168 CARNEGIE INSTITUTION OF WASHINGTON. 



temperature from a relatively short distance away, but where the normal 

 westerly drift prevailed. The higher oxygen content of this water was asso- 

 ciated with a lowered sahnity and hydrogen-ion concentration, as noticed by 

 Dr. A. G. Mayor in another place in this Year Book. As was observed by 

 McClendon for the water of the western Atlantic, the oxygen content was 

 slightly higher at midday than at night or in the early morning. A slight 

 difference was also found between bright sunny and cloudy days, dependent 

 no doubt on the differences in photosynthetic activity of the minute plank- 

 tonic algae. 



The oxygen content of the water in Pago Pago Harbor varied greatly at 

 different times and in different locations. At the end of Blacklock's dock, 

 near our laboratory, at the inner end of the harbor, the range was from 3.503 

 c.c. to 6.11 c.c. per liter. The higher content was coincident with the presence 

 in the water of immense numbers of dinoflagellates, to the photosynthetic 

 activity of which the increased oxygen content of the water may be attributed. 

 On the reefs, when the breakers were running at about a normal condition 

 for half tide, the water often contained as many as 15 c.c. of oxygen to the 

 liter, although oxygen saturation for sea-water of the same temperature and 

 salinity was never more than 5.5 c.c. per liter. 



The Growth-rate of Samoan Coral Reefs, by Alfred G. Mayor. 



In April 1917 reef corals from Pago Pago Harbor, American Samoa, were 

 measured, photographed, weighed, marked by numbered brass tags, and 

 replaced in favorable situations upon the reef-fiats either by tying them with 

 wire to iron stakes or embedding their bases in concrete. In July 1918, these 

 corals were again studied in order to ascertain their growth-rate, and as the 

 specimens selected were of average size it was hoped that an approximate 

 determination of the average growth-rate for each species might be deter- 

 mined. 



It was our object to calculate the weight of stony matter which coral heads 

 of average size add to the reef per annum. In order to do this the weight of 

 the living coral was ascertained both in 1917 and in 1918. Then in 1918 the 

 coral was killed and its animal substance macerated or dissolved in KOH, 

 after which the fresh water was largely abstracted by washing the skeleton 

 in 90 per cent ethyl alcohol ; the coral was then dried in the sun and weighed. 

 This showed that the weight of the dried stony substance in various species 

 of reef corals is on an average about 0.8 that of the same coral head when 

 alive. Various acroporas ranged from 0.07 to 0.9, while branched porites 

 were about 0.8 and large massive porites slightly above 0.9. 



Applying this correction, we find that on these Samoan reefs the gain per 

 annum avoirdupois weight of stony substance in various coral heads of aver- 

 age size appears to be as follows: Acropora, 19 ounces; Porites, 8 ounces; 

 PociUopora, 10 ounces; Pavona, 12 ounces; Psammocora, 3 ounces. Knowing 

 the number of coral heads of these genera upon a reef-flat, we have a means 

 for ascertaining the w^eight of limestone added to the reef each year by the 

 growth of coral upon its upper surface. 



By counting the numbers of coral heads and species of corals on squares 

 of 24 feet on the side (576 square feet), the squares being staked out from 50 

 to 100 feet apart from the shore to the seaward edge of the reef-flat, we are 

 led to the following conclusions: 



On the upper surface of the Aua reef-flat between Breaker Point, Pago 

 Pago Harbor, and the southern end of Aua village, the area of the reef being 

 2,550,000 square feet, the numbers of corals are approximately as follows: 



