Date of Award
Master of Science (MS)
Wildlife and Fisheries Biology
Thomas L O'Halloran
Climate change and subsequent sea level rise are growing pressures challenging salt marsh productivity in estuaries. To persist in the future, marshes must build elevation faster than local relative sea level rise. Marsh accretion rates are controlled primarily through the contributions of above and below ground plant productivity to soil organic matter, and deposition of suspended sediments carried onto the marsh with tides. As a blue carbon system, salt marshes naturally sequester atmospheric CO2 by accumulating organic carbon in soils. Previous work hypothesized that mechanisms of climate change, including elevated CO2, atmospheric temperatures, and enhanced tidal inundation, will increase plant productivity and sedimentation and subsequently soil accretion. In this thesis, I examined aspects of the salt marsh carbon cycle by evaluating effects of leaf sediment coatings on plant productivity and the sensitivity of soil respiration to environmental factors across a marsh platform elevation gradient.Current models of marsh accretion assume suspended sediments only positively influence marsh accretion via soil elevation building. However, we have observed Spartina alterniflora leaves become periodically coated in material that likely derives from suspended sediments. In Chapter 1 of this thesis, I hypothesized that tidal cycles and suspended sediments continuously build leaf coatings on S. alterniflora leaves, reducing gas exchange and plant photosynthesis between rain events. To examine this hypothesis, I measured photosynthesis on both naturally coated and rinsed leaves using a LI-6400XT Portable Photosynthesis System (LI-COR, Lincoln, Nebraska, USA). Leaf-level measurements were made within the canopy footprint of an integrated eddy covariance (EC) tower system that allowed us to apply leaf-level observations of photosynthesis to larger canopy scale measures of productivity (net ecosystem exchange (NEE)). Our results indicated that suspended sediments in the water column did increase S. alterniflora leaf coatings over time in a cycle that resets with rain events. Leaf coatings had a negative effect on gas exchange of the plant, decreasing the initial quantum yield (or light use efficiency (LUE)) of the leaves linearly with a proxy for coating thickness (measured as leaf greenness) at both the canopy and leaf level. These results are significant, as I showed for the first time that leaf coatings can create a negative effect between suspended sediments and S. alterniflora productivity. Further work is needed to determine whether this effect is sufficient to decrease salt marsh productivity and affect marsh elevation gains. Climate change will also expose sequestered soil carbon to higher temperatures, enhancing decomposition and potentially decreasing soil elevation. To better understand the drivers of this decomposition, I evaluated the role of hydrology and abiotic variables on soil respiration in Chapter 2 of this thesis. For one year, I measured soil respiration and water table depth at three plots from high to low marsh using a LI-8100A Soil Gas Flux System (LI-COR, Lincoln Nebraska). Field measurements were made within the EC tower footprint and compared to total marsh ecosystem respiration. Our results indicated that soil respiration contributed approximately 1/3 of ecosystem respiration. The largest drivers of soil respiration included air temperature and relative humidity, and elevational variability of water table depth. By analyzing the tower data by wind direction, we were able to separate contributions of ecosystem respiration from the high and low marsh, and compared these to soil fluxes from sites in the respective region. These results are significant, as the effects of water table depth on soil respiration and relationship to tower NEE measurements have not been reported. This work improves our understanding of soil respiration and carbon cycling in salt marshes. Results from this thesis can be used to improve models used to forecast marsh responses to climate change and sea level rise, which facilitates better planning and adaptation strategies for future marsh conservation.
Furbeck, Michelle E., "Contributions of Leaf Sediment Coatings and Soil Respiration to Blue Carbon Sequestration in a Salt Marsh" (2021). All Theses. 3538.