Ecological Studies on the Productivity of Aquatic Plant Communities IV

Abstract

An attempt was made to describe the distribution of light intensity at different depths in submerged plant communities quantitatively, in terms of the light extinction coefficients of both water and plants.The rate of gross photosynthesis in floating leaves under high light intensities became saturated usually at the range of air-flow rate greater than 1.5 litre/cm2 leaf area/hr. The observed rates of gross photosynthesis at light saturation and respiration were 15-32mg CO2/dm2/hr and 1.1-3.5mg CO2/dm2/hr at 25°-27° and 300 ppm CO2.The rates of daily gross photosynthesis in submerged and floating plant communities were estimated based on the mathematical model in which the effects of daily and depth-dependent changes of light intensity and the profile structure of plant communities were taken into consideration.The rate of daily gross photosynthesis in Elodea Nuttallii communities in summer was nearly proportional to the shoot biomass where the latter was less than 300g/m2, and reached a maximum of about 25g dry matter/m2 land area/ day where the shoot biomass was more than 500g/m2. The total leaf area index in floating plant communities usually ranged between 1m2/m2 and 2m2/m2, and the net photosynthesis became negative in the third layer of leaves. The rates of daily gross photosynthesis and daily net production in a Nymphoides indica community with a leaf area index of 1.8 m 2/m 2 were 26.8 g dry matter/m 2 water surface/day and 15.2g/m2/day on a clear day in June.