Date of Award

Summer 1992

Document Type


Degree Name

Doctor of Philosophy (PhD)


Biological Sciences


Ecological Sciences

Committee Director

Frank P. Day

Committee Member

Robert K. Rose

Committee Member

A. O. Akan


The present research compared and contrasted the physical structure of floating and rooted marshes, their differing responses to open-water salinities and inundation, as well as the nutrient distribution in the porewaters and sediment. The effects of the physical differences in the two marsh types on the ocurrence of the dominant emergent vegetation was discussed.

The main difference in physical structure of the two marsh types was the presence of a mineral, non-buoyant layer at 25-45 cm depth in the rooted marsh, which could serve as an anchor for the overlying highly organic mat layer found in both marsh types. Porewater salinities in floating marshes tracked open-water salinities more closely than they did in rooted marshes. Under the prevailing, mostly fresh conditions, porewaters in the rooted marsh contained significantly higher salt levels. Here also, there was a more pronounced vertical gradient in salt levels than found in floating marshes. With the three years of data it was possible to demonstrate the more extensive exchange of below-ground water with open waters in the floating marshes, rather than the rooted marsh.

Surprisingly, the different hydrodynamics of floating and rooted marshes did not appear to affect inorganic porewater nutrient levels. It appeared that dominant above-ground vegetation determined these levels. The two dominant species of emergent vegetation have clearly contrasting tolerances to ambient salinites and flooding. Thus, the continued persistance of the bore salt-tolerant species in this mostly fresh area is thought to be attributable to the recurring, but infrequent years of high salinity.

The significance of floating marshes in the rapidly subsiding Mississippi River Deltaic Plain, with concomittant increases in ambient salinities is obvious. Their potentially unique responses to these environmental forcing functions deserve closer attention when mitigation measures are conceptualized and implemented. It is quite possible that a majority of the low-salinity marshes in the deltaic plain may be floating.


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