How effectively does a single or continuous iron supply affect the phosphorus budget of aerated lakes?

Abstract

In lake restoration, the redox sensitivity of iron (Fe)–phosphorus (P) compounds has been regarded as detrimental for a sustainable increase in sedimentary P retention since developing low redox potentials release Fe-bound P. Thus, Fe salts alone have rarely been used successfully to inactivate sediment P, and there are no studies on the long-term effects of in-lake Fe applications on P retention. Here, we analyzed for how long, and how efficiently, a single and continuous Fe application can affect the P budget of lakes. Two aerated lakes in Berlin, Germany were compared: Lake Tegel (TEG) experienced a continuous Fe supply via its tributaries, while Lake Gros-Glienicke (GGS) was treated once with Fe in the winter of 1992/1993. By controlling the operation of aerators, their effectiveness on P exchange at the sediment–water interface (non-aerated vs. aerated) was directly compared in spring and autumn between 2008 and 2010. The amount of P controllable by aeration (P control) was experimentally determined by non-aerated vs. aerated sediment cores (at 10 and 16 °C). Core stratigraphy of Fe was observed by high-resolution μX-ray fluorescence analysis. In TEG, the mobility of Fe was limited due to its sulfidic fixation, and thus Fe only accumulated slightly at the sediment surface (Fe/P ratio, ∼3). P control corresponded to only 4 % of the P content of the lake and 18 % of P loading. Hence, aeration only slightly influenced trophy-relevant epilimnetic P. In GGS, the single Fe application still ensures a high P binding ability of sediment since Fe relocated towards the surface (Fe/P ratio, ∼7). P control corresponded to 38 % of the P content of the lake and 74 % of annual P loading. Thus, the P release is not relevant for the P supply to the epilimnion since with the lake’s overturn P is co-precipitated by the hypolimnetically accumulated Fe. When external P loading is sufficiently reduced, as in GGS, amendments to Fe precipitants can increase sediment P retention in a redox-dependent manner over the long term. Thus, the redox-dependent mobility of Fe should no longer be regarded as a disadvantage of Fe-containing precipitants. To compensate for co-precipitation and complexation of Fe with sediment organic matter, a high Fe dosage (≥200 g m−2) is needed.