FACTORS AFFECTING PHYTOPLANKTON DISTRIBUTION IN A DOUBLE-CELL SEWAGE LAGOON (1) (2)

The distribution of phyioplankton in a double-cell sewage lagoon at Hallam, Nebraska, was studied in relation to physical, chemical, and biological factors during the summer and fall of 1965. Sixteen species of algae were recorded in the first and more organically rich of the two physically similar ponds, with 28 species recorded in the second pond. Population sizes were always greater in the first pond due to reduced grazing during the summer and large quantities of ammonia-nitrogen during the fall. The dominant algal species in both ponds on nearly all sampling dates was Ankistrodesmus falcatus v. acicularis. Declines in this population occurred with high organic pollution and heavy grazing. Both ponds had severe reductions in algal numbers during late October due to heavy grazing by the rotifer Brachionus. The distribution of phytoplankton in the Hallam ponds is compared to that of other sewage ponds in the United States, and the general pattern which emerged is discussed.