Date of Award

1993

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Oceanography and Coastal Sciences

First Advisor

William H. Patrick, Jr

Abstract

Marsh loss in Louisiana ($\sim$8,000 ha/yr) is attributed primarily to salt-water intrusion, flooding, and sediment starvation. Soil processes were studied to complement previous work identifying causes rather than mechanisms. Relationships between soil mineral and organic matter were examined to better understand accretion, which counters subsidence and sea-level rise. More soil volume was occupied by organic matter than mineral matter in fresh, intermediate, and brackish soil. Estimated mineral sediment requirements for accretion were 6.7 times greater in salt marsh than in fresh marsh. Sediment requirements probably increase seaward because of sulfate in seawater, reduction of sulfate to toxic sulfides in soil, and the interaction between soil iron and soil sulfides. Decomposition rates were studied because they varied among marsh types. Decomposition was greatest in fresh, and least in brackish soil at all drainage depths tested. Thus, different decomposition rates were attributed to the different plant species that create soil organic matter rather than different field hydrological conditions. Relationships among vegetation, soil, and marsh loss were studied in a deteriorating marsh near Lake Barre. Vegetation was stressed, which was attributed primarily to flooding. However, inadequate soil mineral matter also apparently prevented salt marsh from establishing in brackish areas experiencing salt-water intrusion, which suggested that the sediment regime was not changing although the salinity regime was changing. Excessive flooding resulted from inadequate accretion but accretion depended directly on peat accumulation, which indicated a positive feedback loop of inadequate plant production, inadequate accretion, and excessive flooding culminating in plant mortality and marsh loss. The hypothesis that marsh loss always results from plant stress and subsequent feedbacks between excessive flooding and inadequate accretion was tested at Marsh Island. Pilot and follow-up studies indicated that marsh loss there occurred in healthy marsh via erosion in the marsh interior. Thus, increasing sedimentation, or reducing flooding or salt-water intrusion will not reduce all marsh loss because all marsh loss is not associated with plant stress. Additional study is needed to determine if landscape patterns are related to marsh loss processes.

Pages

178

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