DEPARTMENT OF MARINE BIOLOGY. 169 



About 502,200 heads of Porites grow over 72 per cent of the area of the upper surface of 



the reef-flat and contribute (due to their growth) per annum 251,000 pounds of 



limestone. 

 347,500 Acropora distributed over 56 per cent of the reef-flat contribute annually 500,400 



pounds of Umestone. 

 57,600 heads of Pocillopora scattered over the entire reef-flat contribute 31,700 pounds of 



Umestone. 

 55,900 Psammocora distributed over 30 per cent of the reef -flat contribute each j^ear 11,000 



pounds of Umestone. 

 15,500 Pavona found over 35 per cent of the reef-flat area contribute each year about 



11,600 pounds of limestone. 



Thus the growth of these corals appears to add about 805,000 pounds of 

 lunestone each year to the upper surface of the reef-fiat, and as these genera 

 contribute 95 per cent of the entu'e coral heads of the reef-flat, it is probable 

 that about 847,000 pounds of hmestone are added to the coral reef each year 

 by the growth of the corals over its surface. 



There are, however, certain obvious losses which may be approximately 

 determined. Among these the wash of the breakers due to the prevaihng 

 trade-wind drives a current of about 40 feet per minute over the surface of 

 the reef -flat from Breaker Point northward to the jagged northern edge of 

 the reef-flat, over which the drifted limestone sand spills into deep water. 



In order to determine the annual loss of loose limestone sand lost to the 

 reef-flat by being drifted off its northern edge, 6 barrels (each 2 feet in 

 diameter) were weighted with volcanic rocks and sunk off the reef-flat close 

 to the northern edge, so that the water was about 1 foot over their open tops 

 at lowest tide. It was found that on an average each barrel caught 0.7 

 pound of limestone sand per day, the sand being dehydrated in alcohol and 

 dried in the sun before being weighed. Further tests showed that the barrels 

 caught on an average only 12 per cent of the sand which was carried in the 

 current over their open tops from the edge of the reef-flat. 



The northern edge of the reef -flat is about 1,200 feet vnde, but sand spills 

 over its edge along only about 800 feet of this length, the 400 outermost feet 

 of reef edge being subjected to the constant inward wash of the waves. Using 

 these data, it appears that about 100,000 pounds of sand are washed off from 

 the reef-flat by currents each year. 



There are 290,000 holothurians on the Aua reef-flat between Breaker Point 

 and the edge of the reef off Aua Viflage, and as the acidity of their gastric 

 cavities appears to be the same as that of the holothurians of Florida, each 

 one (according to experiments made at Tortugas in 1917) might be expected 

 to dissolve 10 pounds of sand per annum, or 2,900,000 pounds of sand may be 

 destroyed annually over this reef-flat by solution due to holothurians. 



Thus the corals appear to add annually about 840,000 pounds of limestone 

 to the reef, but 3,000,000 pounds, or nearly 4 times this weight of limestone, 

 appears to be removed annually by holothurians and by currents. Other fac- 

 tors, such as boring algse, moUusca, and fishes (the efficacy of which we have 

 not been able to calculate), add stifl more to the destruction of limestone, 

 although their influence is in some measure offset by the gi'owth of nulhpore 

 algse and Hthothamnion. Alcyonaria, although a decided factor in some reefs, 

 are so rare as to be negligible over the Aua reef-flat. 



In most respects this Aua reef is a tj^pical average Pacific fringing-reef , 

 and our data appear to explain the disappearance of the hthothamnion ridge 

 over the shoreward parts of the reef -flat as the reef grows outward. Moreover, 

 the reef-flat appears to be deepening at present, although the average depth 

 of water over it at low spring tide is less than 12 inches. This does not mean 



