The secret is they're not new.

Trace sulphur aerosols in the stratosphere we did in the 1970s. Not on purpose, just a side effect of high-lubricity jet fuels. Lowered global temperatures by ~1 C, no effect on crop yields. Which stands to reason - no crop plants are noticably luminance-limited, no even sugar cane at any plausible level of use.

Iron enrichment of the oceans happens all the time, when a river cuts into an iron-rich deposit (pyrite especially, but also haematite under the right redox conditions), and when winds erode a desert down to an iron-rich bedrock. This effect is strongest during ice ages, and acts as an unhelpful positive feedback on climate.

Intentional enrichment of High Nutrient/ Low Chlorophyll Southern Ocean waters was done at scale in 2003-2005 austral summers as a pilot project. Plankton bloomed locally, died, and sank. No lasting effects despite extensive sampling over time and space.

Interestingly, blooms become limited by silicic acid availability at high doses, but that's just a matter of choosing the right waters and spacing out the blooms. I can find you guys the papers if anyone wants.

Open oceans have surprisingly terrible primary productivity compared to continental shelves, nowhere for critters to hide. Its really hard to make it worse, and very easy to temporarily improve it.

Policy response: crickets chirp.

Solving the problem "wrong".