Shoot productivity was measured for Typha latifolia and Scirpus americanus. Samples were also subjected to chemical analyses. Tissue concentrations of chlorophylls a and b, carotenoids and most macronutrients declined as the plants aged. Net accumulation of these constituents per square meter usually continued during periods of dry matter increase, even though tissue concentrations were diminishing. Dry matter standing crop was the decisive factor determining quantities of chemical constituents per square meter. Uptake rates for macronutrients were generally not proportional to productivity rates. The most rapid uptake of several nutrients occurred earlier than maximum growth rates.