I have thought about this a lot from the perspective of cheapness, and think that simply having giant teathered robot crawlers pump ocean sediment back up into the top layers of the water collum and inject any missing nutrients into the naturaly found mix. Off shore areas that are shallow enough for wind, oil and other development, could then provide the nessesary power and servicing platforms. so , some sort of power source, undersea electrical cables to each unit, a hose that goes up to a submerged unit that has the discharge, and if needed holds additional elements for dispersal, possibly having a surface floating element will make more sense, or having tanks that sink down to the bottom crawler, and when empty are filled eith air to float them for recovery.