RESTRICTED 



12 



than the lagoon water proper. The reef v/ater therefore overlies 

 the lagoon water initially, producing teidperature differences 

 between siirface and bottom that raay be as uuch as l^r but laore 

 typically are a few tenths of a degree. This is illustrated in 

 Figure I5 , a teiuperature profile obtained by a series of bathy- 

 thermograph lowerings along a line extending westward from bi- 

 kini Island, Sii;iilar variations have been noted in salinity, 

 phosphate, and oxygen. Differences in oxygen are particularly 

 marked, since the reef water is highly oxygenated by surf effects. 



Figures 16 and 17 shov; diurnal variations in tenperature 

 and chemical constituents at tv/o stations. Variations at the 

 surface are only slightly greater than in deep water, which is 

 largely due to rapid vertical diffusion of the products of sur- 

 face heat exchange. The amount of diffusion is of course de- 

 pendent on wind velocity, i'igure 18 shows the maximum observed 

 difference between surface and bottom temperature plotted against 

 wind velocity. The cxirve is extrapolated to zero velocity on 

 the assumptions that (a) the total heat increment is about 300 

 g. cal, per cm-^ per day and (b) the vertical temperature curve 

 is proportional to a normal curve for total energy absorption 

 in moderately clear water, 



3.3 Interchange between lagoon and oceanic water 



The current measurements described in section 3,1 can be 

 used to estimate the volume of water moving into or out of the 

 lagoon at any given tirae, A certain amount of error is unavoid- 

 able in this estimate , but there are tv/o ways to check the gen- 

 eral validity of the results: (a) D\iring a complete tidal cycle 

 the total inflow must equal the total outflow, (b) During any 

 shorter period of time the difference between inflow and outflow 

 must equal the change in the volume of the lagoon as determined 

 by the change in water level. 



Application of current measurements to the problem was 

 carried out as follows: 



(a) The cross- sectional area of current in each channel and 

 pass was determined by measurement of the chart. Cross-sectional 

 areas along the reefs were estimated by measuring the length of 

 the reef and ass\iming the approximately correct depths at high 

 and low tide of I30 and 30 cm. respectively, and an average depth 

 of 80 on. during ebb and flood, 



(b) The average velocity of flow was determined across each 

 reef, channel, and pass at high tide, ebb, low tide, and flood. 



