STUDY ON THE DECOMPOSITION OF PISTIA-STRATIOTES L (ARACEAE) IN CISNE RESERVOIR, URUGUAY

A study on the decomposition process of Pistia stratiotes L. was carri ed out in Cisne Reservoir, Uruguay. For this purpose, leaves and roots were considered separately, and the process was studied in the littor al and the pelagic zone. The litter bag technique was used to estimate dry weight losses at different times. Leaves decomposed faster than r oots in both zones. Pistia decomposed faster in the pelagic zone due t o better oxygen conditions and a more intensive water movement that re moves material from within the bags. Phosphorus, potassium, sodium and magnesium were leached rapidly during the first week. Nitrogen conten t in the leaves initially increased, reaching its maximum in the litto ral zone that coincided with that of bacterial numbers colonizing the litter. The number of macroinvertebrates, dominated by Chironomidae, w as low. A rough estimation of the phosphorus impact on the littoral zo ne associated to the decomposition process shows that after 24 h betwe en 13.9 and 16.8 pg-at.l-1 have been released to the water.