The Effects of Water Depth on the Growth of Two Submerged Macrophytes in an in Situ Experiment

An in situ experiment was designed in which Potamogeton crispus and Hydrilla verticillata were planted in plastic pots 25, 50, 75, 100, 125, 150, 175 and 200 cm underwater to study the effect of water depth on the growth and photosynthetic fluorescence characteristics of the two submerged macrophytes in Lake Gehu, China. The results showed that: 1) Water depth had no significant effect on the germination of both submerged macrophytes. The final germination rate of P. crispus turions was higher than 80%, and the final germination rate of the H. verticillata winter buds was 100%; 2) Water depth affected the survival rate of both submerged macrophytes, and the seedlings at depths of 150, 175 and 200 cm all eventually died; 3) Water depth altered the morphological characteristics of P. crispus and H. verticillata seedlings. (4) Increasing water depth significantly affected biomass (p < .05), with the biomass initially tending to increase, but then decreasing. The root to shoot ratio of both submerged macrophytes decreased with increasing water depth; 5) Chlorophyll contents of P. crispus and H. verticillata increased with water depth from 25 to 150 cm; 6) Water depth had an obvious effect on the photosynthetic fluorescence of submerged macrophytes. The Fv/Fm increased slightly from water depths from 25 to 75 cm, and then decreased significantly when water depth exceeded 100 cm. The ETRmax was significantly different among all treatment groups, and ETRmax was obviously higher at water depths of 25 and 50 cm than at all other depths. These results showed that submerged macrophytes have a limited water depth tolerance range. In Lake Gehu, the recovery of submerged macrophytes will be better in areas with water depths less than 125 cm.