Date of Award

5-2008

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Ecology and Environmental Sciences

Advisor

Stephen A. Norton

Second Committee Member

Aria Amirbahman

Third Committee Member

Ivan J. Fernandez

Abstract

Phosphorus (P) is a limiting nutrient in aquatic ecosystems. Its bioavailability determines the trophic status of lakes. The biogeochemistry of P in surface waters can be controlled by a combination of abiotic and biotic factors. Dissolved inorganic aluminum (Al) and iron (Fe) hydrolyze in streams and lakes to produce Al(OH)3 and Fe(OH)3 in the water column. These hydroxides may also form through photo-oxidation of complexes with DOC, liberation of inorganic metal species, and precipitation, followed by sedimentation. These solid phases readily adsorb P from the water column, reducing the amount that is available for biological nutrition (Kopácek et al. 2000; Huser & Rydin 2005). Microorganisms play a role in the aquatic P cycle through facultative uptake and release of P, which is governed by seasonal oxygen stress (Gächter et al. 1988). Some combination of abiotic (mainly Al and Fe) and biotic (DOC complexation and microbial) factors ultimately controls the transport to, mobility within, and fate of P in a lake. Oligotrophic surface waters are characterized by low primary productivity, mainly due to low concentrations of bioavailable P, a consequence of insignificant internal loading of P from sediment. Lake sediment cores provide a geochemical record of concentrations of Al and Fe hydroxides and associated P that have been deposited over time, and thus an assessment of the stability of the processes controlling P. I dissolved sediments from cores from Little Long Pond, Upper Hadlock Pond, and Mud Pond, Maine using a chemical extraction (Psenner et al. 1988) that sequentially separates the reducible (mainly Fe(III)) and base-soluble (mainly Al) metal hydroxides and associated P. Using 210Pb activity to determine dates of sediment downcore, I constructed a historical profile of the Al-Fe-P geochemistry for each lake. Cores show that P is mainly associated with Al(OH)3 throughout the entire period of record, exceeding the last 300 years. There was very little P associated with reducible Fe, indicating low potential for P release during seasonal anoxic conditions. High extractable Al concentrations relative to Fe and P are the result of chronically acidic conditions and/or high preferential export of Al from soils in these catchments. Evidence of anthropogenic impacts includes increased rates of sedimentation likely due to land disturbances (mainly logging) and erosion within the catchments. Strong correlation of Al and P, and diatom species abundance data, throughout the cores indicate that all three ponds have maintained a stable acidic, oligotrophic status over the last several centuries. Mineralization of sediment organic P was modeled by fitting organic-P and Al-P downcore concentrations to reversible or irreversible first-order transformation models. The calculated t½ of organic P for the three lakes ranges from 24 to 546 years, generally 10 to 100s of times longer than t½ of high-P (eutrophic) lakes (Reitzel et al. 2007; Ahlgren et al. 2005; Penn et al. 1995; and others).

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