CH. X] THE COXDITIOXS OF LIFE IN THE SEA 285 



But we have also seen that the marine animals generally feed 

 on other animals which are smaller than themselves, and these 

 latter upon others which are smaller still, so that the ultimate 

 organised food in the sea consists of the organisms of the plankton. 

 Further it is only the protophyta of the latter that are able to 

 utilise the inorganic salts of the sea water as food ; converting 

 these into organic material with the assistance of the solar energy. 

 The abundance of life in the sea then depends on the abundance 

 of the plankton ; and the abundance of the latter is dependent on 

 the amount of the indispensable food-salts which are Jit the dis- 

 posal of the protophyta. One or more of these substances must 

 be present in minimal proportion, and so must rule the produc- 

 tion of organised material in the sea. What is this substance ? 



It is probable that the abundance of nitrogen compounds in 

 the sea determines the production. Most investigations that have 

 been made shew that there is a relation between the density of 

 the plankton and the proportion of nitrogen compounds. Natterer's 

 estimations shew that the water of the warmer seas is relatively 

 poor in ammonia, and the quantitative plankton investigations of 

 Lohmann, Schlitt, and others indicate that these seas are also 

 poor in plankton. Raben's analyses shew that the water of the 

 North Sea and Baltic contains rather more nitrogen than the 

 water of the Mediterranean area, and we also find that the 

 water of these seas is relatively rich in plankton. Apstein^ made 

 numerous estimations of density of microscopic life in the fresh 

 water lakes of Holstein, and Brandt- shewed that those lakes 

 which possessed the richest plankton also had the greatest amount 

 of inorganic nitrogen. Littoral sea water into which flow rivers 

 rich in the nitrogenous drainage of the land, has, as a rule, 

 a richer benthic fauna than the sea further from the shore. 

 Nowhere do mussels grow so luxuriantly as in estuarine waters 

 rich in sewage matters. Fresh water ponds in which carp are 

 cultivated produce varying "crops," but whether the conditions 

 are good or bad the deliberate addition of nitrogenous drainage 

 makes them better. 



It is rather difficult to determine which of the three food- 



1 Sihswasser Plankton, Kiel and Leipzig, 1896. 

 - "Stoffwechsel im Meeres." 



