Chinese Acad Sci, Donghu Expt Stn Lake Ecosyst, State Key Lab Freshwater Ecol & Biotechnol, Inst Hydrobiol, Wuhan 430072, Peoples R China
Decline of submersed macrophytes in Lake Donghu of China with the progress of eutrophication is assumedly due to low light stress by algae blooming. I conducted a laboratory experiment to study the impact of low-light stress on the growth of Potamogeton maackianus A. Been, a dominant submersed macrophyte of the lake before the 1970s. Plants were grown for six weeks in aquaria with Lake Donghu sediment and enriched water. Light delivered to aquaria was adjusted to simulate the typical Lake Donghu light intensities that exist at several water depths from 0.6m to 1.7m. Biomass growth of the plant was inversely related to light intensity at the simulated depths of greater than or equal to 1.0m (r = 0.96, p < 0.05, n=6) and was negative at the depths of greater than or equal to 1.4m. These results indicate that photosynthetic light saturation and compensation points of the plant in Lake Donghu should be ca. 0,9m and ca. 1.5m depths, respectively. Chlorophyll content, growth of main shoot, total shoot lengths and density of the plant all peaked at 1.2-1.3m simulated depths. These results indicate that P. maackianus responds to low light stress primarily by elongation of shoots, and increase of density. Its biomass growth and nutrient uptake rate did not correlate with the accelerated shoot growth. Below the light intensities of water deeper than 1.2-1.3m, shoot growth rate decreased. The flexible tolerant strategy of P. maackianus to low-light stress suggests that the disappearance of this plant from the lake was not mainly due to eutrophication-induced low-light stress.