Abstract
Biomonitoring using duckweed is an efficient method for assessing overall herbicide effects on river ecosystems. Two species of duckweed, Lenna aoukikusa and Spirodela polyrhiza (Lemnaceae), and Ricciocarpus natans (L.) (Bryophyta) were floated individually for 14 d in a flowthrough aquarium carrying pesticide-polluted water from the Sakura River, and growth was measured at 4, 7, and 14 d. Each test was begun weekly or biweekly and was conducted under continuous light from April to August of 1996 and 1997, Growth of the 3 species was greatly suppressed from spring to early summer, when the river water was polluted with several herbicides. Growth of L. aoukikusa, in particular, was almost completely inhibited. This is attributed to several herbicides based on the analysis of river water samples and a Selenastrum (alga) growth test with herbicides detected in the river, Growth of Selenastrum was also suppressed in May in 3-day tests, which were conducted 3 times a week from April to August. However, the Selenastrum recovered by the end of May, although growth of L. aoukikusa continued to be severely suppressed until late June. Biomonitoring using duckweed can continue when river water becomes muddy after heavy rain because it floats on the water surface
