Sediment geochemistry of Al, Fe, and P for two historically acidic, oligotrophic Maine lakes

Phosphorus (P) may be liberated from lake sediments by reductive dissolution of Fe(OH)3(S) during periods of hypolimnetic anoxia. P, however, remains adsorbed to Al(OH)3(S) regardless of redox conditions. During chronic or episodic acidification of a catchment, ionic Al is mobilized from soils to receiving waters. A fraction of the mobilized Al may precipitate as a consequence of higher pH of the receiving waters. We hypothesized that phosphorus retention in lake sediments is directly related to the magnitude of Al loading in response to low pH in the watershed. We studied cores representing over 200 years of sediment accumulation in historically acidic Mud Pond and Little Long Pond in eastern Maine, USA. Sequential chemical extractions of sediment were used to assess the history of Al, Fe, and P interactions. Mud Pond is a first-order pond with a pH of ~4.7, having acidified slightly in response to anthropogenic acidification from ~1930. The inlet stream to Mud Pond has dissolved Al concentrations often exceeding 500 μg/L, of which more than half is organically-bound. Mud Pond drains into Little Long Pond, a second-order pond with a historical pH of b6, and which has shown little pH or alkalinity response to increases or decreases in atmospheric SO4 2− input. Sequential extractions show that Al and P are predominantly in the 0.1 M NaOH-extractable fraction in the sediments from both ponds throughout the cores. The concentration of the likely biogenic and non-reactive P within the NaOH fraction increases up core from b30% to ~60%. Extractable Fe (b20% of extractable Al) is mainly in the 0.1 M NaOH-extractable fraction, except for the top few cm, which are predominantly in the bicarbonate-dithionite reducible fraction. Accumulation rates of sediment, Al, Fe, and P in both ponds have increased in the last 50–60 yr, but fractions remain in the same proportion. Throughout both sediment cores the molar ratio of specific Al:P fractions greatly exceeds 25, and molar ratio of specific Al:Fe fractions greatly exceeds 3, the thresholds proposed by Kopáček et al. [Kopáček J, Borovec J, Hejzlar J, Ulrich K-U, Norton SA, Amirbahman A. Aluminum control of phosphorus sorption by lake sediments. Environ Sci Technol 2005; 39: 8784–89.] for P release during anoxia. The data illustrate a continuous association of P with Al in both ponds during the last two centuries, likely due to the persistent natural acidity of the catchments