NO. 10 DAWSON : MARINE ALGAE, GULF OF CALIFORNIA 197 



The oceanographical factors affecting aeration, mixing, and diffusion 

 are highly significant in regard to the concentration of oxygen in surface 

 and subsurface waters. In shore water the degree of wave action is a fac- 

 tor of particular importance. In the narrow Gulf waters general surface 

 agitation is relatively low and the absence of strong wave action is in 

 marked contrast to the situation on the outer Pacific coast. In wave-beaten 

 water the oxygen concentration may reach high supersaturation values, 

 which are from all indications very favorable to the development of 

 algae. In the Gulf of California the wave action is much reduced or es- 

 sentially absent, and, though no data are available on the oxygen content 

 of Gulf shore water, the relative paucity of vegetation as compared with 

 the outer California coasts may in part be accounted for by this more lim- 

 ited oxygen supply. 



It will be noted from Table II that in the region of the island mass 

 toward the head of the Gulf there are definitely higher oxygen concentra- 

 tions than anywhere southward in the Gulf. The occurrence of narrow 

 channels between the islands causes rapid movement of tremendous vol- 

 umes of tidal water and mixing of rougher, more broken surface waters. 

 This undoubtedly provides for some additional aeration. Of greater im- 

 portance is the oxygen supply already present in the water at the north 

 which is involved in the movement. A study of the soundings in the upper 

 Gulf reveals that the region north of the island mass is essentially shallow. 

 Very little of the water is deeper than 300 meters, and a great part of it 

 is under 100. The Scripps data show that southern Gulf waters over 

 depths of 1,500 meters or more may be as low in oxygen at 300 meters as 

 .15 ml. per liter. In the northern region the lowest figure given for 300 

 meters is 1.10 ml. per liter. The lower concentrations in surface watei-s of 

 southern parts of the Gulf can partly be explained, therefore, by upwell- 

 ings of deep waters very low in oxygen. According to this interpretation 

 the distinctly higher concentrations in all of the northern waters are at- 

 tributable to the absence of any such deep waters, and high residual con- 

 centrations at all of the comparatively slight depths. 



A comparison of the data on oxygen values with the relative luxuri- 

 ance of vegetation shows a veiy significant correlation. The greatest con- 

 centrations of algae, as to both kinds and quantity, are to be found on the 

 reefs bathed by the highly oxygenated waters of this northern region, par- 

 ticularly those reefs bordering the channels between the islands and lying 

 in the paths of the ever-moving tidal currents. 



On the southwestern side of the Gulf, at Agua Verde Bay, it was 

 notable that on rocky reefs ideally suited for the growth of algae as far as 



