COLONIZATION AND SUCCESSION OF SUBMERGED MACROPHYTES IN SHALLOW LAKE VAENG DURING THE 1ST 5 YEARS FOLLOWING FISH MANIPULATION

Colonization of submerged macrophytes and changes in species composition were studied in shallow Lake Vaeng during the first five years (1987-91) following fish manipulation in 1986-1988 and a resultant significant improvement in lake water transparency. No submerged macrophytes were present in the lake from 1981-1986, during which time the summer mean Secchi depth ranged from 0.6 and 0.8 m. From 1987 to 1990, Secchi depth increased from 0.9 m to 1.8 m and macrophyte coverage consequently increased (1 % of the lake area in 1987, 2 % in 1988, 50 % in 1989, 80 % in 1990 and 90 % in 199 1). At the same time, the macrophytes became taller, and the weedbeds more dense. The macrophytes colonized from the exposed and deeper part of the lake towards the sheltered and more shallow part of the lake, a colonization pattern that was confirmed by transplantation experiments. The delay in colonization of the shallow parts may be caused by waterfowl grazing. The vegetation was initially dominated by Potamogeton crispus L., but there was a gradual change during 1988-1989 and Elodea canadensis Michx became exclusively dominant in 1990-1991.