174 MARINE ANIMALS 



zone an important source of oxygen is the metabolism of plants. In 

 the lower reaches of the lighted zone, plants consume as much oxygen 

 as they liberate, or more; the level where consumption balances pro- 

 duction is called the "compensation point." In the Gulf of Maine this 

 compensation point is mostly around a depth of 25 m. 19 In depths of 

 about 500 m. there is frequently greater oxygen deficiency than at 

 lower depths. Thus in the Atlantic at 500 m. in latitude 10°N. there 

 may be a deficiency of as much as 5-6 cc. per liter; deeper waters of 

 the Atlantic have a deficiency of only 25%. 



In the great oceanic depths, which receive their waters from high 

 latitudes, the oxygen content is only a little below normal. When there 

 is no such bottom current of cold water, carbon dioxide accumulates 

 at the bottom and there is a lack of oxygen. This occurs in some places 

 in the Mediterranean; Natterer found the bottom waters at 3700 m. 

 with only 2.64 cc. of oxygen per liter, at another place at 1005 m. 

 3.85 cc, and at a third at 1210 m., 4.60 cc; but even 2.64 cc. of oxygen 

 per liter will support much life. There are, however, descending cur- 

 rents of surface water in the Mediterranean, which have been observed 

 at isolated places (Aegean Sea, southern Adriatic, northern Balearic 

 Sea) and which extend to great depths, thus affording a limited oxygen 

 supply. 210 In the deep troughs of the Baltic the water stagnates with 

 a resultant lack of oxygen in the bottom waters and with attendant 

 reduction in the number of species at these places. Bornholm deeps, 

 Bay of Danzig, and the Gotland deeps 21 are such localities. 



In many places where there is stagnation of the water at the bottom 

 but a rich surface fauna, the want of oxygen becomes so great that 

 the hydrogen sulphide produced by the decomposition of animal bodies 

 is no longer oxidized and accumulates in the deeper water. The bottoms 

 of isolated bays, lagoons, and harbors accumulate a black ooze filled 

 with hydrogen sulphide. The Bay of Naples and the Gulf of Guinea 

 on the west African coast are examples. Bottom life becomes scarce in 

 such places, and only the most resistant forms persist, like the tubico- 

 lous worm, Spio fuliginosus, in the Bay of Naples. Where the accumu- 

 lation of hydrogen sulphide continues, all life is absent, with the excep- 

 tion of anaerobic forms. The depths of the Black Sea contain increasing 

 amounts of this poisonous gas below 180 m., up to 9.58 cc. in a liter. 

 In the Mogilnoje Sea (Murman coast) the hydrogen sulphide extends 

 to within 13 m. of the surface. 22 Hydrogen sulphide also accumulates 

 in summer at the bottom of certain of the Norwegian fiords and oyster 

 banks, salt-water basins of moderate depth (up to 30 m.) connected 

 with the sea by shallow channels, but in these it is removed by the 

 winter overturn of the waters. When storms or occasional advances of 



