A record of phosphorus dynamics in oligotrophic lake sediment |
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| Authors: | Tiffany A Wilson Aria Amirbahman Stephen A Norton and Mary A Voytek |
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| Institution: | (1) Department of Civil and Environmental Engineering, University of Maine, 5711 Boardman Hall, Orono, ME 04469, USA;(2) Department of Earth Sciences, University of Maine, Orono, ME 04469, USA;(3) Water Resources Division, U.S. Geological Survey, Reston, VA 20192, USA |
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| Abstract: | Historical phosphorus (P) dynamics were studied using sediment cores from three oligotrophic, acidic lakes in Maine, USA.
Long-term oligotrophy of these lakes is consistent with high sediment aluminum (as Al(OH)3) concentrations, as Al inhibits internal P loading, even under reducing conditions. The role of microbially-mediated reactions
in controlling redox conditions was evaluated by estimating microbial biomass and relative abundance of specific functional
groups. Sediments were fractionated using a sequential chemical extraction technique and all lakes met criteria for P retention
based on threshold sediment concentrations of Al, Fe, and P fractions as determined by (Kopáček et al. (2005) Limnol Oceanogr 52: 1147–1155). Sediment NaOH-extractable molybdate-reactive P (rP) and non-reactive P (nrP) represent P
associated with non-reducible phases, and organic matter-related P, respectively. Total P (TP) does not decrease with sediment
depth, as is typical of eutrophic lake sediments; however, nrP/TP decreases and rP/TP increases for all three lakes, indicating
nrP mineralization without any significant upward diffusion and release into the hypolimnion; i.e. diagenesis of P is conservative
within the sediment. Two diagenetic models were developed based on nrP and rP concentrations as a function of sediment age.
The first model assumes a first-order decay of nrP, the rate coefficient being a function of time, and represents irreversible
nrP mineralization, where the produced PO4 is permanently sequestered by the sediment. The second model assumes a first-order reversible transformation between nrP
and rP, representing biotic mineralization of organic P followed by incorporation of inorganic P into microbial biomass. Both
models reflect preservation of TP with no loss to overlying water. The rate coefficients give us insight into qualities of
the sediment that have affected mineralization and sequestration of phosphorus throughout the 210Pb-dateable history of each lake. Similar models could be constructed for other lakes to help reconstruct their trophic histories.
Paleolimnological reconstruction of the sediment P record in oligotrophic lakes shows mineralization of nrP to rP, but unlike
the case in eutrophic lake sediments, sediment TP is preserved in these sediments. |
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