GEOGRAPHIC DIVISIONS OF THE PELAGIAL 



285 



the needle fish Nerophis ophiodon propagate in the Baltic. Besides 

 their decrease in number of species the Baltic animals decrease in 

 size, the farther east they live. 



The Baltic Sea is in general a shallow sea, which increases in 

 depth toward the east; the average depth amounts only to about 67 m. 

 But in a few limited places (see Figs. 76 and 77) the bottom drops 

 off to rather great depths; the most important of these depressions, 

 located south of Stockholm, has a depth of 427 m. In these depressions, 

 the salt content seldom goes below 12% , but in spite of this, they 

 contain only an impoverished fauna represented by few individuals 

 of small size. At depths of 80 m. only 17 species of bottom forms are 

 found, 6 species of worms, 6 of crustaceans, 4 of bivalves and 1 of 

 bryozoans, and indeed only those that occur also in shallow water; 



Fig. 77.— Longitudinal section of the Baltic Sea, from Skagen to Lulea, 

 with isohalines, in %e, according to Ekman; depths in meters at the left. 

 After Brandt. 



at more than 150-m. depth 2 species of worms, Harmothoe sarsi and 

 Priapulus caudatus, occur, and at greater depths than 230 m. all 

 animal life is absent. The reason for this condition lies in the low 

 oxygen content and the accumulation of carbon dioxide. The water at 

 0-60 m. depth is about one-third saturated with oxygen, in the deeps 

 of East Gotland at a depth of 100 m. only one-fifth, and at 200 m. 

 only one-fifteenth. On the other hand, while only 32 cc. of carbon 

 dioxide per liter is present at a depth of 0-60 m., 36 cc. is present at 

 100 m. and 41 cc. at 200 m. Rarely, on the occasion of continuous 

 easterly storms, does the surface water recede in the east and an under- 

 current of water rich in oxygen and salt advances farther toward the 

 east, bringing fresh supplies to the depressions. After such storms 

 richer animal life becomes possible in these local deeps; thus, 3 species 

 of maldanids (Annelida) were taken in the Danzig deeps in February, 

 1904, while only 1 species of annelid normally occurs in the western 

 Baltic Sea. ,i:{ 



Geologically speaking, the Baltic is still young. It has retained a 



