Your geography and site-variables are far outside of my personal experience, but I agree with Ted that your focus should be placed on aeration - and more specifically, "thermal destratification" of the water-column.
IMO, the significant depth of water, relative to the lake's surface-area, poses a far greater issue (as a contributor to thermal/oxygen stratification) than do the phosphorus levels.
Also, reducing the lake's available phosphorus levels would undoubtedly impact food-chain productivity (i.e. creating a new problem while attempting to correct another).
Lastly, it would require a lot of product, expen$ive freight and labor to execute an AlSO4 treatment on the scale needed to offer any reduction in available P; AND it would not alleviate any fish-kills caused by thermal/oxygen stratification.
IMO, destratification-efforts are likely to offer a much better return on the investment.
