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This atlas contains a broad assessment of the results of phosphorus loading reduction in Lake Ontario in the 1970s, including the in-lake phosphorus concentration reduction. It also describes trends of other indicators of recovery from eutrophication. The summer Secchi depths and summer oxygen depletion rates were fairly stable in the 1970s, whereas they would have worsened without phosphorus control. Particulate organic carbon in offshore surface waters during August/September declined steadily by 20% from 1975 to 1981.

Also illustrated are the chemical/biological aspects of the spring time thermal bar, and lakewide upwelling/downwelling in response to winds in summer. In July 1972, there was a prominent lakewide chlorophyll maximum at a depth of about 10m. The spring time diatom crop was located near the lake bottom in summer, as indicated by abundant particulate organic matter and near-bottom release of soluble reactive silica. March/April nitrate+ nitrite had steadily increasing values, from 215 p.g N /L in 1968 to 340 p.g N /L in 1981. There was a residual level of nitrate + nitrite in surface waters during late summer in the later years, amounting to about 100 p.g N /L, which, along with decreased phosphorus and increased N:P ratios, means that troublesome blue-green algal blooms and scums will not occur.

In summary, the phosphorus control program and a fortuitous increase of soluble reactive nitrogen have resulted in very good metabolic conditions in Lake Ontario, with moderate phosphorus and plankton con tent, prevention of troublesome plankton blooms, and excellent oxygen conditions. It is strongly recommended that the phosphorus loading control program for Lake Ontario and upstream Lake Erie be continued, to maintain the presently ideal trophic conditions in Lake Ontario.