 Advocates argue this compaction was aggravated by its use for outdoor concerts once the Sheep Meadow had been restored in 1979.Eroded topsoil that washed into Belvedere Lake resulted in eutrophication that turned it to algal soup each summer.After Belvedere Castle was renovated in the 1980s, marshes were placed on the northern bank of Belvedere Lake, and turtles were introduced there.