Bay. The high productivity of this population of a marine primary producer demands high amounts of organic nitrogen, which is usually the limiting nutrient in marine systems. The purpose of this study was to determine what percentage of Chincoteague Bay Z. marina's nitrogen requirements were being met by nitrogen fixing bacteria associated with the plant's roots and rhizomes. The acetylene reduction technique was employed to measure nitrogen reduction over a 30h period, in August 1991 in Chincoteague Bay, Maryland. The nitrogen reduction rates were so low that even if Z. marina was utilizing all the newly fixed nitrogen, it would only provide the plant with 0.89% of a conservative Z. marina nitrogen requirement estimate.

Proteins and amino acids of the diatom Thalassiosira fluviatilis and the cyanobacterium Synechococcus sp. were quantified during an aerobic decomposition period of 61 days. We developed a micro-method for the extraction and analysis of proteins and amino acids in particulate material. The bicinchoninic acid assay (BCA) and a ribulose-l,5-diphosphate carboxylase standard was used to estimate total protein. Protein degradation did not follow a simple exponential, first order decay due to the presence of bacterial communities. Instead, there were oscillations, which correlated with changes in bacterial metabolism and population numbers. We estimated 40-50 % protein in the cyanobacterium. The diatom appeared to have significantly more protein but we determined that this was an overestimation. Chitin, a major component of diatom cell walls which precipitates with proteins, was found to enhance the BCA reaction. In order to correct for chitin, an assay was performed using a lectin that binds specifically to chitin. For the diatom, every amino acid seems to degrade at the same rate with the exception of arginine. Arginine amounts actually increase in the same time frame as when the bacterial populations increase. For the cyanobacterium, most of the percent composition of amino acids stays the same with the exception of arginine, glycine, serine, and tryptophan. All of these four amino acids also increase around day 10.

Increases in hypoxia and anoxia conditions occur throughout Chesapeake Bay during the summer. Copepods may be able to avoid these conditions by concentrating at the surface. However, their eggs sink into deeper, more anoxia water. The lower oxygen may affect the viability of the eggs and/or the nauplii hatching from them.  I plan to investigate the effects of low dissolved oxygen on the egg, first naupliar, second naupliar, and adult stages of two calanoid copepods common in the Bay, Arcartia tonsa and Oithona calcava. Acartia tonsa, the most abundant copepod in the Bay, may be found in the surface waters. Oithana calcava prefer higher salinity water and thus, may be found in more hypoxia areas. First, this study will compare the differences between the life history stages within both species.  Then, the comparative tolerances of the two species to low DO will be related to their distribution patterns.

Tidal wetlands are highly productive ecosystems, which lie between open waters and the upland area surrounding Chesapeake Bay. The nutrient cycling within these marshes is strongly affected by the presence of phosphorus sorbing mineral phases in sediments. The objectives of this study were to identify the primary forms of phosphorus in sediments of varying salinity tidal marshes of Chesapeake Bay by observing phosphorus profiles and to assess the importance of iron to phosphorus retention in these marshes. Phosphate flux is thought to be controlled by the presence of surface iron oxides (Scudlark and Church, 1989.) Because of their high productivity, brackish marsh sediments contain more total phosphorus and a greater fraction of organic phosphorus: total phosphorus than subtidal sediments from Chesapeake Bay. Marsh sediments act as a nutrient burial zone, although they comprise less than 4% of the total estuarine area of the Choptank River Drainage Basin, brackish marshes are capable of absorbing up to 10% of the total phosphorus loaded into the basin. Nine cores were collected from three main locations on the Eastern Shore of Maryland using a 50 cm McCauley corer. The ability of marsh sediments to retain organic phosphorus and iron- bound phosphorus in varying redox potentials was determined by IN HC1 extraction for 18 hours and further spectrophotometric analyses. Total iron content was measured with an atomic absorption spectrometer.

Feeding experiments involving the harpacticoid copepod, Nitocra spinipes, were carried out during the summer of 1991. Population growth, development, and lipid content of copepods (originally obtained from the lower Patuxent River, Chesapeake Bay) raised on several diets indigenous to the Chesapeake Bay were determined. Copepods were maintained under laboratory conditions and were fed the diets in a freeze-fried form. Lipid analysis was carried out using a thin layer chromatography-flame ionization detection (latroscan) system. Growth and lipid content of N. spinipes fed the lipid rich diatom was significantly greater (p=.0001) than recorded for copepods fed the other diets. Lipid analysis of copepods and their diets revealed that lipid content in N. spinipes reflected that of their diets.

For over sixty years studies have been done of the feeding of ctenophores, and the results have been used to establish the ctenophore's role in food webs and its role in regulating other populations. This study specifically observes the grazing of ctenophores in relation to effects on small ciliate populations (<40μm). Three grazing experiments were completed using natural zooplankton populations and one using cultured ciliates and copepods. Results show that the effect of ctenophore feeding can either reduce or increase ciliate populations. In the experiments with natural populations, when ctenophores less than approximately five grams wet weight were feeding, ciliate populations fell significantly. However, ciliate populations increased when larger ctenophores were feeding. Therefore, a positive correlation between ctenophore wet weight and the rate of growth of small ciliate populations exists. We hypothesize that the small ctenophores feed directly on the ciliates, while the larger ctenophores choose larger zooplankton prey (that themselves feed on ciliates). Thus, due to the larger zooplankton being reduced by the larger ctenophores, the ciliate populations are grazed less and can increase. A final experiment with cultured populations supports this theory since we found that ciliate populations kept with ctenophores increased less than ciliate populations placed with copepods and ctenophores but more than ciliates with copepods alone.

This study also reports a dry to wet ratio which is different from ratios used previously: dry weight =.013312 (&plusmn.000263) wet weight - .000235. At this time no known factor is documented as this cause, however salinity differences are suggested as a possibility.