ANIMALS AND THEIR ENVIRONMENT 



301 



strong mechanical disturbance may be caused by waves and tides, and 

 currents such as the Gulf Stream and the Labrador Current create 

 special conditions of temperature and distribution. Temperature of 

 surface waters varies little during the year in the open oceans (from 7 

 to 12°C. in one situation), but considerably in stagnant seas and bays 

 (3 to 18° in the Baltic Sea). Deep water is always cold. Light of 

 sufficient intensity to aid plant growth penetrates the first 30 to 100 

 meters of water but is detectable at 200 to 600 meters. Concentration 

 of salt is increased ° by evaporation in the tropics, and diminished in 

 summer in polar areas by the melting of ice. Ocean waters are also 

 diluted by rivers, but these streams are more important for the materials 



800 



Fig. 265. — Vertical section through portion of ocean near the shore. Bottom fauna includes 

 animals which are able to move briefly but must periodically come to rest. 



they bring in from the land. The Atlantic and Arctic Oceans receive 

 by far the greatest contribution from rivers, while the southeastern 

 Pacific receives the least. The solubility of oxygen in marine waters is 

 about 20 per cent less than in fresh water, and cold water (either salt 

 or fresh) dissolves more than warmer water does. Deep waters, which 

 are cold and which are replenished by a circulation from the polar 

 regions, therefore have a good oxygen supply. 



Ocean Bottom. — A great majority of marine animals live on, in, or 

 near the bottom (Fig. 265). Near the shores the bottom is lighted; 

 here it is that life is most abundant, and all groups of marine animals 

 are found in this relatively shallow water (200 meters or less). The 

 stock of animals in these coastal waters is generally regarded as having 

 produced all the water-breathing aquatic forms, whether marine or 

 fresh-water. Below the low-tide level there is abundant plant life if the 



