Our Students and Their Research

G. galatheanum and P. minimum are two small, bloom-forming dinoflagellates that have been associated with red tides in Chesapeake Bay. Algal blooms are the consequence of net growth, from increase division rates, or decreased mortality rates. In the past, most attention and research has been directed towards variables that increase growth rates, however, little is understood about the impact of predation, or zooplankton grazing, in terms of development and termination of toxic blooms. This study examines the distribution of the potential grazing pressure on G. galatheanum and P. minimum in the Chesapeake Bay and its tributaries. In situ grazing experiments were conducted in June and July, on (2) separate cruises. Dinoflagellate cultures were stained with CMFDA, a vital green fluorscent stain, and added to treatments with (<200 micron) and without (<1.2 micron) the natural microzooplankton assemblages collected at each station. In 58.6% of the experiments, the grazing coefficient was higher the maximum potential growth rate of G. galatheanum and P. minimum in culture, 0.94/d (Li et al., 1999) and 1.15/d (Grzebyk & Berland, 1996), respectively. Thus, grazing by microzooplankton has the potential to prevent net growth of these dinoflagellates. In June, there was a significant correlation between ciliate abundance (>20 micron) and potential grazing on P. minimum. Grazing pressure appeared to be higher in June than in July, but the difference was not statistically significant. In conclusion, microzooplankton have a high potential to control dinoflagellate growth, although the grazing pressure was rather variable throughout Chesapeake Bay and its tributaries.

The influence of sediment deposition on a meadow (Vallisneria americana) and a canopy (Myriophyllum spicatum) forming species of submerged aquatic vegetation was investigated at Blossom Point, MD. Two depths (5 and 15 cm) of sediment were deposited among these plants. After 7 days, productivity was determined by measuring photosynthesis vs. irradiance curves and biomass change over time. V. americana showed an increase in the rate of O₂ production (alpha) with increasing sediment levels however the maximum O₂ production (Pmax) decreased at the high treatment. M. spicatum's rate of production of O₂ (alpha) followed the light history pattern at the specific sites. The maximum amount of O₂ produced (Pmax) decreased with an increase in sediment deposition possibly because of the increase in phytotoxins at the base of the plant. Biomass data provided no conclusive data possibly due to the short time period of this experiment. In summary, sediment deposition affected the meadow and canopy species through different processes.

This study examined the uptake and assimilation of the trace metals cadmium, chromium, and zinc from suspended sediment by the estuarine copepod Eurytemora affinis. Adult females were exposed to suspended sediment labeled with radioisotopes of Cd, Cr, and Zn at four different concentrations for approximately 44 hours. Activity in the animals was then determined using a gamma counter before depuration and again after a four-hour depuration period. Cadmium uptake showed significant difference from controls at 300 mg/L sediment levels, as well as at 50 mg/L on one occasion. Zinc uptake was highly significant at all sediment levels. No significant differences were observed between non-depurated and depurated copepods, suggesting that perhaps material in guts is an insignificant contribution to total body load.

The hypothesis that humic substances stimulate the growth of dinoflagellate populations was tested in natural assemblages from the Chicamacomaco and Choptank Rivers. All humic additions were from a stock solution of Aldrich Humic Acid. Experiment 1 used samples from both rivers to see the effect of adding humics (HS), at a concentration of 5mg/L on heterotrophic and phototrophic dinoflagellate populations as well as on other parameters such as absorbance, pH, nutrient concentration, and Chl a concentration. Both trophic types of dinoflagellates decreased in density throughout the experiment, however, this decline was up to 50% lower in the HS treatments. This result was not consistent when the experiment was repeated. Nutrient Experiment 1 repeated this procedure for a Choptank sample, but added two treatments: one with nitrogen at 44μM and phosphorus at 1.8μM (NP), and one to which both nutrients and HS were added (HSNP). Heterotrophic dinoflagellates showed positive growth rates, however, they were 50% lower in the HSNP treatment then in the NP treatment. Phototrophs again showed a decline, however, the rate of decrease was slowed by 300% lower in the HSNP treatment than the NP treatment. Data, such as Chl a, indicate other parts of the community may be affected differently by the addition of HS.

We offer a quantitative synthesis of current research on in situ microbial community diversity. The lack of explicit quantitative statements on phylogenetic distribution, and the recent exponential climb in the application of 16S rRNA gene probes to natural systems prompted our research. In this review, studies in several natural systems, both broad and remote, were examined from some leading journals, including Applied and Environmental Microbiology, and FEMS Microbiology. Studies utilizing molecular techniques, such as polymerase chain reaction amplification (PCR) or fluorescence in situ hybridization (FISH), were synthesized according to the reported percentages of rRNA contribution by phylogenetic clades to the total community rRNA amount. Distinct shifts in community profile were observed across marine, freshwater, sediment, soil, and some extreme systems. Moreover, comparison of Eubacteria staining success observed a systematic variance in resolution across these natural systems. This observation, when taken with the small, unbalanced collection of in situ studies, suggests a need for further research to refine our understanding of the extent and function of microbial diversity.

The Atlantic Croaker, Micropogonius undulatus, is increasing in abundance in the Chesapeake Bay thus increasing its importance in the ecosystem. On three TIES (Trophic Interactions in Estuarine Systems) cruises per year from 1995 to 1999, stomach contents of Atlantic croaker were examined to obtain percentage and frequency of occurrence of prey. Over the summer of 1999, 323 croaker were examined and contents were categorized into polychaetes, fish, crustaceans and bivalves. Out of the factors examined during this study, there were three significant factors that play a role in croaker diet. The primary was ontogenetic size classes, with two other factors: one seasonal and one regional difference. There was also a noticeable difference in three fish caught by hook and line off of the pier at CBL. Such dietary changes bode well for the future of croaker, demonstrating high adaptability and an ability to manage environmental instability.

The purpose of this study was to determine the influence of nutrients on bacterial metabolism in salt marsh water with respect to the natural system and under conditions of experimental nutrient enrichments. Water samples were collected from the Monie Bay National Estuarine Reserve in Maryland during June and August 1999. Control and enriched samples were incubated over a 36 hour period in an environmental growth chamber at ambient temperature. Nitrogen and phosphorus were added to samples to achieve 80μM (NH₄) 2SO₄ and 5μM KH₂PO₄ concentrations. Bacterial production rates were measured using the 3H-leucine uptake method (Smith et al. 1992) and bacterial respiration was measured as changes in Oz concentrations using membrane inlet mass spectrometry (Kana et al. 1994), 4 gradient of nutrients was found in the natural system with a corresponding gradient of bacterial growth efficiency and bacterial production. Total nutrient concentrations (N and P) and average growth efficiencies were highest in Little Monie Creek, a tidal creek in close proximity to a chicken farm, and lowest in the Open Bay. Little Monie Creek was not limited by nutrients while the Open Bay waters and Little Monie Creek was not limited by nutrients while the Open Bay waters and Little Creek, a relatively pristine tidal creek were limited by nitrogen, phosphorus, or both. The experimental nutrient additions of nitrogen, phosphorus and nitrogen and phosphorus resulted in increased growth efficiencies for the Open Bay and Little Creek, but not for Little Monie Creek. The results suggest that bacterial growth efficiency is positively related to nutrient concentrations in Monie Bay National Estuarine Reserve.

Grazing preferences for controlled environment of the Calanoid copepod Acartia tonsa under a normal and an enriched nauplii concentration were determined using ambient Choptank River water. The sampling was done on two separate days in the MEERC lab at Horn Point. Two sets of tanks were sampled, 3 control and 3 nutrient tanks. Samples from the tanks were split into 12 1-L bottles, 6 containing copepods (experimental bottles) and 6 containing none (control bottles). Also, bottles 3,4,7,8,11 and 12 contained a 3x the normal enriched nauplii concentration. The results showed that when more nauplii were available, more nauplii and less phytoplankton were ingested. Copepods may switch their grazing preferences during changing environmental conditions and to regulate the population density of their environment.

The potential for ecological impacts, due to additional suspended sediment introduced to the water column, as a result of agriculture and construction, have become more widely recognized in recent years. However, the possible effects of increasing levels of suspended sediment on the general health and reproduction of copepods, a major food source for larval and commercial fishes are not well understood. In this study Acartia tonsa were added to l-L jars containing 0.5 ppm Thalassiosira weissflogii and varying levels of commercially available Kaolin corresponding to suspended sediment levels of 0, 49.5, 147.5, and 382 mg/L. The females were allowed to reproduce for 24hr, at which time the contents of the jars were screened and preserved for counting. Each sediment concentration was tested in quadruplicate. Results varied between trials. In the first trial, egg and nauplii production showed a significant decrease with the addition of sediment. In the second trial there was virtually no difference in reproduction at low levels of sediment, with a significant increase at the highest level. Possible explanations and implications for these results will be discussed.

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.