University of CalifoDiiti PuhJicdtioiis in Zoologij IVoi,. 22 



INTRODUCTION 



The biology of frcsli water is an attractive field for investigation 

 both from the standpoint of ecology and from that of its practical 

 bearings on the problems of fisheries. The fresh water plankton with 

 its varying eoin])oneiits forms a biological complex, or association of 

 both jilant and animal forms which have an intimate relation not only 

 each to the others, bnt also to the varying factors of their environment, 

 such as light, temperature, organic and inorganic substances in solu- 

 tion, and to seasonal change attendant upon the run-off from the 

 watershed. 



The survey of San Francisco Bay undertaken by the United States 

 Bureau of Fisheries in 1912-13 in cooperation with the University of 

 California afforded an opportunity to initiate a survey of the plankton 

 of the San Joaquin River, one of the principal tributaries of that l)ay. 

 My position at Stockton as teacher of biology in the High School 

 afforded some laboratory facilities and also a location near the head of 

 tidewater where the ultimate contributions of the stream of the bay 

 could be examined and where a variety of conditions were present, 

 including both the main channel, and backwaters with varying rates 

 of flow, as well as a canal much enriched by sewage. 



The author is conscious not only of the serious and baffling difficul- 

 ties that attend such an attempt at a continuous piece of work but also 

 of the errors which inevitably creep in, especially in the initial stages 

 of such an enterprise. These errors are, however, in the main, dis- 

 tributed throughout the data and do not necessarily invalidate our con- 

 clusions. Such errors as occur in taxonomy are due to lack of the 

 specialist's knowledge of nomenclature and synonymy, and of his 

 critical skill in the finer distinctions of species and subspecies. The 

 species as I have recorded them are at least groups of recognizably 

 similar organisms. The largest source of significant error in this 

 ecological studj* is the not improbable inclusion in such groups, of 

 small numbers of less abundant or rare species of closely similar form. 



