Master of Science (MS)
Oceanography and Coastal Sciences
The Mississippi River Delta comprises approximately 40% of the coastal wetlands in the conterminous United States, but has experienced more than 80% of the wetland loss in part due to flood control levees that have hydrologically isolated the lower River from the adjacent wetlands for a century. Thus, reconnecting the River to the wetlands through diversions is one of the approaches being used to reduce the rate of wetland loss. However, the diversion structures were originally designed to deliver freshwater rather than sediment. Furthermore, nutrient concentrations in the River water have increased during the past half-century as a result of agricultural intensification in the upper watershed. The goals of this study were to evaluate the effects of selected nutrients on structural and functional components of an oligohaline wetland plant community, to find the major nutrient responsible for significant impacts to the wetlands, and to determine if the current diversion-sediment loading rates effectively reduce any such impacts. Sixty intact marsh sods were collected from a Sagittaria lancifolia dominated wetland near Madisonville, Louisiana, and transported to a greenhouse at Louisiana State University. The sods were assigned one of six nutrient additions (NO3, NH4, PO4, SO4, NO3+NH4+PO4+SO4 [combo], and control) with or without sediment in a completely randomized design. Each treatment was applied monthly to five replicate sods for 25 months. During this time, treatment-effects on above- and belowground biomass, belowground biomass accumulation, total production, plant species composition and richness, as well as bio-physiochemical characteristics (organic matter decomposition, soil shear strength, bulk density, Eh, pH, conductivity, and pore water nutrients) were determined. After 25-months of nutrient addition, the NH4, NO3, and combo treatment-levels increased aboveground biomass while SO4 decreased it; PO4 had no significant effect. Although belowground standing crop was not affected by any of the nutrients, belowground biomass accumulation was significantly lower with SO4 addition; the other nutrients had no effect on biomass accumulation. Sediment addition significantly (p=0.06) increased aboveground biomass by the end of the second year, but no effect was apparent in year 3. Sediment had no significant effect on belowground biomass or biomass accumulation during the study nor was there a significant interaction with nutrients, even though soil bulk density increased with sediment addition. NH4, SO4 and Combo decreased decomposition rate, while soil shear strength increased with added NH4. None of the treatment applications had a significant effect on the number of new species or total species richness. The negative effects of SO4 on aboveground biomass and belowground biomass accumulation suggest that the SO4 of diverted Mississippi River water is potentially a problematic ion, although the concurrent application of nitrogen and phosphorus ameliorated the negative impacts of SO4. These results are informative for wetland restoration activity and suggest that sulfate may be an ion of concern in diversion water, although its impact may vary as a function of other environmental factors. Also, by increasing soil bulk density and promoting aboveground biomass, sediment loading can be beneficial to coastal wetlands, especially where subsidence rates are high and plant flooding is excessive such as in the Mississippi River Delta ecosystem. Thus, the optimization of diversions to carry sediments, along with continued surveillance of potential nutrient effects, would be wise management decisions.
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Poormahdi, Sareh, "Nutrient and Sediment Effects on Oligohaline Marsh Plant Community in Mississippi River Delta" (2014). LSU Master's Theses. 3856.
Mendelssohn, Irving A.