28 University of California Publications in Zoology [Vol.22 



THE PRINCIPAL TYPES OF FEESH WATER PLANKTON 



Among the 396 forms recorded and counted in the preserved 

 material, 201 were thought to he positively identified as to genus, of 

 which number 107 were also satisfactory as to species. The generic 

 designation of most of the remaining 195 forms was regarded as 

 probably correct though some were merely referred to the nearest pos- 

 sible genus or species according to the information at hand. The 

 following table gives the general distribution of these forms among 

 the three stations. 



Algae 



Protozoa 



Rotifera 



Crustacea 



Miscellaneous 



Total 201 69 126 



As might be expected of those found at only two stations most are 

 from stations I and II. Those found at only one station are of the 

 rarer, less conspicuous kinds. 



This table shows the main characteristics i lentioned by Kofoid 

 (1908) as distinguishing fresh water from marine plankton. It may 

 be well to enumerate his main points as verified in the present study. 

 The plankton consists of cryptogams and invertebrates, with some or- 

 ders missing and the others very variable in numbers of representa- 

 tives. Larval forms are very few and the number of invertebrate 

 groups much less than that of the sea. The small size of organisms in 

 fresh water is also a conspicuous feature. There are no large crus- 

 taceans, no coelenterates, no mature mollusks, few worms, no tuni- 

 cates or radiolarians, to make diversity such as that of the sea. In 

 spite of the smaller size of the organisms in fresh water their total 

 mass is much greater than that in the sea. The highest amount recorded 

 here is 18 cubic centimeters per cubic meter; the smallest 0.28 cubic 

 centimeters per cubic meter; while a typical marine product is stated 

 as 0.12 to 0.48 cubic centimeter per cubic meter. The San Joaquin 

 production noted was taken from net hauls only, the maximum from 

 Stockton Channel, the minimum from Smith Canal. Filter paper 

 catches show ten times the recorded volume in some cases. 



