• Iron fertilization is the intentional introduction of iron fines to iron-poor areas of the ocean surface to stimulate phytoplankton production. This is intended to enhance biological productivity and/or accelerate carbon dioxide (CO2) sequestration from the atmosphere.
• Fertilization occurs naturally when upwellings bring nutrient-rich water to the surface, as occurs when ocean currents meet an ocean bank or a sea mount. This form of fertilization produces the world’s largest marine habitats. Fertilization can also occur when weather carries windblown dust long distances over the ocean, or iron-rich minerals are carried into the ocean by glaciers, rivers and icebergs.

Impact

• The changes happen as the iron addition stimulates a race by organisms to capitalize on the resources of sunlight and nutrients. Starting conditions, including nutrient levels and pre-existing populations of plankton, affect which organisms win out.
• Changes at this level may determine what happens to populations of larger predators such as copepods, krill, salps, jellyfish, and fishes.
• There is the chance that the overall increase in food supply could improve the state of the oceans. Fish stocks, many of which have been suffering from decades of overfishing, might actually improve—an outcome that some private companies are banking on. But the fertilized waters might just as easily favor less-useful pathways in the food web, making more jellyfish or algae, especially harmful algal blooms that could have impacts on fish, birds, and even marine mammals up the food chain.
• Another concern is the effect of iron fertilization on other dissolved nutrients in the ocean.
• To grow, phytoplankton take in these nutrients, fixing them in their tissues. So they deplete surface waters not only of carbon (the desired effect), but also of nutrients that support all of the oceans’ food webs.

Source: https://en.wikipedia.org/wiki/Iron_fertilization