Growth, Tissue Composition and Stoichiometry of Duckweed Grown in Low Nutrient Backwaters of the Upper Mississippi River

Introduction Cultural eutrophication of aquatic systems has received considerable attention in our nations waters though much of it in the past has focused on nutrient enrichment problems in lakes. More recent attention has been made on evaluating the impacts of nutrients in riverine systems (Hilton 2006, UMRBA 2011). Recent evidence suggests that nutrient-related impacts in rivers may come in forms other than nuisance blue-green algae blooms that are typical in eutrophic lakes. The development of thick benthic or surface mats of filamentous algae or duckweed in quiescent in riverine aquatic habitats as a result of nutrient enrichment are examples of problems that may develop in these systems. These mats can result in negative impacts to aquatic life and obstruct recreational uses. Such conditions occur commonly during midsummer periods in shallow vegetated areas of the Upper Mississippi River (UMR), especially in off-channel sloughs, backwaters and marshes (Sullivan 2008, Giblin et al. 2010). In recent years monitoring and research has been focused on understanding the factors contributing to the development of filamentous algae or duckweed, generally referred to as metaphyton or free-floating plants (FFPs), in off-channel areas of the UMR (Sullivan 2008, Giblin et al. 2010, Sullivan and Giblin 2011 Giblin et al. Submitted, Houser et al. Submitted). The development of thick FFP mats is dependent on having adequate supply of nitrogen and phosphorus to maintain their growth and development. The sources of these nutrients are complex and variable in large floodplain rivers and includes allochtonous input from natural and cultural sources and internal loading (nutrient cycling). The fate of these nutrients in the river is strongly influenced by biological, physical and hydraulic processes. Further, since FFPs often require substrates for their initial development or quiescent surface waters to prevent their wash-out, FFPs are often associated with beds of rooted aquatic vegetation, especially submersed aquatic vegetation (SAV). FFP research within the UMR backwaters has primarily focused on identifying conditions that limit metaphyton development, especially nutrients. Midsummer metaphyton tissue nutrient content and stoichiometry as well as water column nutrient concentrations have often indicated nutrients were present in excess with limiting nutrient conditions being rare (Houser et al. Submitted). However more recent seasonal analysis of selected Pool 8 backwaters have identified potential water column nutrient thresholds and other physical and biological factors that may be important in influencing FFP growth (Giblin et al. Sumitted). The objective of this work was to further define and …