NOTES ON ZOOPLANKTON 



By 



MARTIN W. JOHNSON 



Together with other oceanographic observa- 

 tions taken during the 1938 regular cruises 

 (Cruises II to VI of the "E. W. Scripps" off the 

 southern California coast), there were included 

 also net collections for the animal plankton. 

 These collections were taken with a regular Nan- 

 sen closing net 3 meters long with an opening of 

 70 centimeters. The bolting cloth used in con- 

 struction of the net was Nos. 000, 0, and 8. 



It was originally planned to take vertical 

 net hauls from 500 to 200 meters and from 200 to 

 meters at each station. However, as indicated 

 below, this plan was carried out at only three 

 stations of Cruise II. Because of the amount of 

 time needed for the joint observations, it was 

 necessary to eliminate the lower-depth hauls. 

 Though collections from depths greater than 200 

 meters are highly desirable, it was deemed that 

 most of the zoOplankton population would be 

 sampled in the 200- to 0-meter hauls. Study 

 of the bathypelagic population is, therefore, de- 

 ferred for the time being. 



A summary follows of the number of stations 

 occupied for each cruise at which net collections 

 were made. Occasionally the wind was too strong 

 to operate the net at all of the scheduled sta- 

 tions, but for each cruise the stations v;ere usu- 

 ally well distributed over the area included. 



Cruise II, April 8-12... 3 stations: 500-200 m. 



and 200-0 m. 



4 stations: 200-0 m. 

 (This cruise was not completed owing to storms.) 



Cruise 111, June 7-16... 25 stations: 200-0 m. 

 Cruise IV, Aug. 16-25... 34 stations: 200-0 m. 

 Cruise V, Oct. 26-Nov. 5... 28 stations: 200-0 m. 

 Cruise VI, Dec. 9-18 27 stations: 200-0 m. 



In addition to the above regular collections, 

 some twenty surface hauls were made with small 

 nets for collection of material for life-history 

 studies. 



Up to this time the collections have been 

 only partially analyzed, and it is possible here 

 to indicate only a few of the outstanding char- 

 acteristics of the population as shown thus far. 



Volumes. - In general, it may be said that the 

 total volumes (when free from diatoms) of plankton 

 caught at each station are characterized by being 

 only moderately rich and rather uniform, especi- 

 ally for adjacent stations. The anomalies are 

 not as a rule haphazard in appearance (fig. 10), 

 but they indicate that extensive, sufficiently 

 large, patches or streaks of zooplankton exist 

 in such a way that several stations may come to 

 fall within a patch. It is difficult to relate 



[27 



the volumes to the hydrographic features, though 

 there is some indication that these general 

 patches are perhaps drawn out in a general north- 

 south direction more or less following the main 

 contour of the currents. 



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Fig. 10. Zooplankton. Net haul volumes (dis- 

 placement). Volumes higher than 15 ml. result 

 from mixture of diatoms except at stations 28A, 

 29, and 31 of Cruise II, where zooplankton con- 

 stituted about 25.20 and 18 ml., respectively. 

 Only the stations at which plankton samples 

 were taken are shown. 



During the April cruise only a few stations 

 were occupied, but it will be seen that total 

 volumes were high in the southern line. At sta- 

 tion 28A this is chiefly owing to a mixture of 

 diatoms which formed about half of the volume, as 

 shown by laboriously separating them from the ani- 

 mals. At station 29 the volume is also influenced 

 by diatoms but to a much lesser degree. The other 

 stations had nearly clean zooplankton. 



The volumes of the June cruise are also con- 

 spicuously influenced by diatoms caught together 

 with the animals. No attempt was made to separate 

 the two in order to obtain a true volume of the 

 animals. ¥here smaller volumes were taken, the 

 catches were nearly pure zooplankton. It may be 

 stated here that, in making net collections, a 

 dilemma is presented in deciding the appropriate 

 mesh aperture to use in nets for general zooplank- 

 ton catches during periods of diatom outbursts. 

 If apertures are sufficiently large to allow es- 

 cape of diatoms, especially larger and filamentous 



] 



