Distribution, Stand Structure and Yearly Biomass Fluctuation of Elodea nuttallii, an Alien Species in Lake Biwa. Studies of Submerged Macrophyte Communities in Lake Biwa(3).

Abstract

The areal distribution, stand structure and biomass of pure stands of Elodea nuttallii, an alien species of water weed invading Lake Biwa since 1961, were studied with emphasis on the remarkable yearly fluctuation in its biomass. The total area covered by the Elodea community was estimated at 895 ha, based on a complete lake survey in 1990 using sonar detection along a number of line transects. Underwater biomass samplings were carried out at two different depths in the northernmost part of the lake during two successive years, representing a light crop year (1988) and a heavy crop year (1989). The maximum biomass and stem density at the shallower site (depth 3 m) in 1989 amounted to 723g dry wt. m-2 and 4, 960 stems m-2, respectively. Those in 1988 were only 68% and 60% of the corresponding values of 1989. Both biomass and stem density decreased at the deeper sites (depth 5m), with the latter remaining fairly constant in both years. A comparison of environmental factors between light and heavy crop years indicated that a mild winter with a minimum monthly temperature above 8°C combined with sufficient sunshine in early spring favored the growth of overwintering plants from which new shoots were produced vegetatively. This resulted in greater biomass production in the following season.