Lake Ontario water chemistry atlas

Abstract

The main content of this atlas is a broad assessment of the results of phosphorus loading reduction in the 1970's, including the in—lake phosphorus concentration reduction, but also trends of some other indicators of recovery from eutrophication. The summer Secchi-depths and summer oxygen depletion rates were fairly stable in the 1970's, whereas they would have worsened without phosphorus control. Particulate organic carbon in offshore surface waters during August/September declined steadily by 202 from 1975 to 1981. Also illustrated are the chemical/biological aspects of the springtime thermal bar, and lake wide upwelling/down welling in response to winds in summer. In July of 1972 there was a prominent lake wide chlorophyll maximum at about 10 meters’ depth. The springtime 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 μg N/L in 1968 to 340 μg N/L in 1981. There was 2 residual level of (nitrate + nitrite) in surface waters during late summer in the later years, amounting to about 100 μg 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 lucky increase of soluble reactive nitrogen have resulted in very good metabolic conditions in Lake Ontario, with moderate phosphorus and plankton contents, and prevention of troublesome plankton blooms, and excellent oxygen conditions too. It is strongly recommended that the phosphorus loading control program for Lake Ontario and upstream Lake Erie be continued, to maintain the presently—idea1 trophic conditions in Lake Ontario