Natural inactivation of phosphorus by aluminum in preindustrial lake sediments

Noncalcareous phosphorus (P)-rich lake sediments typically release P associated with iron hydroxide [Fe(OH)3] during the development of hypolimnetic anoxia. High concentrations of aluminum hydroxide [Al(OH)3] in such sediments (e.g., in aluminum [Al]-treated lakes) can prevent the P release. Here we show that sediment ability to bind P can naturally develop during lake history because of changes in the Al(OH)3 concentration and the Al(OH)3 to Fe(OH)3 ratio in sediment. We reconstructed the development of sediment Psorption characteristics through the Late Glacial and Holocene periods on the basis of sequential fractionation analysis of P, Al, iron, calcium, and magnesium in a ,14,000-yr-long sediment record, fresh settling seston, and bedrock from Plešné Lake, Czech Republic. The most significant change occurred at the Late Glacial–Holocene transition (,10,000 yr before present), when the watershed became forested and soil erosion decreased. The Late Glacial sediment was rich in mineral detritus, derived from the watershed till and bedrock, had most of its P associated with Fe(OH)3 (37%) and calcite and/or apatite (42%), and was probably able to release P during anoxia. In contrast, the Holocene sediment was highly organic, the P release during anoxia was likely negligible, and .90% of P was associated with Al(OH)3. This Al(OH)3 originated from photochemical liberation of Al from dissolved organically-bound Al (Alo) exported to the lake from soils. Because the photochemical mechanism was a more efficient source for Al(OH)3 than for Fe(OH)3, the sediment became a P trap by the beginning of the Holocene. The ability of the sediment to immobilize P increased further during the anthropogenic acidification era because of elevated terrestrial export of ionic Al. Similar sediment fractionation results in cores from six lakes in Maine suggest that this P-immobilizing mechanism is a general process that can occur in lakes with high Alo inputs. Lake and coastal marine sediments, with sorption capacities dominated by iron hydroxide [Fe(OH)3], release phosphorus (P) associated with iron (Fe) during the development of hypolimnetic or in-sediment anoxia because of the reductive dissolution of FeIII solid phases (e.g., Mortimer 1971; Jensen et al. 1995). Some studies (e.g., Prairie et al. 2001; Amirbahman et al. 2003; Golterman 2004), however, show that some noncalcareous P-rich sediments may not release P during the development of anoxia, even as the hypolimnetic Fe2+ concentrations increase substantially. Kopáček et al. (2005a) suggested that these exceptions to Mortimer’s paradigm could be associated with aluminum hydroxide [Al(OH)3] that provides an additional redox-insensitive sorption capacity to sediments. Sediment analyses and experience with aluminum [Al]-treated lakes (e.g., Ulrich 1997; Rydin et al. 2000; Lewandowski et al. 2003) provide important insight for lakes with naturally high or elevated Al(OH)3 concentrations in the sediment. Al(OH)3 can prevent P release into the water column during reducing (low Eh) conditions, because it is insensitive to redox changes, and can bind the P liberated from any reductive dissolution of Fe(OH)3. Laboratory experiments and results of chemical analyses of 52 sediments from different types of European and American lakes show that the ratio between concentrations of Al to Fe hydroxides is a reasonable predictor of the ability of the sediment to release P (Kopáček et al. 2005a). If the ratio is .3, only low amounts of P are released at low Eh conditions. An Al(OH)3 : Fe(OH)3 ratio .3 is typical for sediments of (1) atmospherically acidified lakes with elevated terrestrial Al export from acidic soils (because of acid dissolution of soil Al oxyhydroxides), (2) lakes treated with Al salt in the water column, and (3) alpine lakes with aluminosilicate bedrock and high soil erosion in their watersheds; sediments of these lakes contain soil detritus rich in Al oxyhydroxide (Kopáček et al. 2005a). There are, however, high Al(OH)3 : Fe(OH)3 ratios in some circumneutral forest lakes (e.g., Norton et al. unpubl. data). Such lakes may receive abundant particulate Al(OH)3 due to the hydrolysis and precipitation of ionic Al (Ali) after degassing of carbon dioxide (CO2) from soil solutions emerging to surface waters (Norton and Henriksen 1983; Roy et al. 1999) and from photochemical liberation of organically Acknowledgments We thank R. Schmidt from the Institute for Limnology, Mondsee, Austria, for sampling the Plešné Lake sediment. We appreciate two anonymous reviewers who significantly improved our presentation. We are grateful to the Grant Agency of the Czech Republic (206/06/0410), the U.S. National Science Foundation (DEB0415348), and the EC’s EURO-LIMPACS (European Project to Evaluate Impacts of Global Change on Freshwater Ecosystems) project (GOCE-CT-2003-505540) for financial support. The assistance of students was supported by the MSM 6007665801 project. Limnol. Oceanogr., 52(3), 2007, 1147–1155 E 2007, by the American Society of Limnology and Oceanography, Inc.