Fertilization of Natural Lakes in Michigan

Inorganic fertilizer was applied to two state-owned lakes in northern Michigan, one a 4.3-acre trout lake and the other a 27.5-acre warm-water lake. Two nearby lakes were kept under observation as controls. Fertilizer brought about a plankton-algae bloom the first summer in the warm-water lake and produced a very heavy growth of filamentous algae the second summer. The trout lake responded much less to the added nutrients; the reading of a Secchi disk was at no time less than 8.2 feet. No appreciable oxygen depletion occurred during the winter following the first season of fertilization. Chemical analysis in February of the second winter showed severe oxygen depletion, with oxygen levels at less than 1 p.p.m. at all depths. An almost complete winterkill occurred in both fertilized lakes. No winterkill occurred in the control lakes.