Tatle 6. — Measurements of chlorophyll a made 

 at three depths in the middle and on 

 either side of front 4.; units in mg./m.-' 



[w=warm side; m = middle; c = cool side] 



SUMMARY 



The most marked oceanographic feature of 

 the entrance to the Gulf of California is a 

 frontal system that extends from Cape San 

 Lucas into the Pacific Oceam. This system is 

 formed by three kinds of water: (1) Gulf of 

 California water, warm and high in salinity; 

 (2) California Current system water, cool and 

 low in salinity; (3) equatorial Pacific water, 

 warm and intermediate in salinity. The best 

 studied part of the system, and most stable, 

 is the area off Cape San Lucas. 



The frontal system, at any rate the system 

 off the Cape, is strongest in the spring when 

 the California Current water in it consists 

 largely of cold, upwelled water; the system 

 persists throughout the summer and is weakest 

 in the fall and winter. 



The sharp front near Cape Sam Lucas is 

 formed by warm, high-salinity Gulf water and 

 cool, low-salinity California Current water. 

 Out to sea, the equatorial Pacific water (warm, 

 intermediate salinity) and California Current 

 water form a generally weaker continuation. 

 As a result of mixing and advection of cool, 

 upwelled water from the north, parts of the 

 system are formed of waters not readily as- 

 signable to particular types. 



The interface between the waters forming 

 the system is complicated; near Cape San 

 Lucas it is Z -shaped in the vertical plane, the 

 Gulf water penetrating the California Current 

 water at depths around 30-50 m. 



It is generally believed that plankton is ag- 

 gregated by a convergent front, but our data, 

 though supporting -this view, are not conclu;^ 

 sive. — '"^ 



Chlorophyll a values at the three depths are 

 given in table 6. According to these values 

 the highest concentration of chlorophyll was 

 On the cool side at 40 m. The values for the 

 warm side and the middle are quite similar 

 and both are nnuch lower then those from the 

 cool side, which suggests that the sample in 

 the middle may have been much more on the 



warm side than we supposed. The cool side 

 was much richer in chlorophyll a, and there- 

 fore in phytoplankton. 



The staindardized volume of the zooplankton 

 catch on the cool side was somewhat less than 

 that on the warm side. This does not reflect 

 the gross distribution of chlorophyll a, nor do 

 the distributions of predominant organisms. 

 The composition of the plankton evidently 

 changed in the 13-day interval. A summary of 

 the catches of oblique hauls at front 4 is given 

 in table 5. 



The information in table 5 shows that the 

 composition of the plankton changed within the 

 fortnight. Copepods became much more abun- 

 dant on the cool side, and so did the siphono- 

 phores. Chaetognaths and euphausiids became 

 less abundant on the cool side. Larvaceans 

 persisted exclusively on the cool side. Hetero- 

 pods, which were exclusively on the cool side 

 at front 2, became exclusive to the warm side; 

 ostracods were quite the reverse of the hetero- 

 pods. Salps, formerly on the cool side only, 

 were absent altogether from front 4 (judging 

 from the aliquot). 



We made no attempt to distinguish species, 

 though this nnay actually elucidate the changes 

 better; the data are sufficient to show that the 

 biota at front 4 differed significantly from 

 that at front 2. I presume that this is conse- 

 quent upon the oceanographic changes that oc- 

 curred. 



X 



RECOMMENDATIONS 



With regard to future work, firstly, much 

 more needs to be learned about the area in 

 general and the seasonal variability of the 

 Cape San Lucas frontal system; secondly, 

 new applications of known methods (e.g., 

 multiple -ship operations amd dye-markers to 

 study convergence, divergence, and other 

 frontal movements) should be tried; thirdly, 

 the shortcomings of commonly used methods, 

 as well as the variations used in the present 

 study, indicate the need for new and improved 

 gear, such as the thermistor chain (La Fond, 

 196lb), infrared radiometers, in situ salinity, 

 density, and oxygen recorders which are be- 

 coming available. Attention also should be given 

 to the distribution, in the neighborhood of the 

 system, of organisms likely to be indicators 

 of the different types of water forming the 

 system. 



ACKNOWLEDGEMENTS 



The following oceamographers provided gen- 

 erous help and many useful suggestions, in 

 particular, Joseph L. Reid, Jr. of the Scripps 

 Institution of Oceanography; J.F.T. Saur of the 

 Bureau of Connmercial Fisheries at Stanford 

 University; Klaus Wyrtki of the Scripps Insti- 

 tution and its Tuna Research Prograim; and 

 E. O. Sette, Robert S. Arthur, and Milner B. 



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