SOME RESULTS OF THE INTERNATIONAL FISHERY INVESTIGATIONS. 483 



parts in one million parts of sea water). It is small because it is 

 continually being utilized as food by the diatoms ; otherwise, being 

 continually added to the sea by the decomposition of dead animals and 

 plants, and by drainage from the land, the former would gradually 

 become poisoned by it. The German analyses have shown that this 

 small proportion of fixed nitrogen is not constant. Silica (which is 

 required for the skeletons of the diatoms) and phosphates (which are 

 similarly required for other organisms) are also present in correspond- 

 ingly small and variable amounts. 



Now it is of extreme interest to find that the amount of these 

 ultimate food substances present in the sea is greatest just before the 

 time when the maximum abundance of diatoms occurs, and is least 

 about the time of the minimum. This applies more particularly to the 

 German determinations of silica, but no doubt is also true of the 

 nitrogen compounds. In the winter these food-stuffs have been greatly 

 stored up. Tlien in the spring, under the influence of the rise in tem- 

 perature of the sea and the increased intensity of the sunlight, an 

 immense diatom reproduction takes place. The result of this is again 

 to reduce the proportion of the food-stuffs, and as a consequence we 

 have the summer minimum of diatom abundance. Further accumula- 

 tion of the food-stuffs during this period of relative sterility leads to the 

 autumn diatom maximum. Probably denitrifying bacteria play a not 

 inconsiderable role in producing these variable proportions of nitrates 

 present in the sea. It is known that bacteria exist in the sea which 

 have the power of reducing nitrates to nitrites, the latter to ammonia, 

 and ammonia to free nitrogen. In the latter form nitrogen is, of course, 

 unavailable as food for diatoms. Now it has been shown that these 

 denitrifying bacteria are more active at a high than at a low tempera- 

 ture. In the summer, then, they act on the fixed nitrogen present in 

 the sea and render this unavailable as food for diatoms. In the colder 

 season they are less active, and fixed nitrogen accumulates. It is a 

 surprising thing that the plankton is less abundant in warmer tropical 

 seas, in spite of a higher temperature and better light, than in colder 

 sub-Arctic waters. The explanation lies probably in the more intense 

 action of denitrifying bacteria in those warm waters, whereby the food- 

 stuffs for the plankton are reduced. 



Such considerations suggest the close connexion between the lowly 

 organized plankton organisms and hydrographic phenomena. But can 

 the same connexion be shown to exist between changes in temperature 

 and salinity of sea water, and the changes in the abundance of such 

 highly organized animals as our food-tishes ? These connexions are 

 more difficult to establish. The changing abundance of fishes on tlie 

 fishing grounds is expressed in our imjjcrfect statistical systems, and 



