A FIELD OBSERVATION OF AN IRRIGATION POND WITH A FLOATING-LEAVED PLANT (Nymphoides indica) COMMUNITY

A field observation was curried out in an irrigation pond with a floating-leaved plant (Nymphoides indica) community to make clear the effects of aquatic plants on the ecosystem and water quality. In the observation, the appearance of the plant community and the values of various water quality items were monitored from fall to winter in 2002. Although a lot of nutrients (N, P) discharged from the plant bodies into the pond water due to withering, their concentrations quickly decreased after the temporary increases in the plant community and then phytoplankton cannot greatly multiply. Concerning phosphorus, in particular, this fact resulted from that the recycled phosphorus rapidly fell down as detritus and also being adsorbed by suspended solids after dissolution. In conclusion, it was shown that the recycle of nutrients due to withering of plant bodies scarcely gives negative influence on the ecosystem and water quality in an irrigation pond.