The effects of hypolimnetic aeration on iron-phosphorus interactions

Abstract

Two circular enclosures (8 m dia × 14 m deep) were used to monitor rates of phosphorus sedimentation and regeneration in the presence of oxygenated and anoxic hypolimnetic water and under conditions of low and high iron concentrations. These manipulations showed that during spring turnover, sedimentation of soluble reactive phosphorus (SRP-P) was greater in the enclosure that received hypolimnetic aeration than in the non-aerated enclosure. During the summer (June–September), SRP-P moved from the sediments to the water columns of both enclosures. This suggested that iron (III) substrate might be lacking and so we added iron to the hypolimnetic water of both enclosures. In the aerated enclosure, phosphorus sedimentation occurred almost at once. In the non-aerated enclosure, iron addition had no effect. Sediment trap analysis showed that during the period of iron addition, the rate of TP-P sedimentation in the aerated enclosure increased threefold and the ratio of iron to phosphorus in the sedimented material was 10:1. Throughout the experiment, the non-aerated enclosure lost an average of 0.7 mg SRP-P m −2 d −1 to the sediments and the aerated enclosure lost 6.1 mg SRP-P m −2 d −1. The results appeared to be entirely controlled by hypolimnetic oxygen concentrations and by concentration of SRP-P and T-Fe. This suggests that management of these parameters in combination with hypolimnetic aeration should yield predictable results in any stratified fresh water environment.