Project Title: Effects of Submergence and Nutrient Loading on Greenhouse Gas Production by Coastal Wetlands: A Multi-Scale Analysis
Mendenhall Fellow: Serena Moseman, (508) 457-2332, firstname.lastname@example.org
Duty Station: Woods Hole, MA
Start Date: October 1, 2008
Education: Ph.D. in Oceanography (2008) from Scripps Institution of Oceanography
Research Advisors: Kevin Kroeger, (508) 457-2270, email@example.com; John Crusius, (508) 457-2352, firstname.lastname@example.org; John Bratton, (508) 457-2254, email@example.com; Eric Sundquist, (508) 457-2397, firstname.lastname@example.org
Project Description: Coastal ecosystems currently face unprecedented modifications in nutrient regimes, hydrology, and species composition due to fertilizer use (intensive agriculture) land conversion (urbanization, diking, sedimentation), and introduction of invasive species. Community shifts within wetlands and estuaries occur in response to short term anthropogenic effects (introduced species, diking) as well as long term climate changes (sea level rise, increased temperatures and CO2) that act across a range of spatial scales. Changes in wetland community structures, and the environmental changes that drive them, have significant consequences for the biogeochemical functions of coastal ecosystems and for their broader potential roles in global biogeochemical cycles.
Coastal wetlands rank among the most productive ecosystems on the planet. As such, they may significantly mediate global climate change by sequestering CO2 in recalcitrant plant biomass or anoxic soils. Alternatively, major human impacts on coastal ecosystems, such as nutrient loading, may stimulate the production of greenhouse gases (CH4, CO2, and N2O) and generate potential feedbacks on global warming. The current significance of wetlands such as salt marshes, mangroves, and mudflats as sources of greenhouse gases must be better constrained in order to predict their potential global roles in the future.
In wetlands, plant rhizospheres (roots and surrounding sediments) can be hot spots of activity for microbes performing nitrification and denitrification, which are sources of N2O, as well as methanogenesis and methane oxidation that affect CH4 fluxes from wetlands. Through sediment oxygenation and release of carbon exudates, plants affect the microenvironments in which biogeochemical processes are performed. Several key wetland plants also have aerenchyma.(air filled tissues) that can act as conduits of gases from sediments to the atmosphere (fig. 1).
Figure 1. Biological and biogeochemical processes pertinent to the flux of greenhouse gases from coastal wetland sediments. Key biogeochemical processes are noted in boxes. Arrows indicate direction of gas and nutrient exchange from plants to sediments or the atmosphere.
This research explores spatial and temporal patterns of greenhouse gas fluxes in coastal wetlands across multiple scales in order to understand biogeochemical and ecological mechanisms that lead to their production and release to the atmosphere. In dynamic intertidal environments, biological communities respond to strong elevational gradients in physical and chemical factors as well as temporal variability tidal inundation, temperatures, and sunlight. We seek to understand which biological and abiotic factors most strongly affect the role of wetland ecosystems as sources or sinks of greenhouse gases. In particular, We are focusing on controls of nitrous oxide (N2O), a potent greenhouse gas with more than 300 times the climatic forcing of CO2 on a per molecule basis.Research Questions
Latitutidal variation in wetland biogeochemistry
- Are temperate wetlands (salt marshes) smaller sources of greenhouse gases than their tropical counterparts (mangroves)?
- Are salt marshes and mangroves significant on a global scale?
Regional and local impacts of nitrogen loading on wetlands
- What are the potential impacts of nitrogen loading on greenhouse gas fluxes from coastal wetlands?
- Do impacts depend on the form and duration of nitrogen inputs?
Influence of elevation and community composition on greenhouse gas fluxes
Microbial controls on greenhouse gas fluxes
- Within a wetland, how do changes in elevation constrain the fluxes of greenhouse gases from wetland sediments?
- How do distinct plant and microbial communities in different wetland zones affect greenhouse gas fluxes?
- Which wetland plants are strongest conduits of greenhouse gases?
- How might responses to sea level rise vary across elevational zones?
- What are the significant microbial pathways of nitrous oxide production? Heterotrophic and chemoautotrophic denitrification, nitrification, dissimilatory reduction of nitrate to ammonium may be investigated.
Multiple approaches must be employed to describe and understand variability in fluxes of greenhouse gases from coastal wetlands. We plan to combine mensurative field surveys using an in situ photoacoustic gas analyzer (1412 INNOVA), detailed sediment profiling (with UNISENSE microelectrode sensors), and manipulative studies of inundation and nutrient regimes in field and lab settings. Biogeochemical approaches (including flux chamber measurements, isotopic enrichment studies, and sediment profiles and assays) will be combined with molecular biological studies of microbial communities and ecological studies of plant and microbial interactions involved in the production of greenhouse gases.
Studies are ongoing in ground-water impacted embayments of Cape Cod, Massachusetts, including West Falmouth Harbor (right).
Future studies are planned in a variety of coastal marshes at the Plum Island Long Term Ecological Research Reserve (Massachusetts), Long Island, and mangrove ecosystems in the southeast United States.
Groundwater-derived nutrients delivered to a eutrophic embayment of West Falmouth Harbor, Massachusetts, were hypothesized to enhance N2O fluxes from coastal waters and intertidal zones. In 20 sites throughout the harbor during fall 2007,surface waters were 1.5 to 10 times supersaturated with nitrous oxide (fig. 2), corresponding to an average N2O flux of 16 μmol N2O m-2 d-1. N2O concentrations paralleled those of the groundwater tracer, 222Rn, suggesting groundwater is a N2O source. N2O fluxes from sandy intertidal were also measured in transparent flux chambers during the summer and fall of 2008. Average fluxes were 75 μmol N2O m-2 d-1 throughout the harbor and 565 μmol N2 m-2 d-1 along a 12-m transect in a shallow groundwater discharge zone (fig. 3). Intertidal fluxes exceeded previous reports, likely due to both nutrients and direct contributions of underlying groundwater which was 14 to135 times supersaturated with N2O. This research highlights the role of groundwater as a significant source of nutrients and N2O to coastal ecosystems.
Nitrous oxide fluxes in West Falmouth Harbor, MA
Figure 2. Distribution of nitrous oxide (percent saturation) in water column samples collected by USGS throughout West Falmouth Harbor during fall 2007.
Figure 3. Intertidal N2O fluxes (μmol N2O m-2 d-1) measured in West Falmouth Harbor in summer 2008 (during internship at USGS Woods Hole by Emily Pugh of Indiana State University).
Quantifying the roles of coastal ecosystems as sources or sinks of greenhouse gases improves current understanding of their ecology, biogeochemistry, and-particularly- their potential feedbacks on global warming. Along coastlines worldwide, these ecosystems are critical interceptors of major nutrient and material loads resulting from increasing food and energy production and expanding urban populations. Coastal ecosystems currently play key roles in water quality, coastal stabilization, and support of commercial fisheries. However, their significance may extend to beyond the present to the future through potential feedbacks on global climate change.
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