For years, ocean iron fertilization has been considered both a promise and a curse. We know that iron is essential for phytoplankton and mixoplankton, the tiny organisms that are responsible for nearly 50% of Earth’s oxygen production and form the base of the marine food web. Without iron, chlorophyll cannot form. Without chlorophyll, plankton cannot photosynthesize, grow, or support the ocean’s entire biological carbon pump.
But it is time to shift our mindset. The issue is not whether we should fertilize the ocean. It is whether we should replenish what has been removed.
The oceans are losing iron faster than nature can replace it
Commercial fishing removes up to 30,000 tonnes of iron from the ocean every year (Moreno and Haffa 2014). This corresponds to the iron contained in approximately 77 million tonnes of fish and marine species harvested annually.
To make this tangible: 30,000 tonnes of iron is enough iron to build ~4 Eiffel Towers. Every year.
If these animals remained in the ocean, the iron stored in their bodies would naturally circulate: when predators feed, when whales release nutrient-rich poop, and when animals die and sink (called marine snow). This recycling is an essential part of the ocean’s nutrient engine.
Natural iron delivery is declining in deep seas
Iron also enters the ocean’s deep areas through sand storms, volcanic eruptions or forest fires. When the dust, sand or ash reaches the nutrient-poor areas in e.g. the North Atlantic, it sparks plankton blooms that increase carbon sequestration. But climate change has changed wind patterns that have reduced the amount of dust that crosses the Atlantic. This means less natural iron reaches the regions that need it most.
Meanwhile, coastal nutrient runoff seldom reaches the open ocean. The deep seas remain chronically iron-limited.
In other words: Iron is abundant near coastlines, but not in the deep seas. It’s the deep seas where most of the world’s plankton live that are increasingly starved of iron (e.g. Aflenzer et al. 2023).
Why “fertilization” is the wrong term in today’s world
The legacy term fertilization suggests adding something extra. But the truth is very different: humanity has already removed iron from the ocean system. This has happened through industrial fishing, whaling, and pollution.
What is needed is not “growth stimulation” or “fertilizing the ocean,” but replenishing what is missing.
Ocean iron replenishment uses microgram levels of iron, comparable to a grain of sand in an Olympic pool. This is far below anything we would call ‘fertilization’ on land.
This is about restoring a broken cycle, not forcing the ocean to produce more than it naturally would.
Biomimicking solution with cascading benefits
When done in the right place, at the right time, iron replenishment can:
- support plankton growth
- maintain and restore marine biodiversity
- strengthen the biological carbon pump
- increase surface-water alkalinity and reduce local acidification
- enhance cloud-forming aerosols and help cool overheated regions
- offer conditions for abundant and diverse marine life to return
Given the scale of ocean degradation, doing nothing is not neutral. It is accelerating the harm.
Why we choose the term replenishment
To summarize, the paradigm shift is simple yet profound: We are not “fertilizing” the ocean. We are replenishing a vital element that human activity has depleted.
At Oceanry, we intentionally will adopt the term ocean iron replenishment because it better reflects the science, the ethics, and the purpose: restoring natural nutrient pathways so the ocean can heal itself.
References and more reading:
Aflenzer, H., Hoffmann, L., Holmes, T. et al. (2023). Effect of dissolved iron (II) and temperature on growth of the Southern Ocean phytoplankton species Fragilariopsis cylindrus and Phaeocystis antarctica. Polar Biology 46, 1163–1173 (2023). https://doi.org/10.1007/s00300-023-03191-z
Alldredge, A. L., & Silver, M. W. (1988). Characteristics, dynamics and significance of marine snow. Progress in Oceanography, 20(1), 41–82. https://doi.org/10.1016/0079-6611(88)90053-5
Boyd, P. W., Jickells, T., Law, C. S., Blain, S., Boyle, E. A., Buesseler, K. O., … Turner, S. (2007). Mesoscale iron enrichment experiments 1993–2005: Synthesis and future directions. Science, 315(5812), 612–617. https://doi.org/10.1126/science.1131669
Coale, K. H., Johnson, K. S., Fitzwater, S. E., Gordon, R. M., Tanner, S., Chavez, F. P. … Kudela, R. (1996). A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean. Nature, 383, 495–501. https://doi.org/10.1038/383495a0
De La Rocha, C. L., & Passow, U. (2007). Factors influencing the sinking of POC and the efficiency of the biological carbon pump. Deep-Sea Research Part II, 54(5–7), 639–658. https://doi.org/10.1016/j.dsr2.2006.11.004
Iversen, M. H., & Ploug, H. (2010). Ballast minerals and the sinking carbon flux in the ocean: carbon export mediated by mineral ballasting. Marine Ecology Progress Series, 416, 47–66. https://doi.org/10.3354/meps08788
Moreno, A. R., & Haffa, A. L. M. (2014). The impact of fish and the commercial marine harvest on the ocean iron cycle.PLoS ONE, 9(9), e107690. https://doi.org/10.1371/journal.pone.0107690
Ratnarajah, L., Lea, M.-A., & Law, R. (2014). The biogeochemical role of baleen whales and krill in the Southern Ocean.PLoS ONE, 9(9), e107210. https://doi.org/10.1371/journal.pone.0114067
Silsbe, G.M., Fox, J., Westberry, T.K. et al. (2025). Global declines in net primary production in the ocean color era. Nature Communications 16, 5821. https://doi.org/10.1038/s41467-025-60906-y
Turner, J. T. (2015). Zooplankton fecal pellets, marine snow, phytodetritus and the ocean’s biological pump. Progress in Oceanography, 130, 205–248. https://doi.org/10.1016/j.pocean.2014.08.005
Tagliabue, A., Bowie, A. R., Frew, R., Martin, A., & Strzepek, R. (2017). The integral role of iron in ocean biogeochemistry. Nature Geoscience, 10, 141–151. https://doi.org/10.1038/ngeo2910
