Our Students and Their Research

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.

In aquatic systems, DOC is composed of refractory and labile compounds which are consumed by bacteria and other heterotrophic organisms. The cellular material produced through DOC consumption enters the microbial food web and may contribute significantly to fish or other yields of the system. DOC enters the water column as exudates from primary producers, as excretions from grazers, and as partially decomposed organic material leached from soils, sediments, and marshes. We investigated the distribution of DOC in Chesapeake Bay to examine the relative importance of these different sources within the estuary. DOC concentrations were measured at 10 stations on 7 cruises using the persulfate oxidation method. Property-property plots were used to compare the DOC concentrations with salinity, a conservative property, and chlorophyll a, a non-conservative property correlated with phytoplankton biomass. DOC concentrations declined from 200 μm at the freshwater end-member to 100 μm at the saltwater end-member. During May through July, DOC concentrations in the mesohaline region of the bay exceeded levels predicted by conservative mixing by 80-170 μm. From August through November, however, DOC exhibited conservative mixing. DOC was positively, but weakly correlated with chlorophyll a levels on two cruises, suggesting phytoplankton as an internal source of DOC.

Despite the continuous nuisance of the sea nettle in the Chesapeake Bay, little research on this species has been undertaken, hence this study of predation rates of Chrysaora quinquecirrha on the ctenophore, Mnemiopsis leidyi. Three types of experiments were utilized for this study. These varied by a density factor and experimental time periods, approximately 3-6 ctenophores per 20 liter tank for varying periods of time, 10 ctenophores per 1000 liter tank for 18 hours and 5 ctenophores per 1000 liter tank for 24 hours, respectively, in each case employing 1 sea nettle for each experimental tank. Although the project requires more data in order to be thoroughly analyzed, our results to date indicate that density plays an important role in predation rates.

Eutrophication of the Chesapeake Bay is in modern times an annual event brought about by a variety of factors, especially addition of inorganic nitrogen to the Bay in the form of farmland runoff and dumping of sewage directly into the Bay, This study aims at studying one of the effects of this overfertilization of the Chesapeake Bay—the occurrence of anoxic conditions in the subpycnocline region of the water column and the effects of this anoxia on benthic life. As a means of producing this study, we utilize mathematical modeling via High Performance Systems' Stella (version 2.0), and aim at concentrating on the benthic region of the Bay. Primary and secondary producing plankton in the water column are also modeled so that their direct effects on nutrient and oxygen cycling can be monitored. At this stage in the project, the model represents a closed system, and mimics the general trends of the planktonic components of the Bay. The lower water column approaches anoxia soon after the crash of the phytoplankton spring bloom, resulting in loss of benthic fauna, though not in a manner expected. The model will be expanded to an open system in the future, to include predation of secondary producers, inflow of inorganic nitrogen into the system, and more accurate details regarding benthic fauna metabolic processes, with the hopes of more accurately mimicking field data.

Short-term ammonium and nirate dynamics in three plankton size classes were studied in a mesocosm experiment using tracer and ¹⁵N isotope dilution techniques. A single site in the Choptank River, a sub-estuary of the Chesapeake Bay, was chosen for initial whole water collection. The samples were then size-fractionated and incubated. Subsamples were extracted over 36 hours and analyzed for ambient concentrations, uptake rates, and regeneration rates of the inorganic nitrogen sources.
In each size class population, initial utilization and regeneration of ammonium, preferred over nitrate, was noted. Absence of light for photosynthesis during the night in the smaller size classes caused nitrogen cycling shifts to affect the larger populations. Nitrate preferences dominated the upper fractions near the end of the 36-hour period. Diel variation was not related to nitrogen source preferences.

This study attempts to ascertain the effect of substrate addition on methanogenesis in anoxic estuarine sediments. Box cores of sediment were taken from the upper and middle Chesapeake Bay from which the top and bottom layers were collected, so that a profile of methanogenesis with changing sulfate concentration and changing or by methanogens alone ("noncompetitive"). When sulfate ion was decreased from ambient levelssalinity could be seen. The mud was slurried and assays were made of methanogenesis in the presence of added substrates that are metabolized by both methanogens and sulfate reducing bacteria ("competitive") or by methanogens alone ("noncompetitive"). When sulfate ion was decreased from ambient levels increased methane production from acetate was observed. No significant difference in methane production from methanol was noted between sulfate containing and sulfate depleted samples possible because of the high concentration of added methanol. When noncompetitive substrates were available methanogens were able to coexist with sulfate reducing bacteria in anoxic estuarine sediments. Sulfate did not inhibit methanogens from producing substantial amounts of methane from added trimethylamine dimethylamine and monomethylamine. Estuare methanogens from sulfate rich areas rapidly metabolize methylamines and appear readily able to adapt to acetate metabolism when sulfate levels decrease.