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Seminar Abstract

John Rybczyk - April 23, 2002

 

Predicting the Effect of Sea Level Rise on Coastal Wetland Sustainability: Field and modelling studies

Coastal wetlands exist in a dynamic equilibrium between processes that lead to their establishment and maintenance (mineral and organic matter accumulation) and processes that lead to deterioration, including; sediment compaction, organic matter decomposition, erosion and relative sea level rise (relative sea level rise = eustatic sea level rise (ESLR) + subsidence). Predicted increases in the rates of one of these processes, ESLR, have led to concerns over the sustainability of coastal wetlands worldwide. To add to this potential problem, many wetlands, especially in coastal Louisiana, have been subject to anthropogenic hydrologic alterations that now restrict the movement of the allogenic mineral sediments into these subsiding and eroding systems.

New field methodologies that measure subsidence and changes in elevation relative to a shallow subsurface datum have allowed researchers to partition and quantify many of the processes that affect wetland elevation relative to sea level. With these types of data, we can attempt to identify and predict the fate of coastal wetlands by comparing current and predicted rates of relative sea level rise to measured rates of sediment accretion and then calculating an accretion deficit, surplus or balance. However, these types of calculations must be viewed with caution because they do not take into account possible elevation feedback mechanisms on the processes themselves. Specifically, changes in elevation can result in changes in allogenic sediment deposition, decomposition and autogenic primary production.

For this reason, site specific ecosystem models that incorporate feedback mechanisms, used in conjunction with field measurements of accretion, subsidence and elevation change, can provide a powerful tool for examining the response of wetland elevation to increasing rates of sea level rise. We present here the results of several such integrated field and modelling projects conducted in coastal Louisiana and two deltaic wetlands in Europe. The model utilizes a cohort approach to simulate sediment dynamics (organic and mineral matter accretion, decomposition, compaction, and belowground productivity). These dynamics produce model-generated changes in sediment characteristics and yield total sediment height as an output. Sediment height is then balanced with ESLR and deep subsidence, both forcing functions, to calculate marsh elevation relative to sea level. The model also simulates primary production and mineral inputs, both of which are feedback functions of the model-generated marsh elevation. Field measurements of accretion and elevation change (feldspar markers, CS-137 and Sediment Erosion Tables) not only give some indication of wetland elevation dynamics, but also provide the data for model initialization and calibration.

Our study demonstrates that more accurate predictions of the future of coastal wetlands with rising sea level will be obtained with both short-term measurements of accretion and surface elevation change and long-term modeling. Model simulations revealed that wetland elevation was more sensitive to the uncertainty surrounding estimates of ESLR and subsidence than in changes in autogenic processes such as decomposition and primary production. A series of mineral input simulations suggested that sediment supplement management strategies would be more effective when implemented before increasingly flood stressed wetlands became permanently inundated.

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