THE DISTRIBUTION OF VALLISNERIA-AMERICANA SEEDS AND SEEDLING LIGHT REQUIREMENTS IN THE UPPER MISSISSIPPI-RIVER

Vallisneria americana declined in backwaters of the Upper Mississippi River, U.S.A., after a drought in 1988. To determine whether viable se eds of V. americana occurred in the seed bank of navigation pool 7, La ke Onalaska, the upper 5 cm of sediment was collected from 103 sites i n May 1990. These sediment samples were kept in pots at a depth of 0.4 , 0.8, and 1.2 m in an outdoor pond for 12 weeks. Vallisneria american a seeds germinated from sites throughout the lake, and some seedlings produced overwintering buds by the end of the study. Seeds, spores, or fragments of 12 other species of aquatic plants also germinated. Seed germination trials with fresh and stored seeds in both greenhouse and ponds in which light availability was reduced with shade cloths indic ated that seed germination was insensitive to light level. To determin e the light requirements for seedling survival and bud production, sed iment from Lake Onalaska was incubated in ponds under neutral density shade screens reducing light to 2, 5, 9, and 25% of full sun. Seeds ge rminated under all shade treatments but survival was significantly hig her in the 9 and 25% light treatments, and bud production was restrict ed to these light levels.