Abstract
Effects of light intensity and temperature on the growth of six submerged macrophytes growing in Lake Biwa, Japan, were examined using an experimental growth system. Light intensity and temperature were controlled at five (100, 50, 25, 13 and 1 %) and four (11, 17, 23 and 29°C) levels, respectively, and shoot length, dry weight, and total leaf area were measured at 10 day intervals for 60 days. Potamogeton malaianus died in 20 days at a water temperature 23°C and light intensity of 1 μmol/m2/s. The exotic species Elodea nuttallii died in 30 days at a water temperature 29°C and light intensity of 1 μmol/m2/s. However, Potamogeton maackianus and Hydrilla verticillata survived in all temperature and light conditions. Light compensation points were 12 μmol/m2/s for P. malaianus and Vallisneria asiatica var. biwaensis, and 1-6 μmol/m2/s for P. maackianus, H. verticillata, Egeria densa and E. nuttallii. Comparison of the Relative Growth Rate of each species showed that V. asiatica, H. verticillata and E. nuttallii had high values at a water temperature of 29°C, and P. maackianus and E. densa had high values at a water temperature of 23°C. Egeria densa, H. verticillata, E. nuttalli and P. malaianus showed faster shoot elongation, P. maackianus and V. asiatica slower. When shoots were near the water surface, the elongation speed of E. nuttallii, E. densa and H. verticillata decreased, and active branch extension was noted. However, P. malaianus shoots elongated. We confirmed that each species of submerged macrophyte showed specific responses to minor changes in water temperature and light intensity at around the compensation point, and responses specific to each species to seasonal fluctuations of the water level can be predicted.
