Nutrients sink in the ocean. When they sink below the level of darkness, photosynthesis in plankton cannot proceed, and the nutrients are lost. The idea is to provide floating, wave-powered pumps that will pull the nutrient-laden deep water to the surface where it can fuel the biosphere. If the lit layer in the ocean is productive and alive, the atmosphere can be enriched and fish will be more plentiful.
There is but the smallest chance something like this could work, because economic support is so tenuous, if any can be found at all.
Here is a description of the floating pump:
1. A long tubular hose extends about 1000ft down (or more).
2. A floating platform keeps the top of the hose at the surface.
3. At the bottom of the hose, an anchor cable to the bottom will keep the system from drifting into high-traffic shipping lanes. The cable must be long enough to allow some drifting however, so nutrient-rich deepwater can fertilize an area of the ocean surface rather than just one spot.
4. A very simple and reliable pump mechanism on the platform pulls water up the hose as the platform rides the waves on the surface. The system will have to be very robust to withstand the ocean environment for many years without maintenance.
The pumped deepwater will have more nutrients in it than the rest of the water at the surface, and the nutrients will nourish plankton.
The amount of work done is not large, since the platform will only have to lift the water a very small distance up the hose with every ocean swell.
The platform can also serve as an instrumentation point for environmental monitoring, radio relay, and so on, but this would be a secondary function.