314 MARINE BIOTIC COMMUNITIES 



The great oceanic or pelagic community is similar in some respects 

 to that of large lakes like the Great Lakes of North America. How- 

 ever, it is infinitely richer in diversity of size and form of animals 

 and in taxonomic groups represented. 



Hydroclimate. The conditions of existence in the sea differ from 

 those of fresh water in certain important respects. The occurrence of 

 tides is most important in producing circulation; otherwise they have 

 little or no effect on the pelagic or on bottom communities outside 

 the intertidal area. The most important difference in climatic condi- 

 tions lies in the presence of a large amount of salt, principally sodium 

 chloride, and the occurrence of sulphur compounds in the form of 

 hydrogen sulphide, sulphurous acid, and colloidal sulphur, which often 

 have important relations to aquatic life. The salt present increases 

 the density of the water, and hence variations in salt content are 

 credited with playing a very important role in the climate of the sea. 



The hydroclimatic factors (Wasmund, 1934) are greatly modified 

 by reactions. Greater density of life in the sea produces far greater 

 reaction on the habitat in the way of light reduction, chemical changes, 

 etc., than in lakes. These bioclimatic factors are so important and 

 generally present as to be essentially a part of the hydroclimate itself. 

 The great depths of tlie ocean and the lack of rooted or attached plant 

 life except at irregular intervals along the shore also produce differ- 

 ences between fresh and salt water (cf. Harvey, 1927; Knudsen, 1922). 



PELAGIC COMMUNITIES 



The communities of the sea have been so incompletely studied that 

 it is difficult to outline their arrangement in any adequate manner. 

 On account of their outstanding peculiarities, it seems best to begin 

 wnth pelagic communities. Only those of the North Pacific are fa- 

 miliar to the writer, while most of the work has been done on the North 

 Atlantic. The discussion of marine pelagic communities from a biotic 

 viewpoint has not often been attained, but Murray and Hjort (1912) 

 and Bigelow (1924) have made progress in this direction (see Gran, 

 1912, 1931; Allen, 1921-1932). 



Pelagic Communities of the Enclosed Waters of the 

 North Pacific 



The pelagic communities include the plankton or floating organisms 

 taken together with the swimming animals or nekton. The separa- 

 tion into those two groups as a basis for investigation has led to an 



