The Seto Inland Sea is an important water area not only as a commercial fishing ground but also as a marine aquiculture ground. In this sea, the mechanism of organic production, as well as the ecosy-stem of inhabiting organisms, is complex owing to complicated hydrographic conditions and diversified fauna and flora. However, there is little doubt that almost all the animal production in this sea depends basically upon the primary production referable to marine plants.
This study aims at clarifying the actual state of primary production in this sea. It is principally concerned with the primary production by the phytoplankton with emphasis on its geographical and seasonal variations and on the description of phytoplankton populations.
1. Photosynthesis and its variation with time and space.
The photosynthetic rate of the natural population of phytoplankton (expressed as 'phytoplankton photosynthesis' or simply as 'photosynthesis' in the following) was measured by the usual method, namely the 14C method employing the incubation tank (illuminated at 12,000 lux). It was suspected that, when this method was applied to a coastal water, the suspended particles retained on a filter might absorb β radiation and cause an error in assaying the radioactivity of incubated phytoplankton. Tests were carried out by incubating 100ml of sea water and filtering it through the molecular filter of 24mm diameter, and it was proved that the above mentioned error is negligible at the concentrations of suspended matter less than 0.34 mg (dry weight)/100ml (which correspond to the Secchi disc transparencies of 3.3 m and over). This result indicated that incubating 100 ml of sample water is usually a justifiable procedure in this sea.
(1) Diurnal fluctuation.
(This subject has already been dealt with in a preceding paper58))
Phytoplankton photosynthesis demonstrated notable diurnal variations. Amplitude of the variation was great in summer (the maximum/minimum ratio being 3.0 to 13.6); it decreased vertically towards deeper layers. In contrast, diurnal variation of the chlorophyll αcontent of sea water was of much smaller magnitude.
Although many observational data in the past suggested that diurnal variation in phytoplankton photosynthesis was ascribable to the diurnal variation in the chlorophyll content of sea water, the foregoing result indicates that the diurnal rhythm of phytoplankton photosynthesis in the Seto Inland Sea was closely correlated with the 'photosynthetic rate per unit amount of chlorophyll α content' rather than with the chlorophyll α content itself.
(2) Geographical variation
(A part of this subject has been dealt with in a preceding paper56).)
Phytoplankton photosynthesis was measured several times at a series of fixed stations arranged on a line traversing the Seto Inland Sea, obtaining values ranging from 1.0 to 17.1 mgC/m3/hr; simultaneously measured chlorophyll α contents varied from 0.2 to 7.3 mg/m3. Both variables varied geographically and seasonally. In many cases their geographical variations followed their own peculiar patterns, rather than such patterns as were expected from the distribution of water temperature and chlorinity.
(3) Seasonal variation.
(This subject has already been dealt with in a preceding papers54).)
Seasonal variation of phytoplankton photosynthesis was studied by taking monthly measurements for a period of one year at a station located in Bingo-nada, a region which could be regarded as representative of the Seto Inland Sea. The measurements varied over a wide range of 0.3 to 15.4 mgC/m3/hr with the mean at 3.9 mgC/m3/hr. They were generally high during summer (in May, June, August and September) and low during winter.
Monthly measurements of chlorophyll α content varied between 0.2 and 2.4 mg/m3 with the mean at 1.0 mg/m3; highest values occurred in January and September.
Monthly values of photosynthesis per unit amount of chlorophyllαcontent tended to be greater at higher water temperatures.
2. Taxonomic composition and propagation rate of phytoplankton
(A part of this subject has been dealt with in a preceding paper55, 57).)
Neritic species of planktonic diatoms usually predominated in the phytoplankton which was the major primary producer in this sea. Dominant species were those which had been commonly found in the coastal waters of Japan. Population density and species composition of phytoplankton varied not only geographically but also, to a greater extent, seasonally. Population density ranged between 106 and 109 cells/m3.
It was a frequent occurrence that phytoplankters other than diatoms propagated rapidly to produce the 'red tide' temporarily. For example, a dinoflagellate, Exuvaella sp., propagated abundantly in September, 1965. On this occasion, field observation was carried out repeatedly and the average population growth rate at the peak of population level was estimated at about 0.3 fissions per day (i.e., population density doubles in about 2.5 days) from the results of cell count as well as from the chlorophyllαdata.
Experiments in which sea water was incubated after addition of various nutrients showed that nitrate-N, silicate-Si, trace metals and vitamins might act as a limiting factor for the propagation of phytoplankton in the Seto Inland Sea.
3. Assessment of primary production
Primary production by phytoplankton was computed from the photosynthesis data obtained by the tank method by use of Steemann Nielsen's equation (1952); necessary corrections were made regarding the depths of sampling layers and thickness of euphotic zone.
Monthly value of primary production thus computed for a representative station varied between 0.03 and 1.21 gC/m2/day during a year with the mean at 0.33 gC/m2/day; it was high in August and September when water temperature was high, and low in February when water temperature was low. Annual production at this station was assessed at 120 gC/m2/year. Primary production for the whole Seto Inland Sea was calculated at 6×103 ton C/day or 2×106 ton C/year; the latter figure is equivalent to 2 ×107 ton (wet weight) of phytoplankton per year.
The foregoing results indicate that in the Seto Inland Sea, as compared with the open ocean, the euphotic zone was endowed with very much higher chlorophyll α content and phytoplankton photosynthesis, but that the primary production per unit area was not much greater owing to lower transparency and smaller thickness of euphotic zone. This condition may be regarded as one of the chief characteristics of the primary production of this sea.
4. Primary and related productions in the Seto Inland Sea
Although the phytoplankton was the major concern in the foregoing, macroscopic marine plants also are primary producers in the Seto Inland Sea. Furthermore, this sea is rich in the suspended particulate matter, the standing crop of which often amounts to more than several times that of phytoplankton. One can not neglect the important role that the suspended matter plays in organic production, particularly in the production of filter-feeding animals. Organic productions referable to these factors was estimated from the data appearing in the literature, and their bearing on primary production was examined. Examination was also made of the correlation between primary production and commercial fish production.
Annual harvest of commercially valuable seaweeds (such as Undaria, culture laver Porphyra, etc.) from the Seto Inland Sea was found to be equivalent to one half of the average daily production of phytoplankton. Annual growth of other wild marine plants (such as Zostera, Ulva, etc.) was estimated at about 1.5% of the annual production of phytoplankton. It may be said from the above that macroscopic plants contributed only a small fraction to the primary production in this sea.
The suspended organic matter occurring in the Seto Inland Sea may be referable to two main sources: namely, (1) the primary production and (2) the organic matter of land origin that is carried into the sea by rivers. Annual supply of organic matter from the latter source was estimated at about 14% of the phytoplankton production. Suspended organic matter may originate also from the extracellular products of phytoplankton and from the bacterial chemosynthesis, but the supply from these sources can be regarded as negligible in amount. It is therefore concluded that the main portion of the suspended organic matter occurring in this sea originated, either directly or indirectly, from the primary production by phytoplankton.
The total catch of those fishes whose growth was judged as having taken place in the Seto Inland Sea was estimated at 1.8×104 tons per year, which corresponded roughly to 1 % of the annual phytoplankton production on wet weight basis. When computed in terms of energy, the above mentioned fish catch represented 0.70 % of the primary production by phytoplankton. This value, although considerably high, is comparable with the values reported for Tokyo Bay and Lake Suwa.