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Abstract

Since the 1980s, several in-lake restoration methods for eutrophied lakes have been developed based on artificially induced calcite precipitation and lime additions. The aim of these measures is to improve water quality and reduce the incidence of cyanobacterial blooms by increasing P adsorption and removal through natural calcite precipitation. The authors compared the relative efficiencies of lake marl application and natural calcite precipitation in immobilizing P, Mn and Fe in Lake Arendsee, northeastern Germany, and their subsequent value as a tool for managing eutrophication.

Microprobe investigations of element distributions in calcite minerals of laminae precipitated annually during several years before and two years following the restoration suggest that recently grown biogenic calcites were more efficient at the co-precipitation and immobilization of P, Fe and Mn. The lake marl applied as 16% calcite/water mixture proved unsuitable as a restoration tool because of its coarse grain size mainly in the range of 20–1000 μm and associated organic matter which reduced its suspension time and sorption reaction rates in the water column. In Lake Arendsee or in similar eutrophic hardwater lakes with fast internal P recycling by rapid degradation of autochthonous organic matter in the water column and at the sediment surface (sedimentary TOC:TP ratios of 200 or more), biogenic calcites provide almost permanent P, Fe and Mn retention in the sediments. The study shows that epilimnetic additions of pure CaCO3 seed crystals comparable to natural calcite precipitation are the most effective for improving water quality. Small CaCO3 crystals exhibit a larger reactive surface, and thus, incorporate adsorbed dissolved nutrients and trace metals more rapidly during growth. However, in-lake water quality management strategies work best only if external point and non-point nutrient sources are reduced or diverted.