TRANSPORT PATHWAYS OF PHOSPHORUS IN VARIABLE HYDROLOGIC SYSTEMS

Agriculturally derived phosphorus (P) is the root cause of poor surface water quality in Midwestern watersheds. Phosphorus enhances phytoplankton growth in water, resulting in increased deposition of organic matter to deeper water. This in turn stimulates respiration, thereby increasing the consumption of oxygen, which leads to eutrophication of the water body. Today, many of Iowa's lakes no longer support their designated uses as a habitat for aquatic life and a recreational water body because of the eutrophication problem.

Abundance and mobility of P in two contrasting hydrologic environments were compared in this study. Silver Lake of northeast Iowa is characterized by cyanobacterial blooms in the summer with dissolved oxygen (DO) of less than 2.0 mg/L. Lake sediments show high levels of P with a concentration gradient in which P decreases with depth. The average P concentrations in the sediments are 848 µg/gm at 0-5 cm, 666 µg/gm at 10-15 cm, 420 µg/gm at 20-25 cm, and 293 µg/gm at 30-35 cm below sediment surface. Munns Creek watershed in north-central Iowa is well aerated with DO values ranging between 5.3 and 13.9 ppm. The average P concentration in surface soil is 30.2 µg/gm of soil. However, samples recovered from 6 inches below the surface have an average concentration of 57.6 µg/gm of soil. This increase in concentration is attributed to the high adsorption of P to the higher particle surface areas in deeper but still well oxygenated condition. On the other hand, the high level of P in Silver lake sediments is caused by its mobilization from particles in low oxygen condition and then accumulation in pore water for not having adequate escape routes. It is evident from the above results that redox conditions play a very important role in determining the fate of P in shallow hydrologic environments.