The concentration of nutrients in Nymphoides indica (L.) O. Kuntze, Pontederia cordata L., Andropogon bicornis L. and Utricularia breviscapa Griseb was analyzed at monthly intervals from January 1980 through February 1981. The macrophytes strongly influenced the organic nitrogen and phosphorus concentration of the water, mainly in those months of intense plant decomposition after a dry period. The monthly changes in chemical composition of the macrophytes and in water are discussed.