This volume of Advances in Microbial Ecology marks a change in the editor ship of the series. The Editorial Board wishes to take this opportunity to express its gratitude to Martin Alexander, the founding editor and editor of the first five volumes, for his enterprise in establishing the series and in ensuring that Advances has become an outstanding focal point for the identification of new developments in the rapidly expanding field of microbial ecology. With the publication of this volume, we welcome Howard Slater to the Editorial Board. The policies of the Editorial Board remain the same as before. Most con tributions to Advances in Microbial Ecology will be solicited by the Board. However, individuals are encouraged to submit outlines of unsolicited contri butions to the Board for consideration for inclusion in the series. Advances is designed to serve an international audience and to provide critical reviews on basic and applied aspects of microbial ecology. Contributions in the present volume are predominantly concerned with the ecology of aquatic microorganisms, but encompass a variety of approaches to this area. The exception is the chapter by J. W. Doran on the role of micro organisms in the cycling of selenium. G-Y. Rhee discusses the effects of envi ronmental factors on phytoplankton growth. The factors limiting the produc tivity of freshwater microbial ecosystems are considered by H. W. Paerl.