24 THE BIOLOGY OF THE SEA-SHORE 



" in a shallow bay like Ballynakill Harbour, a large proportion 

 of the water is renewed from outside at each tide. . . . Bally- 

 nakill Harbour is partially surrounded by high hills and 

 several rivers flow into it. After heavy rain the fresh water 

 streams down on to the surface of the bay and in calm 

 weather forms a distinct layer of low salinity on the surface." 

 Further, it was found that the difference between the 

 salinity at the surface, and at a depth of one fathom varied 

 directly as the rainfall, and inversely as the amount of wind ; 

 the lowest surface salinities were found on calm days after 

 rain. The density of the water of rock pools may oscillate 

 from a little above i to about 1*220. Further, the solu- 

 bility of oxygen diminishes with increase of salt and still 

 more with increase of temperature, with consequent effects 

 on animal life. Nitrogen exchange in the sea can be modified 

 by certain bacteria which can fix free nitrogenous compounds 

 (nitrates, nitrites, and ammonia salts). In the warmer 

 seas in particular bacteria are of importance, and in the region 

 of the Great Bahama Banks chalk mud is precipitated by 

 B. calcis. Such bacteria swarm in the shallow coastal waters 

 and have the power of secreting and precipitating lime, and 

 at the same time of converting nitrogen combinations 

 (Drew, 191 3). Loeb also dwells upon the importance 

 of the bicarbonates as regulators in the development of 

 marine organisms, by keeping neutral the water in which 

 these animals live. 



Vegetation in Relation to the Alkalinity of Sea- 

 water. — The effect of vegetation in promoting alkalinity 

 and the importance of this to the animal life is discussed 

 by Shelf ord (1916), who discovered a surprising difference 

 (which is constant) between the waters of the two sides of 

 Friday Harbour, San Juan Island, Puget Sound. The 

 water on the south side is always more acid than that on 

 the north side, the latter being usually neutral or slightly 

 alkaline and at times, especially at the end of a sunny day, 

 decidedly alkaline. Water pumped from the acid side into 

 tanks has never been satisfactory for aquarium purposes, and 

 animals do not develop well in acid water. The oxygen 



