VERTICAL DISTRIBUTION OF AVAILABLE PHOSPHOROUS IN THE BOTTOM SEDIMENTS OF A BIOLOGICALLY INACTIVE LAKE

  Chad Fields, Ariana Houge, Heather Bailey, Mohammad Iqbal, and Edward Brown

   Lake watersheds often play a crucial role in phosphorous (P) cycling between the biosphere and the hydrosphere. P 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 and stays in the bottom sediments, thereby damaging the ecosystem.

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

This research was supported, in part, by the Roy J. Carver Charitable Trust and the Iowa Space Grant Consortium.