Abstract
This work, also, was conducted together with previous ones to study percentages of vegetational cover and covers of aquatic weeds in the same creeks.
The mean of percentages of vegetational cover in investigated creeks was 27%, although they were variously different by creeks. Vegetational covers of about three-quarters of creeks were less than 50%, of 11% creeks were 50 to 75%, and of 16% ones were 75 to 100%, which were mainly consisted of covers of waterhyacinth [Eichhornia crassipes (Mart.) Solms] or other troublesome weeds.
In Chikugo Plain, the highest percentage of cover in creeks was 12% of waterhyacinth, the second was 6% of hairly subspecies of knotgrass (Paspalum sp.), and the third was 3% of waterchestnut (Trapa spp.) group. On the other hand, in Saga Pl., the highest was 6% of knotgrass (Paspalum distichum L.), the second was 5% of duckweed [Spirodela polyrhiza (L.) Schleid. and Lemna paucicostata Hegelm.] group, and the third was 5% of waterhyacinth. Covers of any aquatic species were variously different by creeks as percentages of vegetational cover. Covers of most weeds were less than 50% in 90 to 100% creeks where they were found. However, those of wathrhyacinth, hairly subspecies of knotgrass and parrotfeather (Myriophyllum brasiliense Camb.) were more than 50% in 17 to 29% creeks where they were found. These three species appeared to have adapted to thrive vigorously in creeks of the area.
In Chikugo Pl., 21% creeks were dominated with waterhyacinth, 10% with hairly subspecies of knotgrass, and 3% with parrotfeather. On the other hand, in Saga Pl., 13% ones were dominated with knotgrass, 9% with waterhyacinth, and 4% with hairly subspecies of knotgrass. In either Pl., 54% creeks were not dominated with any aquatic species.
All investigated creeks were classified into two groups, that is, main and subsidiary ones, according to their roles in irrigating water in the paddy area. It was found that subsidiary creeks were generally more thickly covered with aquatic weeds than main ones, and, however, mains were more extensively covered with weeds than subsidiaries in local areas where aquatics grew most severely.
