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.

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The distribution of macrophytes in Lake Kawaguchi, Japan, was surveyed in August 1999 using a sampling anchor from a boat. The survey revealed that the present aquatic vegetation was composed of 17 submerged species, and no floating-leaved plants were present. The diversity of submerged plants and their vertical growth limits decreased progressively from the west to the east end of the lake. At the time of the survey, the dominant species was Elodea nuttallii (Planch.) St. John, which had once grown explosively in the mid-1980s in the lake, and except for Vallisneria asiatica Miki, all of the species commonly observed in the lake seem to have diminished their growth greatly. Among them, Potamogeton compressus L., which was the most dominant species in the lake before the invasion of Elodea nuttallii, had decreased most severely. It is suggested that the present state of aquatic vegetation in this lake was much influenced by the persistent growth of Elodea nuttallii as well as by environmental factors.

The Ozegahara mire is the largest raised peat bog in central Japan. Nymphaea tetragona and Nuphar pumila var.ozeensis were the dominant floating-leaved plants in the bog pools. N. tetragona is known to have specific spatial distributions in pools. Forty pools in the Kamitashiro area were surveyed in August 2017 to document the present status of macrophytes. To clarify the factors that cause these specific spatial distributions of N. tetragona in the pools, the growth status of N. tetragona (coverage index, number of floating leaves, and width of leaf blades) and bottom sediment properties (fabric matter, ash, and total phosphorus content) were measured in three study pools in the Kamiatashiro area in August 2018. The survey revealed that an increase in N. tetragona and a decrease in N. pumila var. ozeensis has continued since the 1970s. The results from the three study pools showed that there were no clear differences in the growth status of N. tetragona (coverage index and number of leaves) and in the bottom sediment properties (fabric matter, ash, total phosphorus content, and the thickness of the decomposed peat) between the shore side bottoms and at the central bottoms in the study pool where N. tetragona was distributed all over the bottom (overall type). On the other hand, an obvious increase in the growth status of N. tetragona and the values of the bottom sediment properties were observed toward the central bottoms in the study pools, where N. tetragona was distributed all over the pool, avoiding the shore side bottom (overall without shore side type). The results indicated that the absence of N. tetragona on the pool bottoms was attributed to sediment nutrient infertility.

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