This project used three different experiments to test the hypothesis that deposition of small amounts of sediment in wetlands alters plant diversity and soil properties. Greenhouse experiments using potting soil and Sagittaria latifolia were used with 0.5–2 cm sediment treatments and controls. Thirty 2 m X 2 m plots were established in a freshwater palustrine marsh, and the plots were monitored before and after the addition of 1, 2, or 4 cm of sediment to half of the plots. Large vegetated cores of native soil were removed from the wetland and monitored in the greenhouse before and after the addition of 2 cm of sediment. Greenhouse experiments showed that less than 2 cm of sediment had no effects on the growth of S. latifolia, while 2 cm of sediment significantly increased aboveground biomass but had no effect on matrix potential or oxidation-reduction potential. Field plots showed significant differences in bulk density, organic matter, and phosphorus resulting from addition of sediment but did not show differences in water levels, temperature, plant diversity, biomass, or stem density under any amount of added sediment. Soil cores showed a significant difference in bulk density and in matrix potential near the soil surface under wetter conditions but showed no significant differences in temperature, evapotranspiration, oxidation-reduction potential, or aboveground biomass. The lack of effect seen in the field plots is likely the result of a low effective loading rate relative to the greenhouse experiments and the dense plant cover and thick debris layer of the wetland. Observed changes in bulk density would alter the hydrologic functions of the wetland if these depths of sediment occurred over a larger area. The tolerable level of sediment for a wetland depends on the structure and function of the individual wetland.
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Vol. 24 • No. 2