PNNL

Abstract

Lake 227 of the Experimental Lakes Area (ELA) in Ontario, Canada has been fertilized with phosphorus (P) since 2 1969, which resulted in a rapid transition from oligotrophic to eutrophic conditions. Sediment cores collected 3 from the oxygenated epilimnion and the mostly anoxic hypolimnion of this unique lake contain a historical record 4 of the changes in sediment P speciation and burial rates across the trophic transition. To elucidate these changes, 5 results of chemical extractions were combined with 210Pb sediment dating, and with 31P NMR, Mössbauer and 6 XANES spectroscopies. Prior to 1969, organic P (POrg) was the major sedimentary P sink in Lake 227. 7 Eutrophication of the lake coincided with marked increases in the burial rate of total P (TP), as well as in the 8 relative contribution of the NaHCO3-extractable P pool (humic-bound P, PHum). Together, PHum and POrg account 9 for =70% of total P burial in the sediments deposited since artificial fertilization started. The PHum fraction likely 10 comprises phosphate complexes with humic substances. The strong linear correlation between P and iron (Fe) 11 extracted by NaHCO3 implies a close association of the two elements in the humic fraction. Mössbauer and 12 XANES spectra further indicate that most Fe in the post-1969 sediments remained in the Fe(III) oxidation state, 13 which is attributed to the stabilization of reducible Fe by organic matter, in part via the formation of phosphate-14 Fe(III)-humic complexes. Importantly, our results show that the eutrophication experimentation of Lake 227 15 caused the accumulation of a large reservoir of reactive sediment P, which may continue to fuel internal P loading 16 to the water column once artificial fertilization is terminated.