and vigorous reef growth only takes place within 90 

 feet of the surface. Possibly the need of water currents 

 to bring food and of wave action to provide oxygena- 

 tion are also involved in restricting vigorous reef growth 

 to these depths. In deep water, where wave action and 

 currents are less effective or absent, sediment may also 

 prevent coral growth. 



The adverse effect of sediment is most pronounced in 

 sheltered water. In shallow water near shore there is 

 also likely to be excessive evaporation leading to unduly 

 high salinity, or land drainage of fresh water v/hich 

 results in dangerously low salinity. These factors, to- 

 gether with the need for wave action account for the 

 restriction of vigorous reefs to the windward sides of 

 islands and for the greater growth of coral on the sea- 

 ward side of the reef where it is completely exposed to 

 heavy seas. 



i9 



