Discussion - Extended drawdown effects on a lakeshore

Extended drawdown effects on a lakeshore
propagule bank

Stephanie Guisti, Beverly Collins, and Tom Hinton

Discussion

Wetland emergent and aquatic vegetation at the upper Par Pond waterline did not persist after drawdown (Fig. 6). Over the 4-year drawdown period, the vegetation on the exposed shore underwent a successional sequence toward an upland community (Narumalani 1993; Jensen et al. 1997; Mackey and Riley 1997). When Par Pond was refilled, flood-intolerant species were eliminated (Fig. 7) and wetland vegetation redeveloped along the original shoreline. Vegetation development in the experimental mesocosms suggests that propagules of wetland plant species either remained viable at the original Par Pond waterline during the extended drawdown or were carried up with rising water.


Hydrologic regime, but not water source, influenced species composition and richness of vegetation in the experimental mesocosms. Vegetation from CF cores was species-poor and dominated by aquatic, floating-leaved and emergent perennials (Table 2). Species richness increased with decreased flooding frequency; pots that were not constantly flooded had a mix of wetland and upland species. In wetlands, species and seed bank richness often decreases with an increase in water depth (Wilson, Moore, and Keddy 1993; van der Valk, Squires, and Welling 1994). Richness increases with decreasing depth due to satisfaction of seed germination requirements in shallow water or moist soil rather than in flooded conditions (van der Valk and Davis 1978; Mitsch and Gosselink 1993; Wilson, Moore, and Keddy 1993).


Both water level and water source influenced species composition and plant growth and, therefore, per-pot cover and mass of vegetation in the experimental mesocosms. Among water level treatments, the less species-rich continuously flooded (CF) pots had lower cover and lower mass than the other water level treatments. Among water source treatments, plant cover and mass were greatest in the more fertile Savannah River water. The higher concentrations of nitrate in the Savannah River water (Table 1) may account for the greater growth and production of above-ground foliage compared to the other water source treatments (Grevilliot, Krebs, and Muller 1998).