Abstract
Lake 227 of the Experimental Lakes Area (ELA) in Ontario, Canada, has been fertilized with phosphorus (P) since 1969, which resulted in a rapid transition from oligotrophic to eutrophic conditions. Sediment cores collected from the oxygenated epilimnion, and the mostly anoxic hypolimnion of this unique lake contain a historical record of the changes in sediment P speciation and burial rates across the trophic transition. To elucidate these changes, results of chemical extractions were combined with 210Pb sediment dating, and with 31P NMR, Mössbauer, and XANES spectroscopies. Prior to 1969, organic P (POrg) was the major sedimentary P sink in Lake 227. Eutrophication of the lake coincided with marked increases in the burial rate of total P (TP), as well as in the relative contribution of the NaHCO3-extractable P pool (humic-bound P, PHum). Together, PHum and POrg account for ≥70% of total P burial in the sediments deposited since artificial fertilization started. The PHum fraction likely comprises phosphate complexes with humic substances. The strong linear correlation between P and iron (Fe) extracted by NaHCO3 implies a close association of the two elements in the humic fraction. Mössbauer and XANES spectra further indicate that most Fe in the post-1969 sediments remained in the Fe (III) oxidation state, which is attributed to the stabilization of reducible Fe by organic matter, in part via the formation of phosphate-Fe (III)-humic complexes. Importantly, our results show that the eutrophication experimentation of Lake 227 caused the accumulation of a large reservoir of reactive sediment P, which may continue to fuel internal P loading to the water column once artificial fertilization is terminated.
Original language | English |
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Article number | e2020JG005713 |
Number of pages | 22 |
Journal | Journal of Geophysical Research: Biogeosciences |
Volume | 125 |
Issue number | 8 |
DOIs | |
Publication status | Published - Aug 2020 |
Funding
The authors acknowledge the insightful comments and suggestions of two journal reviewers. Financial support for this research was provided by the Canadian Excellence Research Chair (CERC) Program. The authors also wish to thank the IISD Experimental Lakes Area (ELA) of Northern Ontario for providing field site access, sampling equipment, and on-site laboratory facilities. Especially, we acknowledge ELA scientists and staff responsible for the artificial fertilization of Lake 227 over the past 50?years, with particular mention to David Schindler, Michael Paterson, and Lewis Molot. The Environmental Molecular Sciences Laboratory (EMSL) is acknowledged. EMSL is a user facility funded by the U.S. Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. We are grateful for the assistance of Corey Liu at the Stanford Magnetic Resonance Laboratory for the P-NMR analysis. The Canadian Light Source is financially supported by NSERC, NRC, CIHR, and the University of Saskatchewan.
Keywords
- advanced spectroscopy
- eutrophication
- legacy phosphorus
- phosphorus cycle
- phosphorus speciation
- water quality