A quantitative relationship between zooplankters and attached animals on aquatic plant in the littoral zone of Lake Biwa

Abstract

The species composition and population numbers of zooplankton and attached animals in littoral vegetation zone in Lake Biwa were investigated during spring and summer of 1966 and 1967.
The plankters were collected with 0.5 liter cylindrical water sampler, and attached animals with vinyl-sack.
In the Yamanoshita Bay where the investigation was carried out intensively, littoral vegetation developed was as follows : In the marginal zone there were emerged plants (Phragmites, Zizania and Carex), among which patches of sub-merged plants were observed. In the outer part, submerged plants, such as Elodea, Potamogeton, etc., developed widely.
Much more numbers of species of zooplankton were found in the littoral zone than in the offshore area. In the former, abundant species were Chydorus sphaericus and Alona guttata, whereas Daphnia longispina and Conochilus unicornis in the latter.
Between the littoral stations there was no remarkable difference in the total density of zooplankton, but the species composition was remarkably different each other : i.e., Eodiaptomus japonicus and Bosminopsis deitersi were numerous in open littotal area, whereas Chydorus sphaericus, Mesocyclops leuckarti, Euchlanis dilatata and Philodina roseola were numerous in vegetational belt.
Zooplankters living in vegetational belt were divided into two types by their modes of life as follows : completely free-swimming species and species which sometimes attached on aquatic plants and swam freely in other times.
In the latter type of species, much more number of individuals remained attached and only few individuals swam freely at least in the daytime. For instance, attached number of Chydorus sphaericus was 60-120 times higher than the swimming one per unit water column.
The total population number of attached animals changed according to the vegetation type. Attached animals usually occurred more abundantly in Potamogeton belt than in Elodea belt, and much less in Phragmites belt than in the former two submerged plants area.
It was observed that the changes in number of free-swimming animals at small open space between Potamogeton leaves were strictly parallel with that of attached ones on the submerged plant. In such area, attached animals will be supply-base for the planktonic ones.