Abstract
Submerged aquatic plants play an important role in the conservation of aquatic environments, and the preservation and improvement of biodiversity in lakes and reservoirs. However, land reclamation, construction of lakefront bank, and water contamination owing to rapid economic growth led to the disappearance of submerged plants in Lake Kasumigaura and Lake Inbanuma in the 1990s. Although restoration projects using soil seed bank have been actively conducted since the 2000s, ensuring sustained growth of submerged plants has been difficult. This study examines the effects of various environmental factors on the growth of submerged plants based on these restoration project results and discusses the future of restoring submerged aquatic vegetation. In Lake Inbanuma, 26 species of submerged plants, containing local genetic information, have germinated and regenerated from the propagule bank collected from the bottom of the lake. When the soil seed bank was spread over the aquatic plant restoration zone (30－70 cm below the water surface) enclosed by wave dissipation structures, submerged plants disappeared within a few years owing to invasion by other plants, such as Typha domingensisis, accumulation of sediments and fluid mud , and feeding damage inflicted by crayfish and aquatic birds. Typha domingensisis grew until depths of approximately 70 cm. Further, previous results suggested that germination and regeneration of submerged plants are inhibited at depths greater than 80 cm, owing to the limited amount of light reaching the bottom of the lake. These findings indicate that the environment conducive for stable growth of submerged plants is extremely limited at the current level of water contamination. Therefore, improving water transparency by controlling the sources of pollution in each watershed, and developing effective measures for controlling animals inflicting feeding damage are urgent requirements. In addition, the research findings suggested that intermediate levels of disturbance to prevent the accumulation of sediments and fluid mud is required for sustaining the growth of submerged plants. In future, the development of a new simulation that incorporates diverse environmental factors based on previous findings, followed by demonstration experiment, is required for the development of an optimal environment for the restoration of submerged plants.
