Ecology of submersed aquatic vegetation in Lake Pontchartrain, Louisiana.

摘要

Submersed aquatic vegetation (SAV) dominated by Vallisneria americana and Ruppia maritima declined 75% from 1953 to 1992 in Lake Pontchartrain, Louisiana. In response to this decline, I studied SAV to determine factors controlling abundance and species composition and to develop appropriate restoration goals. I conducted annual line intercept surveys (1996–2002) at six sites and compared SAV depth distributions to monthly water quality measurements from the preceding year. I assessed monthly biomass between March 2000 and May 2002 to determine seasonal trends in biomass, density, and flowering.; My results indicate that annual mean water clarity determines SAV colonization depth (Z_col = 2.3 K_d), mean water level controls SAV minimum depth (Z_min = 0.3 m), shoreface slope (&thetas;) determines distance from a shore to Z_col, and annual mean salinity controls species composition.; Environmental gradients (turbidity, nutrient, water color, littoral energy, underwater profile, and salinity) determine potential SAV habitat. Meteorological events (El Niño, La Niña, and hurricanes), anthropogenic stressors (shell dredging, and urban and agricultural runoff), and shoreline modification interact with natural gradients and are responsible for the short- (annual variation) and long-term (historic decline) changes in SAV coverage. In 1999, a rapid increase in Ruppia maritima occurred which was driven by an increased water transparency associated with a low rainfall La Niña climate trend. However, increased salinity caused Vallisneria to decrease.; I developed a linear model, X = (2.3 − 0.3 * K _d)/(sin &thetas; * K_d) to predict habitat increase from bathymetry. Approximately 600 ha of potential habitat will be gained for every half-unit decrease in K_d (vertical absorption coefficient) on the north shore. South shore restoration requires nutrient reduction and shoreline restoration.; Due to differences in seasonal biomass, growth form, and salinity and light tolerance, the relative competitive advantage of Ruppia versus Vallisneria changes with season and salinity. Ruppia produces abundant seeds (20,000 seeds/m2), exhibits rapid propagation, and high peak biomass (130–320 g/m 2). High turbidity and epiphytes negatively affect newly germinated Ruppia and tropical storms damage mature plants. The sustained abundant Ruppia growth through 2002 indicates a positive response to restoration efforts.