They don't talk about pollution, some pollution will drop off while coagulating microplastics can be much higher. The whole ocean is basically a fractionating column. Of course they are going to want to dump the salt in the bottom to complete the mass transfer loop of the upwelling water. This is going to mess up the whole thing.

Humans should be operating in closed water systems. We would have to do that anywhere else we go, we should be turning Earth into well run spaceship.