Comment by bearsandsalmon

Some more ideas that might work:

- Explosives. Drop large explosives deep into the lake periodically. It may kill a lot of life, but if you need to stir up organic matter at the bottom, it’s more efficient than attempting to stir it.

- Migrate life from similar depths at other lakes. If it’s going to die if you don’t, maybe it’s worth the effort.

- Drill. Just drill a deep hole until you get to a water source or heat.