The writer has made a limnological investigation of five lakes and the rivers connecting with them in the Hokuriku district, with special reference to the relation between the salinity of the waters and the plankton species found there. The results obtained are as follows :
1. Forms of the waters
A. Lakes
1) Kahoku-gata…Mesohaline water
2) Kita-gata…Fresh water-polyhaline water
3) Imae-gata…Oligohaline water
4) Kiba-gata (south-west) Shibayama-gata…Fresh water
B. Rivers a. Kakehashi-gawa
1) at Sumiyoshi-bashi…α Mesohaline water
2) at Shiodome-bashi at Mukaimotoori-shinbashi…Oligohaline water b. Kushi-gawa
3) Muramatsu-cho…Fresh water-oligohaline water c. Shinhori-kawa
4) at Genpei-bashi at Shiodome-suimon…Fresh water
2. Plankton
The plankton consist of 151 species, 20 Rotifera, 2 Cladocera, 6 Copepoda, 1 Bryozoa, 13 Protozoa, 31 Chlorophyceae, 3 Chyanophyceae and 75 Diatomaceae.
The dominant and subdominant species occurring in this district are Sinocalanus tenellus, Paracyclopina nana, Pseudodiaptomus inopinus, Diaphanosoma brachyurum, Thermocyclops taihokuensis, Bosminopsis deitersi, Keratella valga, Brachionus calyciflorus var. dorcas and Ceratium hirundinella.
Generally speaking, the plankton fauna in these waters, are divided into two groups according to the main representatives. One consists of Paracyclopina nana and Sinocalanus tenellus in L. Kahoku-gata and L. Kita-gata, both of which have the water of high salinity, and the other is represented by Diaphanosoma brachyurum, Bosminopsis deitersi and Thermocyclops taihokuensis in the water of low salinity, such as, L. Kiba-gata, L. Imae-gata and L. Shibayama-gata.
As seen from the tables the zooplankton in the waters in question are richer in quantity than the scanty, phytoplankton, thongh the latter is rich in the number of species.
Among the phytoplankton, Merismopedia sp., Anabaena sp., Pediastrum tetras, Ankistrodesmus falcatus var. mirabilis, Schroederia setigera, Scenedesmus dimorphus, S. denticulatus, S. longispina, S. maximus, Actinastrum sp., and Coelastrum sphaericum seem to adapt to brackish water.
The diatoms are widely distributed in this district. The representative species found at Yoshizaki of L. Kita-gata is Melosira octogona and that at the Shiodome-bashi of the Kakehashi-gawa River is Nitzschia obtusa and N. obtusa var. scalpelliformis; that at Katayamazu by L. Shibayam-agata is Melosira varians. None of representative species is found in the other stations.