Declines in submersed aquatic macrophytes, notably Vallisneria
americana Michx., recently have been observed in portions of the
Upper Mississippi River (UMR). Coincidentally, Myriophyllum
spicatum L. appears to have become more common, frequently occurring
in locations formerly occupied by Vallisneria or other submersed
aquatic species. Mechanisms causing these changes in the abundance
and composition of aquatic vegetation are unknown. However, a
3-year drought may have affected nutrient transport and
phytoplankton production, thereby influencing growth and
reproduction of Vallisneria and other macrophyte species. Other
factors may potentially affect populations of submersed macrophytes
within the UMR. Foremost among these are suspended sediment
concentrations, flooding, herbicides, and grazing.
