90 Carbon-Dioxide Content of Sea-Water at Tortugas. 



relatively shallow, plant life appears to be the chief agency in causing 

 a daily variation in the CO2 content, and also a slight deficit in the 

 average content from the theoretical. Even here, however, the 

 departure from the equilibrium conditions required by the equation 

 of Fox, which is based entirely on the physical factors of temperature 

 and salinity, is not great. It is to be inferred that the slight deficit 

 in CO2 is accompanied by an excess of oxygen in the water, increased 

 activity in the animal-vegetable cycle, and increased precipitation 

 of calcium carbonate. 



The diurnal changes caused by photosynthesis are in the same 

 direction as those caused by the diurnal temperature changes, assum- 

 ing complete adjustment of the equilibrium between the carbon diox- 

 ide in the water and that in normal air. This equilibrium can scarcely 

 reach a full adjustment each day, however, even in the relatively 

 shallow water of the lagoon, but if it did it would not account for all 

 of the observed variation in carbon-dioxide content. Theoretically, 

 all surface sea-water should show^ slight diurnal changes in its gas 

 content, but the mixing of the water and experimental errors have 

 heretofore prevented the demonstration of such changes. 



I have attempted to discover whether Dole's figures^ on total 

 carbon dioxide at Tortugas reveal diurnal changes, but unfortunately 

 the data were gathered solely with reference to the tides and do not 

 give the time of day exactly, so that it is impossible to use them for 

 this purpose. As far as can be determined, there is no general cor- 

 relation with the tidal fiow^, but if the effects of photosynthesis were 

 considered, it is possible that some relationship to the tides could 

 be determined. However, as photosynthetic action extends to 

 considerable depths, it appears doubtful whether unaltered water 

 gains access to Tortugas by tidal action, the ke3''s being situated on the 

 western part of the Florida platform. There is thus no opportunity 

 for an up-welling of deep w^ater rich in CO2, as noted by Mayor in the 

 open water of the Pacific. The source of the water in the flowing 

 tides should be determined, if possible, at the time further samples are 

 taken to test these points. Wood-Jones has showm how markedly 

 the whole development of the Cocos-Keeling Islands is affected by 

 the prevailing wdnds and currents of the open Indian Ocean. 



The average diurnal variation in CO2 found around Loggerhead 

 Key was about 4.3 per cent of the total CO2. There was greater 

 variation than this on some days, as much as 5.6 per cent. Moreover, 

 the absolute quantities of CO2 found varied from day to day, the 

 mean being 0.0895 gram per liter at 27.2° C. On one rainy, cloudy 

 day the CO2 found averaged 0.091 gram per liter, w^hereas on a fine 

 day it averaged 0.0885. This is as w^ould be expected with regard 

 to the photosynthetic effect of sunlight, but the data are still too few 



1 Carnegie Inst. Wash. Publication No. 182, p. 73 (1914). 



