A study on the primary production was carried out in the western half of the Seto Inland Sea (except Suo-nada) in June, 1963, and in the eastern half in September, 1963. The results of the quantitative and qualitative examination of phytoplankton standing crop (diatoms and dinoflagellates) and the consideration on the interrelation between primary production and phytoplankton are dealt in this paper.
The total cell number of diatoms was relatively small in the western half (especially in Bingo-nada and Hiuchi-nada) in June, and relatively large in the eastern half (especially in Bisan-seto and its neighbouring area and in the southern part of Osaka Bay) in September. Species composition of diatoms varied from station to station, but in general, Skeletonema, Leptocylindrus and Chaetoceros were dominant in the western half, and Thalassiosira, Skeletonema and Chaetoceros in the eastern half.
Dinoflagellates occurred abundantly at St. E-4 and E-8 (in Harima-nada). At other stations they were usually scanty and accounted for only small proportions of phytoplankton, but at 0.5 m layer of St. W-9 and transparency layer (T layer) of St. BG-2, dinoflagellates and diatoms occurred in about equal numbers.
The quantitative interrelationship between primary production and phytoplankton population was considered 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. Variables marked with* were calculated both for "diatoms only" and for "diatoms and dinoflagellates combined".
The following general statement applies to the great majority of the stations, with the exceptions of 0.5 m layer of St. E-4 and all layers of St. E-8.
a) There is a tendency that the chlorophyll a content per cell decreases as the total cell number increases.
b) It appears that the photosynthesis per unit chlorophyll a was at a normal level for the season, because the observed values were of the order that had been recorded ·in Bingo-nada during warmer months of the year (previously reported).
c) The photosynthesis per cell, or the average photosynthetic activity of each cell, varies proportionally to the chlorophyll a content per cell.
d) In many cases, a large value of total photosynthesis was associated with a markedly large "total cell number". In such cases, the photosynthesis per cell was comparatively low, but the cumulative photosynthesis was high because of a large number of cells. And it seems that the variation in the magnitude of total photosynthesis from station to station can be ascribed to the variations in the photosynthetic activity of chlorophyll a (i. e. photosynthesis per unit chlorophyll a) and in the total cell number.
e) The foregoing interrelations between the photosynthesis and phytoplankton was held irrespectively of the species composition of phytoplankton. On the other hand, the total photosynthesis was much higher at St. E-8 than at other stations. Gniaulux polygramma (Dinoflagellata) occurred abundantly at this station. There is strong indication that this species showed much stronger photosynthetic activity per cell than diatoms. A similar case was observed at 0.5 m layer of St. E-4. At 0.5 m layer of St. W -9 and T layer of St. BG-2, it was observed that the photosynthesis per cell of dinoflagellates (Prorocentrum micans or Ceratium fusus) was about the same as that of diatoms.