植物プランクトンと大形水生植物との拮抗的関係について

抄録

An antagonistic relationship between large rooted aquatic plants and phytoplankton has been long recognized intuitively and empirically according to the facts that even in eutrophic lakes the smaller the amount of rooted aquatic plants is, the larger the amount of phytoplankton or transparency of lake water becomes, and vice versa. This relationship must be an important factor in the course of hydrarch succession of lakes. In the present paper, the authors are going to clarify the antagonistic relationship objectively, and then to analyse the relationship experimentally. The main results obtained are as follows.
1. In some eutrophic lakes, the standing crop of rooted aquatic plants and phytoplankton were determined in the summer season when the maximum values were attained. The standing crop of phytoplankton was remarkably larger in Lake Suwako than in Lakes Tega-numa or Inba-numa, while the amount of rooted aquatic plants per unit surface in the former lake was smaller than in the latter lakes. Transparency was smaller in the former lake than in the latter lakes.
2. In a series of pot cultures with or without rooted aquatic plants of several kinds, such as Hydrilla verticillata var. Roxburghii, Vallisneria spiralis and Nymphoides indicum, the same phenonmena as seen in the lakes were also observed regardless of the difference in species.
3. The concentration of nutrient salts in the water of lakes and pot cultures was lower than that of minimum requirement of phytoplankton for normal growth. The fact that the growth of phytoplankton is inhibited by rooted aquatic plants in the initial stage of culture even under sufficient supply of nutrient salts, indicates that the antagonistic relationship mentioned above is not always resulted by the lowering of the nutrient factor. It was confirmed that the changes in underwater illumination and water temperature by rooted aquatic plants are not the main reason why the antagonistic relationship occurs.
4. A series of culture experiments of Chlorella sp. in the water of Lake Teganuma was carried out under the conditions of enriched or non-enriched nutrient salts and with or without heat-treatments at 125°C for 20 minutes. The results indicate that the lake water containes a substance inhibiting the growth of Chlorella, and moreover that the inhibition is lost easily by heat-treatment. The concentration of this substance seemed to decrease with the march of season, and no inhibiting effect was observed no longer in early winter.