As a section of a study on the primary production in Bingo-nada (central part of the Seto Inland Sea), quantitative examination of living plankton was carried out at two stations (BG-1 and BG-2). This paper deals with the results of the examination, and considers the quantitative relationship between the primary production and the standing crop of planktonic diatoms.
Some outstanding features of the seasonal changes in the total cell number and specific composition of planktonic diatoms are shown in Text-figs. 1-3. The total cell number of diatoms increased remarkably in and after April, 1963 at BG-2 (especially at the 0.5m depth). Species composition of diatoms varied so much from month to month that dominant species alternated frequently.
The occurrence of zooplankton is shown in App. Tables 1 and 2. Acartia clausi and Oithona nana dominantly occurred only at BG-2. From the difference of dominant copepod species (Text-fig. 4), the environmental condition at BG-2 seems to be stronger in inshore character than that at BG-1.
The quantitative relationship between primary production and living planktonic diatoms was discussed by correlating the following variables to one another: the total cell number, the total photosynthesis, the photosynthesis per cell, the chlorophyll a content per cell, and the photosynthesis per unit chlorophyll a.
a) There is tendency that the chlorophyll a content per cell drops as the total cell number increases. Chlorophyll a content per cell is high when the total cell number is scanty.
b) The photosynthetic activity of chlorophyll a seems to be stronger in the warmer season, and weaker in the colder season.
c) The photosynthetic activity per cell fluctuates according to the variations in both the chlorophyll a content per cell and the photosynthetic activity of chlorophyll a. In the most cases, however, its fluctuation was caused by the variation in the chlorophyll a content per cell. Accordingly, there was observed a tendency that the photosynthetic activity per cell becomes weaker as the total cell number increases and becomes higher when the diatom cells are scanty.
d) In most examples, an increase in total photosynthesis was observed concurrently with a marked increase in the total cell number. In such cases, the photosynthetic activity per cell was comparatively weak, but the cumulative photosynthesis was high because of the large number of cells. On the other hand, there were cases in which an increase in total photosynthesis was observed inspite of a rather scanty cell number; in such cases the photosynthetic activity per cell was very strong.
e) The several interrelationships mentioned above were held irrespectively of the species composition of planktonic diatoms. Accordingly, it seems that an increase or decrease in the total photosynthesis in Bingo-nada is controlled not by the occurrences of any particular speices of phytoplankton, but by the variations in the photosynthetic activity per cell and in the total cell number.