366 



DEPTHS OF THE OCEAN 



Plankton less 

 abundant in 

 the open sea 

 than in coastal 

 waters. 



species is very considerable, yet the total quantity of individuals 

 is surprisingly small compared with what we might find, for 

 instance, off the coasts of Europe. In the Skagerrack one 

 often gets plant- cells in tens of thousands or even hundreds 

 of thousands in every litre of sea-water from the upper layer, 

 and, what is more, they are much larger and more nutritive 

 than the stunted forms which make up the bulk of this ocean 

 plankton. 



It cannot be denied that our investigations are as yet too 

 incomplete to justify us in framing laws for plant production in 

 the ocean. Still the great expeditions which have made 

 researches in the open sea have given us a general conception 

 of the conditions prevailing over wide stretches of water at 

 certain seasons ; on the other hand, careful investigations of the 

 variations in the plankton throughout the year have been 

 carried out at a number of coast stations, while our international 

 researches have resulted in a great deal of material being 

 collected at all seasons from the North Sea and adjoining 

 areas. Though these investigations have not all been devoted 

 to studying quantity, they have nevertheless enabled us to 

 form some idea of the annual variations. 



One thing at any rate we may learn even from this in- 

 complete material. The development of the plankton is much 

 more irregular than it would be if merely such simple factors as 

 warmth and light controlled production. It is not in the 

 warmest waters that the greatest amount of organic substance 

 is to be found. On the contrary we get larger masses of plants 

 in temperate seas than we have ever yet come across in 

 tropical or subtropical areas,^ at any rate so far as the open 

 ocean is concerned. Even when we come as far north as 

 the coast of Norway we find that it is not in the hottest months 

 of summer that the plankton attains its maximum, but in the 

 early part of the spring or the end of autumn. Now it is 

 certainly true that the quantity of vegetable matter present at 

 any given moment is no direct measure of production. Ac- 

 cording to the law of Van 't Hoff, metabolism always takes place 

 quicker ceteris paribus at a high temperature than at a low 

 temperature, and a plant-cell in the tropics may perhaps produce 

 more organic matter than a similar cell would do in the North 

 Sea in the same space of time. The small tropical plants may 



^ The "Challenger " met with diatoms in the Arafura Sea in as great abundance as in the 

 Antarctic regions, but neritic in character (see lists of species in Summary of Results, 

 Chall. Exp., pp. 515 and 733). 



