Effects of floating-leaved plant, Trapa natans, on the water qualities in Lake Inba

Abstract

　Recently, the floating-leaved plants, especially Trapa spp., have been increased in many shallow and eutrophic lakes in Japan. In this study, we investigated the seasonal changes in water qualities in relation to the presence / absence of Trapa natans in the eutrophic Lake Inba. In the Trapa growing seasons (from July to September), the percentage of dissolved oxygen and turbidity were lower in the Trapa beds than the open water without Trapa. Dissolved oxygen significantly decreased near the bottom especially at night. Turbidity was significantly lower both at the surface and bottom layer in the Trapa beds only in the Trapa growing seasons. Low oxygen and turbidity in the beds might be due to the reduced water currents and the suppression of photosynthesis by phytoplankton under the low light conditions. There were no significant differences in nutrient concentrations between Trapa beds and the open water, though high concentrations of NH₄-N in the Trapa beds were observed in August and September. Decay and decomposition of Trapa and anoxic condition near the sediment might have caused the release of inorganic nitrogen into water. While in the non-growing seasons, there were no significant differences on water qualities between the stations. Differences in water qualities were more remarkable between seasons that total phosphorus concentration was high from July to September, while NO₃-N and NO₂-N were high in other months. Since those seasonal changes in nutrient concentrations were common in Lake Inba, the effects of Trapa spp. on nutrient concentrations were much smaller than the other factors affecting seasonal variations.