Distribution of phosphorus in a biologically restricted lake in Iowa, USA

Abstract

The impact of soil runoff phosphorus on the Silver Lake of Iowa was assessed in this study. Currently, the lake cannot support its designated uses as a recreational water body. Extensive algal blooms characterize the lake in late summer, lowering the dissolved oxygen content in water (<2.0 mg/L). The goal of this study was to map the long-term buildup of phosphorus in the lake sediments and come up with recommendations for restoration strategies. Sediment cores were taken from 20 sites along five transects in the lake. In 10 of these 20 sites, P content exceeds 600 μg/gm of sediments. In general, a strong vertical gradient is observed in which P decreases with depth. The average P levels in the contaminated sediments are 848 μg/gm at 0–5 cm, 666 μg/gm at 10–15 cm, and 420 μg/gm at 20–25 cm. Further examining the cores, the average P content at 30–35 cm depth was calculated as 293 μg/gm. The above results indicate that sediments in Silver Lake are heavily contaminated with soil runoff P, mostly coming from the surrounding croplands and an active hog lot on the southeastern lakeshore. In addition to deteriorating aesthetic value, the P loading into Silver Lake from the watershed holds a potential threat to the local population by way of introducing cyanobacterial biotoxins to the environment. The average rate of sedimentation in the lake has been 1 cm/year in the last 30 years. The annual loading of P from land to lake was calculated as 1.44×10 3 kg (1.42 tons). This incoming mass of P was found distributed in the lake as 1.15×10 3 kg (80%) in bottom sediments, 21.3 kg (1.5%) in suspended particulates, 70.6 kg (4.9%) in the dissolved phase, and 198 kg (13.6%) used in growth of biomass and mineral complexes.