Light Availability and Growth of Wildcelery ('Vallisneria americana') in UpperMississippi River Backwaters. Long Term Resource Monitoring Program

摘要

Large beds of Vallisneria americana declined in the backwaters of the UpperMississippi River after a drought that occurred between 1987 and 1989. One hypothesis for this decline is that low light availability may have decreased net photosynthesis to the extent that overwintering tubers were not formed. Following the decline, light availability remained low. To determine what light levels would be necessary for the re-establishment of Vallisneria in the Upper Mississippi River, the long-term growth of plants in a backwater lake and in an experimental pond was measured while the surface and subsurface light were monitored continuously. plants grown from tubers transplanted to 0(dot)5, 1(dot)0 and 1(dot)5 m depth in the lake grew and produced tubers only at 0(dot)5 m depth (9% of surface light). At 1(dot)0 m. light availability was less than 1% of the surface light. Plants grown from tubers in experimental ponds with four shade treatments (2, 5, 9 and 25% of surface light) for the same growing period produced replacement-weight tubers in 9% light. For a longer growing season, plants also produced replacement-weight tubers in treatments with at least 5% of surface light. An average light-extinction coefficient of 4(dot)64 m was calculated for the backwater lake based on continuous data collected during 94 days during the growing season from eight widely separated sites.