| Abstract: | Historic data on soluble silica in the offshore waters of Lake Michigan were compiled and analyzed to determine whether the data supported a decrease in silica concentration which had been hypothesized previously on the basis of other studies. Although the data base was limited and no data were obtained for offshore waters (depths >40 m) prior to 1954, the available data support the conclusion that the silica concentration decreased rapidly after 1954. The thesis is developed that rapid silica depletion occurred in the 15-year period from 1954 to 1969 when the winter maximum concentration decreased from approximately 4.4 to 1.4 mg SiO2 · 1?1 and the summer minimum decreased from approximately 2.2 to <0.2 mg SiO2 · 1?1. The decrease in silica concentration is attributed to increased production and sedimentation of diatoms that resulted from increased anthropogenic phosphorus loading. Total phosphorus in offshore waters was probably <10 μg P · 1?1 during the period of rapid silica depletion. The rapid decrease in silica concentration with relatively small phosphorus enrichment demonstrates that the tight coupling of biological and geochemical processes drastically affected the biogeochemistry of silica in a large, aquatic system within a relatively short period of time. |
