fighting dead zones with aerating buoys and other large-scale geoengineering

I understand that this is a well-documented but useless phenomenon: due to differences in density due to temparature and salinity between surface and deeper water in the open sea, a vertical flow in a "chimney" made by sinking one end of a tube will continue on its own.

(Not able to find a source for this with a cursory googling; perhaps it is bogus; please send link if you have one.)

Perhaps this concept could be used to encourage flows between deep and surface waters in ocean hypoxic zones.

A government agency could provide a buoy every few square miles which would hold up the top of a large vinyl tube going down several hundred feet, after that there would be a cable going down to an anchor on the bottom. the chimney effect would occur, leading to increased mixing of surface and deep waters and providing oxygen to the depths where it is needed.

Experimentation would need to be done to determine (1) if this will work at all (2) the optimum depth for both the top and bottom of the tube (3) flow against tube size data (4) how to construct and deploy twenty thousand of these things in such a way that they will last decades.

Deploy ten thousand a year, retiring and replacing them when they don't work any more? What would happen to them to stop them from working? enough barnacles accumulate so the float doesn't float any more and the whole thing sinks?

Or...

What is the specific density at the bottom of the hypoxic zone? If it is denser that at the bottom of the basin (or rather, at the edge of the shelf) perhaps a bank of huge siphons could be placed on the floor of the gulf and hung over the edge, allowing the hypoxic zone to drain away.

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