I wouldn;t give up on the idae so quickly
Large areas of the oceans are effectively deserys, because they have no nutrients for life to survive on. In many places, the bottleneck is the availability of iron. Add that, and plankton can gorw. Add plankton, and the rest of the oceanic food chain, which depends on it, can move in.
It might not be massively effective at long-term carbon sequestration, but on the other hand, could have a marked effect on ocean productivity. Essentially, add iron, get more fish.
Interestingly, it has been hypothesised that whales may play an important role in the cycling of iron in marine ecosystems. Because they move up and down the water column, they feed in the depths where iron is more plentiful, and excrete the waste matter nearer the surface, thus transporting nutrients. This is an important counter-argument to that which certain countries put forward for controlling whale numbers by culling in order to increase fishery reserves. It turns out the opposite may actually be the case, and allowing whale numbers to recover could lead to greater ocean fertility, and thus productivity.