37 



Equatorial Pacific bottom water. Trawls taken in the region marked by 

 the 1 and 2 ml L. isopleths in fig. 9 in the southern Gulf were exceedingly 

 rich, and roughly of the same composition as those taken in slope depths 

 elsewhere along the coast of Middle America. However, fewer numbers 

 and kinds of animals were taken in the central Gulf basins which have 

 lower oxygen values. 



The oxygen concentrations on the continental shelf portions of the 

 Gulf of California are not quite so important in influencing the faunal 

 assemblages, as they are at least over 1 to 2 ml/L. As shown in fig. 10, 

 there are areas along the shelf where upwelling does occur, and oxygen 

 may occasionally be depleted fairly close to shore. This is especially true 

 north of Mazatlan, where the oxygen-minimum zone on the bottom comes 

 quite close to shore despite the width of the shelf. This condition seems to 

 influence the faunal assemblages which change considerably north of 

 Mazatlan in depths of from about 65 to 120 meters (36 to 65 fms.). Since 

 the general character of the sediments also changes, it cannot definitely 

 be stated that oxygen alone is the controlling factor. 



Bottom Water Temperature Characteristics. 



Surface water temperatures to a depth of 400 meters have been discussed 

 by RoDEN (1958) and Roden and Groves (1959). Using Roden and 

 Groves' data, plus unpublished values obtained from various recent 

 Scripps hydrographic cruises in the Gulf region, it was possible to con- 

 struct a bottom isotherm chart of water temperatures for the Gulf, ex- 

 clusive of shallow inshore areas (fig. 11). As can be seen from this figure, 

 there is an orderly progression from shore to the deepest portions of the 

 Gulf of from more than 14°C. to 2°C. in the deep southern sections. From 

 between San Lorenzo and Tiburon Islands, south to Puerto Vallarta, the 

 edge of the continental shelf is fairly well marked by bottom temperatures 

 of between 14' and 10° C. From shelf-edge to the bottom of the central 

 basins, temperatures gradually change to about 4°C. However, the south- 

 ernmost basin from Los Mochis south, and the upper end of the Middle 

 American Trench area have bottom water temperatures of 2°C. or less, 

 which are characteristic of the bottom water of the equatorial Pacific. 



One exception to the rule of decreasing temperature with descending 

 depth is the deep channel between Angel de la Guarda and San Lorenzo 

 Islands and Baja California, and the moderately deep Tiburon Basin. 

 Here, as was demonstrated in the case of bottom oxygen values, temper- 

 atures are nearly uniform from surface to over 1500 meters, resulting from 

 the intense tidal mixing in this channel and basin. For instance, in Roden 



