Abstract
The present paper deals with the relation of the primary production of lakes to the trophic degree, with special reference to the nutritive salt conditions.
1) The ratio of the production of phytoplankton to that of rooted aquatic plants are examined in some Japanese lakes. If the whole area of a lake is concerned, rooted aquatic plants which take part in the primary production can be neglected except the shallow lakes where the aquatic plants are growing luxuriantly.
2) The phytoplankton production and the mean chlorophyll content in the euphoric zone are small in deep lakes and large in shallow lakes.
3) There are two types in seasonal variation of chlorophyll content in the euphoric zone, i. e. the one is seen in the shallow eutrophic lakes where its maximum content is found in midsummer, and the other is that of the deep oligotrophic as well as mesotrophic lakes where two maxima occur, one in autumn and the other in spring.
4) The total nitrogen and total phosphorus contents of lake waters were measured during the circulation period. The inverse curve linear relation could be found between the mean depth of lakes and the content of each element as well as of chlorophyll.
5) The content of different mineral elements in lake waters was compared with the minimum requirements of various phytoplanktonts for the corresponding elements. It was shown that the lake waters contained the least nitrogen and phosphorus for the requirement of phytoplankton. It could be found a fairly direct linear relationship between the chlorophyll content and the total phosphorus content of the surface waters of lakes, when they were plotted on the logarismic scales.
6) The relation between the daily production rate of phytoplankton and the total phosphorus content of the lake water was calculated theoretically. On the basis of the results thus obtained, the balance of phosphorus was calculated between the part incorporated in the newly produced plankton bodies and that contained in the lake water. From these results it could be surmised that most part of phosphorus incorporated in the newly produced plankton bodies, especially of those of the eutrophic lakes, should come liberated from dead plankton bodies and mud.
