ECOLOGY OF THE MURRAY ISLAND CORAL REEF. 



39 



EXPERIMENTS UPON DILUTION OF SEA-WATER. 



Another factor which might be supposed to afifect the welfare of the 

 inshore corals of the southeast reef is dilution of the sea-water during the 

 showers of the rainy season, from November to May.^ Table ii shows the 

 results obtained by placing corals in large aquaria in sea-water diluted 50 

 per cent by mixing it with an equal volume of rain-water. 



Table ii. — Results obtained by placing corals in jo per cent sea-water, made by 

 diluting sea-water with an equal volume of fresh (rain) water. 



Thus the common reef-flat corals can survive for 4^ hours in 50 per cent 

 sea-water, but only about half of them can survive 11 1 hours of this treat- 

 ment, and only three, Porites murrayensis, Porites mayeri, and Cceloseris 

 mayeri, can withstand 24 hours of this diluted sea-water.^ In Samoa I found 

 that openings in reefs opposite stream-mouths are places where corals are 

 killed by silt and dilution and thus could never obtain a foothold in such 

 places. The most resistant coral in this respect is the massive Porites. In 

 19 1 5 about 30 inches fell upon Maer Island from November to May and less 

 than 3 inches from May to the end of October. 



Samples of the water from definite stations over the reef-flat and from 

 the open sea were taken at various times of the day and states of tide and 

 these were titrated with one-tenth molecular silver-nitrate solution, using 

 potassium chromate as an indicator. These experiments showed that there 

 is no appreciable concentration of the water, even at low tide, over the shore- 

 ward parts of the reef-flat due to evaporation, nor was the water at any 

 place diluted by an influx of spring water from the shore, due probably to our 

 visit being near the end of the long dry season which begins in May. The 

 concentration of the waters over the reef-flat was within the limits of error 

 of experiment identical with that of the ocean close to but just outside of 

 the reef, the average of 11 tests giving 36.1 per cent of salinity for both the 

 outer ocean and the reef-flat. 



'Mr. John Stewart Bruce states in letters that the rainfall on Maer Island from January i to April 30, 

 1914, inclusive, was 30.17 inches, and from May I to June 30, 1914, it was 3.51 inches. In 1915 the rainfall 

 was 32.66 inches, of which 24.65 inches fell between January I and April 30; the remaining 8 months having 

 only 8.01 inches. 



*In the summer of 1914, Dr. Vaughan made similar experiments upon 17 species of corals at Tortugas, 

 Florida, and our conclusions are in substantial agreement. See Vaughan's report in Carnegie Inst. Wash. Year 

 Book No. 13, p. 224. 



