The influence of lime and biological activity on sediment pH, redox and phosphorous dynamics

Abstract

The addition of powdered limestone to intact sediment cores from oligotrophic, acid Lake Hovvatn caused pH to increase, redox potential (E₇) to drop, and permitted net precipitation of phosphorous (P) from the water column. Significant pH increase was found to a sediment depth of 6 cm and a maximum increase in pH from 4.9 to 6.5 was found at a depth of 0.5 cm when dosed with 36 g m⁻² of lime. Such pH increase creates important changes in sediment equilibrium chemistry and enhances habitat suitability. In the case of Hovvatn, however, sediments would consume only 5 kg of the 91 tons of applied limestone. Superficial sediments remained oxidized, but below 0.5 cm, E₇ in limed sediment declined significantly more than in unlimed sediments, with a maximum difference of 102 mV versus −66 mV at a depth of 6 cm in unlimed and limed cores, respectively. Abiotic reactions account for 82 ± 54% of this reduction and the remainder is due to the oxidation of organic matter by bacteria. Precipitation of CaSO₄, reduction of the sediments by organic compounds at elevated pH and inhibition of the downward diffusion of O₂ by the limestone powder are potential abiotic mechanisms which could drive E₇ down. Enhanced P release was not found at lowered E₇, and supernatent TP concentrations dropped from 11.7 to 4.4 µg P l⁻¹. More P was swept from solution in cores which recieved larger lime doses. The presence of chironomids caused sediment pH to increase by as much as 1.2 pH units, presumably due to NH₄ release, reduced sediment E₇ by as much as 171 mV and facilitated TP release during the first 17 d of core incubation. Field measurements of vertical distributions of sediment pH and E₇ before and after the liming of Hovvtn corroborated laboratory findings.