Although the specific research projects outlined below are no longer active, many of the research questions that were addressed in them are still being persued. Questions like "How do wetlands respond to environmental changes?" and "How does biogeochemistry work in these ecosystems?" are still relevant and pertinent today. Much of the data associated with these projects have been analyzed and can be found through the Publications page.

Former projects:

Saltwater intrusion in Delaware River tidal marshes

Anaerobic metabolism in tidal wetlands

Microbiology of iron cycling

Salt marsh responses to increased flooding and wrack deposition

Tidal freshwater marsh C and N cycling

The rhizosphere is a dynamic environment because the release of oxygen from roots (radial oxygen loss) introduces oxygen into otherwise anoxic wetland sediments and soils and initiates a series of redox reactions that can affect the speciation of elements such as iron, carbon, and sulfur. One result of these redox reactions is the formation of iron oxyhydroxide  precipitates (iron plaque) on plant roots. Iron oxidation kinetics suggest that most plaque formation in circumneutral environments is chemically driven, but the discovery of lithotrophic Fe(II)-oxidizing bacteria (FeOB) raises the possibility that microbes play a significant role in plaque formation.

We have investigated the microbial role in iron oxide formation using pure cultures of strain BrT, a lithotrophic FeOB isolated from the rhizosphere of Typha latifolia (cat-tail). In bioreactors, we measured rates of Fe(II) oxidation in the presence and absence of iron oxidizers and found that the bacteria could account for up to 50% of total (biological + chemical) Fe oxidation. In microcosms where radial oxygen loss from Juncus effusus (soft rush) was the only oxygen source, we studied Fe(II) oxidation kinetics in the presence and absence of Fe(II) oxidizers. Over the short-term (ca. ~ 1 wk), Fe(II) oxidation rates were 1.3 to 1.7 times greater in the presence of FeOB than in FeOB-free microcosms. In contrast, FeOB did not appear to affect longer-term (3-6 wk) rates of Fe(III) plaque accumulation.

This work was conducted in the lab in collaboration with Drs. David Emerson (now at Bigelow Laboratories), Pat Megonigal (Smithsonian Environmental Research Center) and Johanna Weiss (now at Northern Virginia Community College).

Due to their position at the interface between terrestrial and marine environments, tidal salt marshes will be one of the first ecosystems to feel the effects of a rising sea level. In a manipulative experiment conducted at a mainland marsh site on the eastern shore of Virginia, a high salt marsh community was exposed to two disturbances that are expected to result from a rise in relative sea level: 1) increased frequency of inundation and 2) enhanced deposition of wrack (mats of dead marsh plant stems and litter). We showed that flooding and wrack deposition each lowered net ecosystem carbon accumulation relative to non-manipulated control plots. Because the sequestration of primary production is important in maintaining marsh elevation in the irregularly flooded high marsh, these disturbances may be a first step in initiating the transition from organic-rich high marsh to mineral-rich low marsh.

This work was conducted in collaboration with David Miller (former MS student at WiIliam and Mary) and Dr. Iris Anderson (Virginia Institute of Marine Science), using experimental plots established by Dr. Bob Christian (East Carolina University) and students.

Extensive tidal freshwater marshes are a dominant feature of the upper reach of many estuaries along the mid-Atlantic coast of North America. To understand the role that these marshes play with respect to estuarine carbon cycling, we asked two broad questions: 1) What are the sources of carbon to these marshes? and 2) What are the ultimate fates of this carbon? We addressed these questions for a tidal freshwater marsh on the Pamunkey River, Virginia.

Harvest-based estimates of marsh primary production are often unreliable due to a lack of information on rates of biomass turnover and translocation. As an alternative to harvest methods, fluxes of carbon dioxide have been used to measure photosynthetic rates for both macrophytes and sediment microalgae. Additionally, carbon dioxide and methane fluxes measured in the dark can be used to calculate total respiration rates. Gas flux measurements were used with a process-based model to calculate annual rates of marsh macrophyte and microalgal photosynthesis and respiration. The gas flux results were then combined with biomass harvest and literature data to create a conceptual mass balance model of macrophyte-influenced carbon cycling in the marsh. Annually, total marsh respiration exceeded gross photosynthesis, suggesting the input of additional carbon source(s) to the marsh.

Because there is a large decrease in water column turbulent energy as marshes are flooded, tidal marshes are often sites with high sediment deposition rates. Short-term sediment deposition rates (biweekly to monthly) measured using sediment collection tiles were spatially and temporally variable but were sufficient to balance the combined effects of marsh respiration and relative sea level rise. Sediment core inventories of the radioisotope beryllium-7 showed that the spatial patterns of sediment deposition were not due to erosion and redistribution within the marsh. Accretion rates calculated from cesium-137 and carbon-14 dating were substantially less than annual deposition rates, with a decrease in accretion rate with increasing time scale. The metabolism of a labile sediment fraction can explain a portion of this observed decrease in accretion rate, with the remainder likely due to periodic storm-induced erosion and historical variability in sediment deposition rates.

Seasonally, the concentration of dissolved inorganic carbon (DIC) was measured in a marsh tidal creek to quantify the exchanges between the marsh and estuary. At low tide, DIC concentrations were 1.5 to 5-fold enriched relative to high tide concentrations, indicating an input of DIC from the marsh. On an estuary wide scale, the export of marsh-derived DIC could explain a significant portion of excess DIC production in the adjacent York and Pamunkey River estuaries and indicates that estuarine DIC concentrations in excess of conservative mixing between freshwater and marine end members does not necessarily indicate that an estuary is net heterotrophic.

To date, most studies that have examined nutrient exchanges in tidal freshwater marshes have done so in highly polluted or eutrophic systems. In parallel with studies on carbon cycling (described above), we examined nitrogen cycling in a relatively pristine (i.e. low nutrient) tidal marsh on the freshwater Pamunkey River, Virginia. A process-based mass balance model of N cycling for this system revealed that nitrogen cycling in the system was largely conservative. Gross mineralization of organic N was the largest nitrogen flow and accounted for more than enough N to support total macrophyte and microalgal primary production. Efficient microbial utilization of porewater ammonium helped retain N within the marsh. Uptake of N by tidal marshes may be important in controlling estuarine nutrient concentrations, but may play only a small role in supporting marsh primary production.