| The submersed plant Vallisneria americana declined in Upper Mississippi River backwaters after a drought in 1988. In that year overwintering tuber production may have failed when net photosynthesis declined with increasing temperatures and turbidity. To determine if light limitation and increased respiration could account for the decline, minimum seasonal light requirements were estimated for growth and tuber production, and plant respiration and photosynthetic characteristics measured.;Seasonal light requirements for plants growing from tubers were measured in field transplant and pond studies. In Lake Onalaska, plants grew only at 0.5 m depth (9% of ambient light). A split-plot pond study was designed assuming light as the primary limiting factor for growth; minimum light requirements could be estimated by determining the light level above which sediment fertility significantly affected growth. Plants were grown in lake sediment, or in lake sediment diluted 1:4 with sand, with 2, 5, 9, or 25% light. Underwater light levels were measured continuously. Plant biomass was measured over 5 months. Tuber production depended primarily on light, requiring an average daily minimum photosynthetic photon flux density of 67 ;Increasing temperature significantly increased leaf dark respiration rates. Instantaneous light compensation points (typically 10 ;Seed bank studies were done to determine if revegetation could occur naturally from the seed banks. Sediment samples were collected from 103 sites in Lake Onalaska, Pool 7, Upper Mississippi River, and maintained in an outdoor pond. V. americana seeds germinated and produced tubers from sites throughout the lake regardless of sample location, sediment characteristics or depth. Light requirements for seedlings to produce tubers were estimated by examining seed bank samples incubated for five months in 2, 5, 9 or 25% of ambient light above the water surface. Seeds germinated in all shade treatments, but produced tubers only in 9 and 25% light. |
