Progressive conversion of sediment mobile phosphorus to aluminum phosphorus

Abstract

Welch EB, Gibbons HL, Brattebo SK, Corson-Rikert HA. 2017. Progressive conversion of sediment mobile phosphorus to aluminum phosphorus. Lake Reserve Manage. 33:205–210.Sediment core data show that the 2004 alum treatment of Green Lake, Seattle, Washington, was not only initially effective at forming aluminum phosphorus (Al-P) and reducing mobile-P, but that the process of forming Al-P continued during the following 11 years. Increases in Al-P concentrations were determined by differences in levels at 0–10 and 0–20 (or 20+) cm between cores taken in 1998 (7 years after the first treatment), 2004 (6 months after the second treatment), and 2015. Most of the increase in Al-P formed since 1998 occurred between cores taken 6 months after the 2004 treatment and 2015. That increase was 60% at the deepest site and 84% at a shallower more central site. An increase in Al-P between 2004 and 2015 also occurred at 2 remaining shallower sites. Mobile-P (Fe-P and labile-P) decreased during this period, associated with increased Al-P, but the magnitude of change was less, probably because mobile-P had been largely depleted and P had diffused from deeper sediment. The increase in Al-P resulted in a decrease in the Al-added (dose) to Al-P–formed ratio in the top 25 cm at the deep site from 15:1, 6 months after the 2004 treatment, to 3.7:1, 11 years later. At the shallower more central site, that ratio decreased from 57:1 to 24:1 using the top 10 cm, or 28:1 to 10:1 in the top 20 cm. Higher ratios at the shallower site than at the deep site may be due to focusing of floc to deeper areas.