490 DEPARTMENT OF THE NAVAL SERVICE 



must be far less than their respiration; there can hardly be any possibility of con- 

 tinued propagation here. Thus even the comparatively simple question as to the limit 

 of depth at which assimilation predominates over the destructive metabolism of chloro- 

 phylliferous organisms offers in itself considerable difficulty, and can hardly be solved 

 by mere investigation of the vertical distribution of the organisms concerned. 



The investigations in question did, however, furnish various important results 

 which could serve as basis for further research; among these, the following may be 

 particularly mentioned: — 



(1) The diatoms, which appear to be the most rapidly growing of all producers 

 in the sea, develop in great quantities in the coastal waters of Northern Europe during ' 

 the later winter months (February-April) while the temperature of the surface layers 

 is still at or slightly above the minimum for the year. They are restricted to a cold 

 surface layer with comparatively low salinity, lying above water layers of considerably 

 higher temperature. The .greatest quantities are found in places where there is a 

 continual inflow of water either from the land, e.g. from the Baltic, and from the 

 rivers of Northern Europe, or fi-om the ice limit of the Arctic ocean, but where the 

 fresh water has nevertheless become so mixed with salt that there is a certain degree 

 of stability in regard to osmotic pressure. The supply of foodstuffs brought down by 

 the river water seems to play an essential part in this production. 



(2) The great production of diatoms in the coastal waters ceases in May-June, 

 when the surface layers become warmer. At this time of year, there may be a second- 

 ary minimum in the total quantity of phytoplankton, until the mobile algae, chiefly 

 Ceratium, propagating in the warmest season of the year, have reached such numbers 

 as to replace the diatoms. Local and temporary maxima of diatoms may, however, 

 occur also in summer, especially in the vicinity of the coast after a rainy spell, when a 

 rich supply of foodstuff in solution has been carried down from the land. In summer, 

 then, the conditions of life must be unfavourable to diatoms, the marked isolation 

 between the warm, light surface layer and the waters below preventing any introduction 

 of nutritive matter by vertical circulation or mechanical mixture, and also because 

 they are ill able to keep floating in the water layers of low specific gravity, which are 

 also, owing to their high temperature, of comparatively slight viscosity. These obstacles 

 can be surmounted by the mobile Ceratium, though these also exhibit a distinct 

 tendency to sink down towards the limit of the surface layer — but not by the diatoms, 

 save where circumstances are particularly favourable with regard to nourishment, as 

 by abundant supply of food from the land. In the autumn, when cooling begins, when 

 the autumn gales and also the purely mechanical equilibrium of the water layers 

 drives the surface water towards land, so that both vertical circulation and a stronger 

 mechanical mixture can take place, then the conditions are more favourable. At this 

 season, in October-November, we find a new maximum, and the diatoms can then, 

 at any rate, in some years, again occur in quantity. 



The characteristic feature, then, of North European waters, considered as feeding 

 grounds for pelagic plant life, is the rich supply of river water, which in a mixed 

 state flows over the surface in thick layers in the vicinity of the coasts. This supply 

 is probably the most important source of the great productivity of the entire area of 

 sea, but it is just in the hottest season, when the surface layer is most markedly 

 isolated from the remaining water masses, that it offers the least favourable conditions 

 lor production. 



The waters about the east coast of Canada present numerous points of resemb- 

 lance in oceanographical resi)ects to what we flnd off Northwiestern Europe, and it may 

 therefore be interesting to try how far the working hypotheses above set forth may be 

 applicable to conditions on the coast of America. I was therefore happy to avail 

 myself of Pr. Hjort's kind offer to collect a series of water samples in and outside 



