The mathematical model WEED was derived to study the productivity of the submerged macrophyte Myriophyllum spicatum in the littoral of Lake Wingra, Madison, Wisconsin. The model coordinates four biomass compartments with selected physiological processes evaluated for a stratified meter—squared water coloumn. Light and temperature are the primary environmental forcing functions. The model considers growth form (depth distribution of biomass), total biomass/meter—squared, the depth profile of photosynthesis and macrophyte contributions to dissovled and particulate organic matter pools in the water column. The model output was tested against field data for seasonal patterns of standing crop, depth distribution of biomass and depth profile of photosynthesis for a rooting depth of 150 cm. WEED has permitted the simulation of macrophyte growth response to changes in environmental conditions and has outlined areas of needed research. See full-text article at JSTOR