Preliminary Evaluation of Submersed Macrophyte Changes in the Upper Mississippi River

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 UMRS. Foremost among these are suspended sediment concentrations, flooding, herbicides, and grazing.