In 1968, USGS began a program of basic research in San Francisco Bay that has complemented the research and monitoring elements of the Interagency Program. Although the USGS program ranges its focus of study. From year to year, it has elements of continuity because some measurements have been made routinely for decades. One of these elements has been a study of the spring phytoplankton bloom in South San Francisco Bay. Here I present data from multiple sources to explain why such emphasis has been placed on this biological phenomenon.