ノリ生育に対する環境，特に水流の影響に関する研究

The laver, Porphyra tenera, 'nori' in Japanese, is extensively cultivated in Japan, where dried laver, prepared in the form of thin sheets, is beloved as a delicacy and one of the most popular foods. In many coastal areas of the country, inhabitants earn important part of their livelihood by engaging in the culture industry of this seaweed. The laver farms are distributed along the Pacific coast, and generally located in bays and inlets, but especially in the Seto Inland Sea, they are limited to the estuaries at the mouths of rivers. The growth of laver is affected by such various environmental factors as water temperature, salinity, nutrient contents of sea water, etc. The author investigated into the reason why the laver farms in the Seto Inland Sea are located exclusively in estuaries by comparing the hydrographic conditions of these farms with those of the laver farms in Korea, in which country the laver farms are mostly located in those inshore waters where no stream empties. It was recognized through this investigation that, in the Seto Inland Sea, the growth oflaver is controlled by water currents no less than by water temperature and nutrient contents of water.

The author then studied the effect and the optimal range of each of the important environmental factors by carrying out culture experiments mainly in the laboratory and partly in the field. The results of these studies are summarized below.

(1) In the Seto Inland Sea, environmental conditions of the offshore waters are unsuitable for the growth of laver, because water temperature is relatively high (about l0°C in the winter or the peak season of laver's growth) as compared with the optimal temperature range for this seaweed (i.e., 6-10°C), and because the concentrations of nutrients in sea water are rather low and very changeable (Tables 1,3,4,5, and 6). In contrast, in estuarine waters, water temperature drops in the winter, water is rich in plant nutrients which are supplied by the river water, and moreover, water currents of favorable velocities are developed (Tables 7, 8, 9 and 10). It is in these waters that laver farms are located.
(2) When water temperature is high, growth of laver is greatly affected by water current. Water currents transport nutrients to, and remove excreted metabolites from laver fronds.
(3) In estuaries laver fronds are exposed to changing salinities. When laver fronds are experimentally exposed to a sudden change of salinity, their growth is accelerated if the time of exposure is very short (less than 15 min.), but retarded if exposure lasts longer (Table 12 and Text-fig. 3).
(4) It was found, through the transplanting experiments from the Seto Inland Sea to Korea and between the localities in Korean waters, that laver is rather tolerant to the changes of environmental conditions (Tables 13 and 14).
(5) The velocity of water current suitable for the growth of laver ranges from 15 to 30cm/sec, but the optimal velocity varies in relation to other environmental factors as well as the condition of laver frond (Table 17) as indicated below.
(i) Optimal range of current velocity is narrow for young fronds, but becomes wider for older fronds (Table 17). When fronds are growing thick, they require rapid currents of velocities nearly 30 cm/sec (Tables 18 and 19).
(ii) 7 cm/sec is the most suitable current velocity for the spores to attach. After attachment, however, growth is equally good within the velocity range from 7 to 25 cm/sec (Text-figs. 7 and 8).
(iii) Rapid water currents have ill effects upon the growth of laver frond at low salinities; at the specific gravity (ρ15) of 1.018, optimal velocity is 20cm/sec (Table 20).
(iv) Optimal current velocity for the growth oflaver varies very markedly according to the nutrient contents of sea water. It is about 20cm/sec in the ordinary sea water, but greater velocities (nearly 30cm/sec) are required if the concentrations of nutrients are lowered by diluting ordinary sea water with the NaCl solution of the same salinity. If ordinary sea water is enriched by adding nitrates and phosphates, water flow of 15cm/sec is sufficient for the best growth of laver. In the sea water enriched by adding ripe manure, laver fronds grow equally well over the velocity range from 5 to 20 cm/sec (Table 21).
(v) At favorable temperatures (below 10°C), the range of suitable current velocity is wide (i.e., from 10 to 30cm/sec). When water temperature is relatively high (10-14°C), especially when nutrients are not available in sufficient amounts, rather rapid current (30cm/sec) is need. Laver fronds do not survive longer than 2 weeks at the water temperatures over 16oC (Text-fig. 9).
(6) Growth of laver frond is affected also by light intensity. The growth increases with the increase of light intensity up to 10,000 lux, although the rate of the increase differs according to the light source (i.e., the spectrum of incident light). However, it seems that there exists an upper limit of suitable light intensity. This is inferred from the results of the growth experiment in which laver was cultured at different water depths throughout the growing season: the growth was best near the surface in December and January, but the zone of best growth moved downwards as the sun light became stronger with the further progress of the season (Tables 23, 24 and 25).
(7) The roles that water current plays in controlling the growth of laver are considered on the basis of the results of various experiments.
(i) As was shown in Section VII of the text and mentioned in summary (5) (iv), water current of nearly 30cm/sec is necessary for the maximum growth of laver frond if concentrations of plant nutrients are low in sea water, while a current only 5cm/sec is sufficient if the concentrations of nutrients are high. It seems therefore that the lower limit of the current velocity that is necessary for the maximum growth of the frond depends on the nutrient contents of sea water. On the other hand, it seems that there is an upper limit of current velocity which the laver frond can tolerate physiologically. Water currents faster than this limit do not have favorable effect upon the growth of the frond (Text-figs. 12 and 13).
(ii) According to the results of the experiments in which laver fronds were cultured in the medium which presumably contained the external metabolites of this seaweed in excessive concentrations, it appears that the metabolic wastes of the laver, once liberated into the sea water, do not have any harmful effect upon the growth of the frond (Table 26). The culture medium employed in this experiment was the one in which laver fronds had been cultured in high densities; after these fronds were removed, the medium was enriched by adding nitrate and phosphate and used for the experiment.
(iii) When laver fronds are cultured without renewing the culture medium, their growth is retarded. The harmful effect of such a culture condition is ascribable to the deficiency of plant nutrients, the rise of pH and the growth of bacteria and other microorganisms, rather than to the accumulation of laver's metabolic wastes. The rise of pH, which is particularly remarkable (Text-fig. 11), is probably due chiefly to the consumption of carbon dioxide as a result of photosynthesis rather than to alkaline substances which laver fronds may excrete (Table 29).
(iv) It is concluded from the foregoing evidences that, from the viewpoint of the growth of laver, the major function of the water current is to transport dissolved plant nutrients to the fronds, that current velocities of 10-30cm/sec are needed for this function to be fulfilled, and that water currents of such velocities automatically accomplish such other beneficial functions as checking the rise of pH, removal of
metabolic wastes, etc.
(8) It was deduced from the foregoing results that in the Seto Inland Sea commercial laver culture need not be limited to estuarine waters and that other parts of the Sea can also be utilized as laver farms if there is suitable water current. In order to test the validity of this idea, laver culture was conducted on an experimental scale in the Onomichi Suido (Text-fig. 15) where the inflow of fresh water is very limited and commercial laver culture had never been attempted. According to our preliminary survey, the meteorological and hydrographic conditions of this strait did not appear unsuitable for the growth of laver. The results of this experimental culture indicated, as expected, that laver can be cultured commercially in this strait (Table 39).
(9) The over-all results of the present study are presented in a condensed form in Table 40. This table shows how the variations in the four important environmental factors (i.e., water temperature, dissolved nutrients, illumination and water current) affect such qualities of the laver frond as color, luster, toughness, flavor, etc. In addition, the relative importances of these four environmental factors are evaluated from the viewpoint of the growth of laver fronds, and the result is expressed in the form of the ratio as below:
water temperature : illumination : dissolved nutrients : water current
= 30 : 10 : 35 : 25 or 35 : 10 : 35 : 20
It is hoped that this ratio, although it is a crude approximation, will serve as a guide when one tries to select sites suitable for laver culture.