Storms as agents of wetland elevation change: their impact on surface and subsurface sediment processes

Abstract

Direct measures of the impact of major storms on wetland sediment elevation are rare. Recently developed techniques have enabled simultaneous, quantitative observations of surface and subsurface processes affecting sediment elevation. An analysis of ten wetland sites revealed the following patterns of sediment elevation change after storm passage: (1) elevation change equivalent to sediment accretion or erosion, (2) elevation loss in spite of sediment deposition, or in excess of erosion, and (3) elevation gain greater than the amount of sediment accretion. These observations suggest that storms influence sediment elevation not only by sediment deposition and erosion but also through subsurface processes of sediment compaction, root growth and decomposition, and water flux. Wetlands receiving a substantial deposit of sediment did not always realize an equivalent elevation gain. Some realized a net loss in elevation as a result of sediment compaction apparently caused by the weight of the sediment deposit or the tidal surge waters, or both. Sediment elevation collapsed in two mangrove forests with highly organic substrate when the storm killed the forest. In two marshes, elevation gain exceeded deposition apparently through increased sediment water storage or plant root growth via nutrient enrichment from storm sediment deposits. The elevation responses were either temporary or permanent on an ecological time scale (> 8 years). In one organic marsh substrate, compaction was followed by expansion, only to be compacted again by another storm. Thus the elevation response of coastal wetlands to major storms varied depending on local substrate conditions and degree of storm impact.