VERTICAL DISTRIBUTION OF AVAILABLE PHOSPHORUS IN THE BOTTOM SEDIMENTS OF A BIOLOGICALLY RESTRICTED LAKE

Lake watersheds often play a crucial role in phosphorus (P) cycling between the biosphere and the hydrosphere. Phosphorus arrives in natural aquatic ecosystems as a consequence of agricultural land use. Soil runoff P does not have an escape route to the atmosphere. It accumulates in the bottom sediments, causing sustained impacts to the ecosystem.

In June 2001, sediment cores were taken from 15 sites in Silver Lake of northeast Iowa. Extensive algal blooms characterize the lake throughout the summer, lowering the dissolved oxygen content in water (< 2.0 mg/L). Currently, the lake cannot support its designated purpose as a recreational facility. The average P concentration in the top 2 inches of sediment is 630 µg/g (range: 11 µg/g to 1.3 mg/g). In 10 of these 15 sites (66%), P concentration exceeds 600 µg/g. In 11 of 15 sites (73%), a strong concentration gradient is observed in which P decreases with depth in sediment. At these 11 sites, average P concentrations are 848 µg/g at 0-2 inch, 666 µg/g at 4-6 inch, and 420 µg/g at 8-10 inch. Further examining 8 cores, the average P concentration at 12-14 inch depth was calculated as 293 µg/g. It is evident from the above results that sediment in Silver Lake is heavily contaminated with soil runoff P. The strong vertical gradient indicates that sediment contamination has increased in recent years, which has dramatically restricted biological activities in the lake. It seems that any strategy for restoring Silver Lake should include dredging or immobilizing P in the sediments.