But will we be able to eat the different organisms that will thrive in this environment? What happens when today's staple crops no longer produce the yields we need to sustain our civilization?
I think it's much more plausible that we'll go the route Asimov predicted in "Caves of Steel" - genetically modified and processed yeasts, which are much more calorie dense than seaweed, and much better suited to the sort of industrial process that feeding 10+ billion people requires.