| Marsh Loss Mechanisms in Areas with High River Inflow and Slow Subsidence: How Estuarine Marshes Can Erode Even in the Virtual Absence of Wave and Tidal Energy |
PRESENTATION
J.A. Nyman(1) and T.E. McGinnis II(2) jnyman@lsu.edu
(1) School of Renewable Natural Resources, LSU AgCenter, Baton Rouge, LA 70803
(2) Louisiana Department of Natural Resources, Lafayette, LA
Abstract
The central Louisiana coast is similar to many estuarine marshes elsewhere in that subsidence is relatively slow and riverine inputs of freshwater and sediments are relatively high. Marsh loss occurs, albeit very slowly, but why does it occur at all under these conditions? Successive aerial photographs and/or satellite imagery from central Louisiana and elsewhere reveal that much of this marsh loss occurs adjacent to small, scattered ponds farthest from larger tidal channels. We previously showed that this pattern of marsh loss was unrelated to plant stress, and appeared to proceed via erosion. Here we report the results of (1) studies to determine factors controlling soil strength and erosion in marsh soils adjacent to ponds, and (2) a comparison of erosion rates between managed and unmanaged marshes. We found that soil strength was unrelated to amounts of mineral sediments or dead organic matter in soil but was related to live organic matter; i.e., living roots, in soil. We also found that the effects of natural drought and management on erosion depended upon pond depth. Erosion was unaffected by natural drought and management where ponds were deeper than the live rooting depth of adjacent emergent vegetation. Factors that reduced water level, such as management or natural drought, reversed erosion where ponds were shallower than the live rooting depth of adjacent emergent vegetation. These relationships among soil strength, live rooting depth, and pond depth might explain marsh loss patterns in many areas, and might develop anywhere sealevel rise causes marsh vertical accretion to exceed pond vertical accretion. |
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