Coastal marshes serve important ecologic and economic functions, such as providing habitat, absorbing floodwaters and storm surges, and sequestering carbon. Throughout the northern Gulf of Mexico, coastal marshes are disappearing due to wave attack, sea-level rise, sediment export, and subsidence. Marsh area increases when sediments accumulate at the marsh shoreline, accrete vertically, and when non-marsh areas are colonized by marsh vegetation. Marsh shoreline erosion results in net marsh loss when transgression rates at the marsh-water edge exceed upland-marsh migration. The balance between marsh destroying and marsh creating processes determines the long-term survivability of a marsh system. Thus, processes of shoreline change are important considerations when evaluating the overall health and vulnerability of coastal marshes.
Shoreline erosion can be measured using remotely sensed data in a geographic information system. Using shoreline position delineated from aerial imagery, historic maps, and field surveys, shoreline change analysis estimates long- and short-term shoreline movements to identify erosion or accretion for coastal marshes at the Grand Bay National Estuarine Research Reserve (GBNERR) and Wildlife Refuge (GBNWR) on the border of Mississippi and Alabama, USA. However, these techniques do not directly provide information on sediment deposition on the marsh surface. To understand sediment deposition, four study sites provide in-situ measurements of sediment deposition using sediment plates and sediment tracers (silica beads) that were collected every three months.
Analysis of the shoreline change data and in-situ sediment data for the GBNERR showed that in 2017, all of the shorelines at the study sites are eroding at rates between –0.50 m/yr and –3.39 m/yr, an average rate of –1.45 m/yr. Positive sediment deposition rates were measured from 5–20 meters inland of the marsh shoreline during each season (3-month period) (0.19 ± 0.05 cm [Fall], 0.26 ± 0.11 cm [Winter], 0.48 ± 0.12 cm [Spring], 0.63 ± 0.15 cm [Summer]), indicating sediment deposition increased with every season. Sediment tracer (silica-bead) counts confirmed that sediment was transported onto the marsh surface from eroding marsh shorelines. Higher energy sites had more beads deposited on the sediment plates than the low energy tidal creek site, due to the different wave and tidal conditions between the sites. Increased wave and tidal energy correlated to increased sediment transport further into the marsh.
The relative importance of this marsh cannibalism for the long-term marsh survival depends on factors, such as wave attack, as they control the rate of persistent lateral marsh loss. This findings in this thesis suggests that material from eroding marsh edges contributes to the ability of the interior of marshes to maintain their elevation with respect to rising sea levels.
|Commitee:||Plant, Nathaniel, Rains, Mark, Smith, Kathryn|
|School:||University of South Florida|
|School Location:||United States -- Florida|
|Source:||MAI 58/01M(E), Masters Abstracts International|
|Keywords:||Deposition, Erosion, Grand Bay National Estuarine Research Reserve, Marsh, Mississippi, Sediment tracer, Seiment plate|
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