When artificial fertilizers were added to unstratified, soft-water Crecy Lake (20 ha), New Brunswick, in three years (1946, 1951, and 1959), the lake was sufficiently self-regulatory that the artificial enrichments had only a temporary effect on organic production. The induced eutrophication proved largely reversible, with minor evidence of hastened senescence of the lake over an 18-year period. Each fertilization provided 210 ppb (parts per billion) of nitrogen (N), 390 ppb of phosphorus (P), and 270 ppb of potassium (K) to the water if the fertilizers were equally distributed throughout the lake.The highest concentrations of total P, determined daily after the fertilizations in 1951 and 1959, were 220 ppb after 2 days and 192 ppb after 1 day, respectively. The concentration of inorganic P declined rapidly from about 150 ppb the day after the fertilizations in 1951 and 1959 to < 10 ppb at a percentage loss per day of 5.5. Concentrations of organic P were highest 5 (80 ppb, 1951) and 8 (43 ppb, 1959) days after the fertilizations and declined to equilibration above prefertilization values for 3–4 months, and then to prefertilization values during the next years. Phosphorus lost in the drainage from the lake, assessed only in 1951 and 1959, was equivalent in these years to 8.5 and 12% of the amounts added in the fertilizers.Limited observations indicated an increase of nitrate N in the water immediately after fertilization, but then a rapid loss. Deficits in dissolved oxygen were greatest under ice in the winters immediately after algal blooms induced by the fertilizers, but were serious in only about 30% of the volume of the lake. Specific conductance of the water exhibited minor change with fertilization.Pronounced algal blooms, not previously recorded for the lake, followed each fertilization. Years of fertilization, and the next, were marked by blooms of Anabaena. Intensified growth of Spirogyra usually followed closely the blooms of Anabaena. In the 3rd year after fertilization algal growth was weak, at prefertilization densities. During blooms of Anabaena the numbers of zooplankters and bottom organisms, and growth of submerged, rooted aquatic vegetation, decreased. Zooplankton and bottom fauna were most abundant 2–4 years after fertilizations. Of the rotifers, three out of eight species became abundant. Of the planktonic microcrustaceans, Leptodiaptomus was always the most numerous; Diaphanosoma and Daphnia were more numerous after the last fertilization (1959) than they were earlier. Among the bottom organisms, Hyalella, chironomids, Amnicola, and sphaeriids increased most in abundance after the fertilizations.