196 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 3 



With this in mind, careful attention was given to collection localities to 

 which return trips at other seasons might be made feasible. After the re- 

 turn of the Hancock Expedition it was found that one of the richest algal 

 stations visited during the winter season could be reached from the Sonora 

 side of the Gulf. This was Turner's Island, a tiny, rocky island ofiF the 

 south end of Tiburon, within 25 miles. In direct line, of the fishing village 

 of Kino. A return visit to this island at the height of the warm-water 

 season of midsummer confirmed completely the suspicions regarding sea- 

 sonal alternation of the floras. 



In attempting to explain the general distribution of algal types and 

 floras in the Gulf, the author is fully aware that in the present account 

 only suggestions can be made. Ecologic data are still exceedingly scanty. 

 Even so easily recorded a fact as temperature is available for only a small 

 part of the year, and that almost entirely from measurements in the open 

 sea. Fortunately, Sverdrup and the Scripps Institution staff have made 

 available a series of oxygen values showing the concentration of oxygen in 

 the sea water of the Gulf of California at 53 stations and at many depths. 



Unlike terrestrial plants, which live in a partly atmospheric partly 

 subterranean environment, the algae, in their aquatic medium, are subject 

 to different primary environmental factors. Next to temperature, oxygen 

 is probably the most significant primary factor. In contrast with lane 

 plants, when marine algae carry on photosynthesis the CO2 removed from 

 sea water is immediately replaced by the dissociation of bicarbonates and 

 carbonic acid. This buffering action of sea water prevents any deficiency 

 in the CO2 content which might be limiting to photosynthesis. The oxy- 

 gen supply, therefore, except when photosynthesis is proceeding at an ade- 

 quate rate, becomes a critical factor, and its availability in the sea water 

 of different habitats becomes significant as a factor in the general devel- 

 opment and distribution of marine vegetation. 



From the oxygen data at hand (see p. 341) we find values in sur- 

 face waters ranging from 5 to 9 milliliters per liter from south to north 

 in the Gulf. These differences are attributable to a number of climatic 

 and oceanographical factors. 



The solubility of oxygen varies inversely with the temperature. Thus, 

 the warmer waters of the southern regions contain much less oxygen at 

 saturation than do the colder northern waters (in winter). The extreme 

 seasonal temperature differences at Turner's Island serve as an instance in 

 which the solubility of oxygen in 15° C. winter water is approximately 

 35 per cent greater than in the 31° summer water. 



