Changes in biomass and spatial distribution of Elodea nuttallii (Planch.) St. John, an invasive submerged plant, in oligomesotrophic Lake Kizaki from 1999 to 2002

Distribution of pure Elodea nuttallii vegetation was surveyed from 1999 to 2002, immediately after the most recent expansion of the species in Lake Kizaki, Japan. During 2001 and 2002, areas of E. nuttallii vegetation rapidly diminished and the summer plant height decreased wherever the vegetation remained. The organic matter content, total phosphorus, and extracted P of the sediment from the vegetation bed were measured. A linear relationship was observed between the extracted P in the sediment and the biomass. The extracted P significantly decreased in the shallow littoral vegetation bed, where the biomass clearly diminished. A fertilization experiment using the shallow littoral sediment collected in the vegetation bed was conducted in 2001. In this experiment, apical shoots of E. nuttallii were planted in pots with fertilized sediment (nitrogen, phosphorus, and potassium additions). The growth of E. nuttallii shoots was significantly enhanced by enrichment with phosphorus alone. The ecological implication of sediment phosphorus limitation is discussed in relation to the cause of decline in the E. nuttallii population in Lake Kizaki.