AN ANNOTATED KEY TO THE 



IDENTIFICATION OF COMMONLY 



OCCURRING AND DOMINANT GENERA OF 



ALGAE OBSERVED IN THE 



PHYTOPLANKTON OF THE UNITED STATES 



By Phillip E. Greeson 



ABSTRACT 



In early 1979, a retrieval was made for all phytoplankton data contained in the com- 

 puterized data file of the U. S. Geological Survey. The retrieval revealed the analytical 

 results of 17,959 samples collected and processed between October 1973 and October 

 1978. Of the approximately 500 genera of freshwater algae reported in the United States, 

 the U.S. Geological Survey observed 321 genera in the phytoplankton. Fifty-two genera 

 were considered to be commonly occurring and 42 genera were considered to be com- 

 munity dominants. The report lists, describes, and provides a detailed taxonomic key to 

 the identification of 58 genera of algae considered either commonly occurring or domi- 

 nant. Also included is a summary of environmental conditions under which each algal 

 genus was observed, as well as a glossary and an extensive list of selected references. 



INTRODUCTION 



Taxonomy is the science concerned with the orderly arrangement of 

 organisms in some scheme of likenesses and differences among the 

 various individuals. The present taxonomic system was developed 

 mostly during the 18th and 19th centuries. During the mid-1800's, tax- 

 onomy played an additional and equally important role, that being the 

 basic tool in the analysis of evolutionary relationships among 

 organisms. This is commonly called the science of systematics. It was 

 in the early- 1900's when investigators began to realize that organisms 

 not only had certain inherent relationships but also were sensitive to 

 changes in the environment. 



Since the beginning of the 20th century, numerous publications have 

 discussed algae as indicators of environmental conditions. Perhaps the 

 most important contribution on ecological indicators was by Kolkwitz 

 and Marrson (1908), who introduced the concept of indicator species. 

 This resulted in the "saprobian" system for classifying organisms in 

 organically polluted rivers. 



After the paper by Kolkwitz and Marrson, many similar papers were 

 published; all discussed indicator species. The importance of biological 

 indicators as an index of pollution was described by Forbes (1913), 



