ON THE COMPETITION FOR LIGHT BETWEEN AMERICAN WILDCELERY AND SAGO PONDWEED AT 
HIGH AND LOW NUTRIENT AVAILABILITY: A MODELING APPROACH

Elly P.H. Best, Gregory A. Kiker, and William A. Boyd 
U.S. Army Engineer Research and Development Center, Environmental Laboratory, 
Vicksburg, MS 39180-6199.

A simulation model has been developed that focuses on the ability of two 
competing submersed macrophytes, respectively canopy-forming and meadow-forming, 
to maintain their biomass at different environmental conditions. Vallisneria 
americana (American wildcelery) serves as the example for non-canopy-forming 
plants, and Potamogeton pectinatus (sago pondweed) for canopy-forming plants. 
The model can be used to predict changes in the species composition of submersed 
vegetation as a result of changes in the availability of resources in shallow 
freshwater bodies. In the model, the two plant species compete for light and 
exhibit different species-characteristic relationships between plant tissue 
nitrogen (N): phosphorus (P) ratio and reduction in plant biomass production. 
      
Competition for light proved to be a far more important determinant of species 
composition of the vegetation than the availabilities of N and P in the 
sediment. Intraspecific competition for light did not occur in wildcelery in a 
temperate climate, typical for the Upper Mississippi River System (UMRS) at La 
Crosse, WI.  However, it was observed at densities > 8-9 plants m-2 in a near-
subtropical climate, typical for the freshwater systems near Davis, CA. It 
occurred in sago pondweed at plant densities >4-5 plants m-2. Coexistence of 
both species in mixed stands occurred only at wildcelery: sago pondweed plant 
density ratio’s of 28:2 to 26:4 without N and P limitation of growth, 
irrespective of climate. At density ratio’s higher than 28:2 wildcelery wins, 
and at density ratio’s lower than 26:4 sago pondweed wins. The density ratio 
range at which coexistence was possible increased with water turbidity between 
extinction coefficients of 0.43 and 2.00 m-1. Epiphyte shading at a level of 25 
percent of observed maxima in the UMR allowed coexistence in clear water, but 
prevented it in turbid water. Under N limiting conditions for both species, sago 
pondweed wins the competition, but under P limiting conditions for sago pondweed 
wildcelery wins. Coexistence was expanded by fertilization with both N and P.

These results indicate that sago pondweed has a high potential of replacing 
wildcelery when allowed to colonize gaps in dense wildcelery stands. N limiting 
conditions strengthen and P limiting conditions weaken the competitive potential 
of sago pondweed relative to that of wildcelery, while raised N and P 
availabilities enhance the potential for coexistence of both species. These 
notions can be used as a basis for management of submersed macrophytes. Model 
calibrations and simulation results are currently verified experimentally .

Keywords: aquatic plant growth, competition model, light, nutrients, Mississippi 
River