On Changes in the Sea and Their Relation to Organisms. 233 



tension changes very slowly and may be practically the same when it 

 reaches the surface the second time as when it left the surface the first 

 time. Palitzsch's data are shown in table 8. 



It is not intended to create the impression that the air regulates the 

 CO 2 tension of the sea. There is 20 to 30 times as much total CO 2 in 

 the sea as in the air. If there were no C0 2 in the air and this condition 

 could be maintained by some external agent, all of the C0 2 in the sea 

 would pass into the air, thus reducing the carbonates and bicarbonates 

 to hydrates. In other words, the total C0 2 of the sea is available for 

 regulating the CO 2 of the air. From the available data, however, it 

 appears that the partial pressure of C0 2 in the air at sea-level is less 

 than the average C0 2 tension of the sea. The reason for this anomaly 

 is as follows: Part of the carbon fixed by photosynthesis at depths of 

 less than 300 meters gravitates to greater depths and is oxidized, thus 

 increasing the C0 2 content of deep water. Some of this deep water is 

 constantly being carried to the surface by vertical currents, but on 

 reaching the light the CO 2 is reduced again by photosynthesis and the 

 original state of affairs is regained. Thus the sunlight interposes a 

 barrier between the air and the main body of sea-water, so that the air 

 contains less CO 2 than it would if the sea were more rapidly stirred. 



Thus photosynthesis has an effect on the CO 2 of the air that is not 

 entirely counteracted by the respiration of animals, because animals 

 live not only at the surface but at all depths. The amount of photo- 

 synthesis must affect the CO 2 content of the air, but so many factors 

 affect photosynthesis that probably no one factor is the limiting factor 

 for the whole surface of the sea. Photosynthesis varies directly with 

 both CO 2 tension and illumination, and either of these might become 

 a limiting factor under certain conditions. There is an optimum pH 

 range for plants, and since the temperature of the sea-surface as a 

 whole is dynamically constant, there might be constructed an optimum 

 C0 2 -tension map of the sea. But the range of pH compatible with the 

 life of seaweed is rather broad and may be different for different species 

 and has not been worked out for the whole life-history of a single 

 species, and besides, there may be some other limiting factor. It 

 seems probable that the limiting factors are different in cold and warm 

 seas. Photosynthesis is more than doubled by a rise of 10 in tempera- 

 ture, but the number of grams of plant tissue per square meter of sea 

 surface is less in warm seas than in cold seas. The limiting factor in 

 very cold seas may be temperature. The limiting factor at Tortugas 

 may be fixed nitrogen, since I found less than 0.02 mg. of fixed nitrogen 

 per liter of sea-water. There is a more abundant growth of eel-grass 

 on the west side of Loggerhead Key than on the east, and sewage and 

 garbage contaminates the water of the west side. On the other hand, 

 the water of the west side is less agitated by the wind. At the Marque- 

 sas, fixed nitrogen is washed into the sea from decaying organic matter 



