116 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL.3 



northern island group may be controlled by two principal sets of condi- 

 tions: (1) those of substratum, and (2) those of water movement. 



It was observed throughout the upper two thirds of the Gulf, as in 

 other marine areas, that shallow, sandy, or muddy ba^^s and shores are 

 relatively poor in algae as compared to rocky shores, which provide firm 

 bases for holdfast development. Regions of abundant sedimentation are 

 unsatisfactory, as shown by the decreasing richness of development both 

 toward the mouth of the Colorado River and along the coastal plains at 

 Bahia Gonzaga and Bahia Tepoca, where alluvial sedimentation occurs 

 during the rainy season. (The former yielded only 34 species on shore 

 and 8 by dredge, the latter 22 species on shore and 22 by dredge.) 



As to the effect of water movement, one may best note the way in 

 which the northern island group blocks the Gulf to general mass move- 

 ment, leaving only narrow channels for the passage of the tremendous 

 volume of tidal water. The daily rise and fall of tides in the upper Gulf 

 is spectacular, as much as 32 feet in some places. The continuous move- 

 ment of water through the channels is very rapid, and thorough aeration 

 of the relatively sediment-free water apparently takes place. This greater 

 purity of the flowing water seems favorable to algal growth, and we find, 

 as compared to the above figures, the striking totals of some 80 species 

 collected at Turner's Island and 130 from Puerto Refugio. On the other 

 hand, in the quiet water of the sandy or mucky bays and harbor lagoons 

 around Guaymas, where renewal is minor and sediment accumulation 

 continuous, the number of species is low. Across the Gulf from Guaymas 

 and to the south, little could be determined except that the winter flora is 

 very inconspicuous and the summer flora, according to accounts by the 

 natives, relatively abundant. 



In the perennially warm, tropical waters around La Paz, the algal 

 flora is of a very different type from that found farther up the Gulf, 

 where the water is subject to periodic heating and cooling of a much 

 greater degree. The tropical algae are smallest in quantity and species; 

 the rocky shores in most places are almost completely bare of plant life. 

 The tropical water of the Gulf is not calcareous in nature, such as is that 

 of the Caribbean region, and, though corals are plentiful in the lagoons, 

 they are of an individual rather than of a reef-building nature. Thus, 

 under conditions which in other such warm localities might yield an 

 abundance of calcareous green algae, almost none are found. Only a few 

 emaciated specimens of Halimeda turned up in the dredge. 



In order to secure midsummer data on the algal situation in the Gulf 

 to compare with the midwinter data already on hand, the author decided 



