Downwelling is too expensive, upwelling is the way to go, but not by laying airpipes at the bottom of the lake. Every aquarium hobbyist knows what is called an air-lift, a vertical pipe sucking the water from the bottom by having air pumped in 1/4 from the surface with the use of a ventury system. The air bubbles being confined inside the pipe will push the water upwards in a constant and very efficient way as well as a water pump can do. In the case of this lake, the 10" PVC vertical pipes with ballasts & floats could be sucking the anoxic water from any depth to have it being aerated once it reaches the surface of the lake and if this process in continued for long enough all the anoxic water could be replaced by oxygenated water in weeks or days. This is very basic hardware, easy to use & install, economic and super reliable and efficient; why would you want to do this any other way?

@PhilDor - that rings true for me. Why was it not even mentioned as a possibility, even if were found not to be (I'd doubt) far more effective?

The relative costs here make no sense. If you can reduce the pollution at source, that's a one off cost of whatever tech or operating system change is required, which can be defrayed over an appropriate period of say, ten years. Indeed, it might even be a cheaper way of operating than wasting nutrients, as happens at present. But installing any type of pumping system represents a continual cost, in perpetuity, with associated fuel costs and carbon emissions. Bite the bullet, stop the pollution at source rather than trying to cover it up. In any case, what happens when the entire water column is toxic?

I had the same thought as PhilDor. In addition to pushing anoxic water up this system will aerate it along the way increasing the benefit. The system could be driven by solar or wind energy there by almost eliminating any additional pollution caused by conventional energy production.
My only concern with this is what effect moving this oxygen-less water to the surface will have on the upper water table and visa-versa.

A thought - If you increase the oxygenation - any risk of increasing the algae blooms, the ones depleting the oxygen? Logical solution is stop the additional nutrients entering the system.

Another thought what is the natural consumer of algae?

Wow, I used to swim in this water!
Philidor has it right, BUT, why don't we go back to more organic ways of planting; see the video "Back to Eden".
https://www.backtoedenfilm.com/
The more we wait, the more expensive it will be...

Phildor...hope there are people with inte llegence who read your post and act on it....bravo

Build a holding tank upstream to capture part of the water flowing into a lake. Run a pipe from that tank down into the lake, through a manifold that branches to pipes that let out directly into the deepest parts of the lakebed. Gravity does all the work and water with a higher oxygen level gets fed to the deepest levels.

There are some excellent suggestions here, including one from Carolyn Farstrider that suggests going to the source and getting the pollution stopped from flowing into the water. I note that adults usually pick up after themselves, and so perhaps getting the farmers to clean up their act might be a good target. Use some of that federal money, or maybe creating some tax incentives, on improving the existing system,

One question- isn't algae a plant? Do plants not consume CO2 and release oxygen?

Someone will have to explain to me how an algae bloom causes anoxia.