Volcanic ash falling on the ocean can trigger significant blooms of phytoplankton, the microscopic marine algae that form the base of the marine food web, according to multiple studies analyzing natural events and laboratory experiments. The findings confirm that ash, rich in iron and other essential micronutrients, acts as a powerful, albeit episodic, fertilizer for vast ocean regions that are otherwise nutrient-limited, with important implications for the global carbon cycle.
This process of natural fertilization is particularly effective in what are known as High-Nutrient, Low-Chlorophyll (HNLC) zones, where the growth of phytoplankton is held in check by a lack of bioavailable iron. While nutrients like nitrogen are plentiful in these areas, the scarcity of iron prevents large-scale growth. Volcanic eruptions provide a dramatic, large-scale infusion of this missing ingredient, setting the stage for rapid and widespread increases in marine primary productivity. By studying these events, scientists gain critical insight into marine biogeochemical cycles and how sudden nutrient inputs can alter ocean ecosystems and potentially influence climate.
An Unlikely Source of Ocean Life
The open ocean is often compared to a desert, with enormous stretches where life is limited by the availability of key resources. For marine phytoplankton, the most common limiting nutrient is nitrogen. However, in major oceanic regions like the Southern Ocean and the North Pacific, nitrogen is abundant, yet life fails to thrive. For decades, this paradox puzzled oceanographers until the “iron hypothesis” emerged, identifying the lack of the micronutrient iron as the critical missing factor. Iron is essential for photosynthesis and other metabolic processes in phytoplankton.
The primary natural source of iron for these remote HNLC regions is atmospheric dust blown from continents. This supply is slow and diffuse. Volcanic eruptions offer a starkly different delivery mechanism: a rapid, concentrated deposition of nutrient-rich material over a potentially huge area. The ash ejected from certain types of volcanoes, particularly those in subduction zones, is a potent source of iron and other trace elements that can dissolve in seawater. This makes volcanic fallout a powerful natural mechanism for stimulating ocean productivity on a scale rarely seen with other natural events.
Evidence from a Natural Laboratory
A clear demonstration of this phenomenon occurred in August 2008 following the eruption of the Kasatochi volcano in the Aleutian Islands. The eruption ejected a massive plume of ash and sulfur dioxide roughly 11,000 meters into the atmosphere. Fortuitously, the ash cloud was caught in a storm system that swirled over the North Pacific, distributing the material over an unusually large area of the ocean, a region known to be iron-limited.
In the weeks that followed, scientists using satellite data witnessed one of the largest phytoplankton blooms ever recorded in the region. The concentration of chlorophyll, the pigment used by phytoplankton for photosynthesis and a key indicator of their abundance, increased by 150 percent across a vast swath of the ocean downwind from the volcano. This natural experiment provided unambiguous evidence linking the volcanic ash deposition directly to a massive biological response. It was a clear confirmation that a single volcanic event could fertilize the ocean on a basin-wide scale.
Tracking the Ash and the Bloom
The scientific effort to connect the Kasatochi eruption to the bloom involved multiple lines of evidence. Satellites first tracked the aerosol particles in the atmosphere, watching the ash plume spread across the ocean. Subsequently, ocean-color satellites detected the dramatic spike in chlorophyll concentrations in the surface water directly beneath the path of the ash cloud. The case was strengthened by data from oceanographic buoys, gliders, and a Canadian research vessel that happened to be in the area, all of which captured elements of the event. This combination of remote sensing and direct in-water measurements allowed researchers to conclusively link the cause—the ash—with the effect—the phytoplankton bloom.
The Chemistry of Ash Fertilization
The effectiveness of volcanic ash as an ocean fertilizer lies in its chemical composition and the speed at which its nutrients become available to marine life. Laboratory experiments have shown that ash from subduction zone volcanoes begins releasing an array of nutrients, most critically iron, almost immediately upon contact with seawater. This process occurs on a minute-scale, far faster than previously understood. This rapid release means that phytoplankton in low-iron areas can begin utilizing the new nutrients within days, leading to the kind of explosive growth observed after the Kasatochi eruption.
Further studies incubating seawater with volcanic ash confirmed these findings. In experiments using water from the Southern Ocean, another major HNLC region, 18 of 23 incubations treated with ash showed significantly more phytoplankton growth and higher chlorophyll levels than control samples. This direct experimental evidence supports the observations from natural events, confirming the potent fertilizing power of volcanic material.
Beyond Just Iron
Interestingly, some experiments have revealed that volcanic ash can be an even more effective fertilizer than solutions containing only iron. In some Southern Ocean water samples, incubations with volcanic ash produced more phytoplankton growth than samples to which only iron was added. This suggests that ash provides other limiting micronutrients in addition to iron. The findings point to a potential “co-limitation” where the lack of multiple trace elements, such as manganese, might be restricting phytoplankton growth. By providing a cocktail of these essential micronutrients, volcanic ash can unlock a level of productivity that adding iron alone cannot achieve.
Broader Climate Implications
The stimulation of massive phytoplankton blooms has direct relevance to the global carbon cycle and climate. Through photosynthesis, phytoplankton draw vast amounts of carbon dioxide (CO2) from the atmosphere. When these organisms die, a portion of them sink into the deep ocean, carrying their carbon with them. This process, often called the biological carbon pump, is a critical natural mechanism for sequestering atmospheric CO2. Therefore, widespread ocean fertilization by volcanic ash could temporarily enhance this carbon drawdown. Over geological timescales, periods of intense volcanic activity may have played a significant role in modulating atmospheric CO2 levels and global climate.
A Complex and Evolving Picture
While the link between volcanic ash and phytoplankton blooms is now well-established, the complete ecological and biogeochemical consequences are still being explored. Research has shown that the biological response is not limited to photosynthetic organisms. The addition of ash can first stimulate the growth of heterotrophic bacterioplankton—microbes that consume organic matter—which are then followed by the larger phytoplankton bloom. This indicates that volcanic ash can alter the entire microbial community structure, with cascading effects throughout the marine food web.
Furthermore, while the blooms are effective at absorbing CO2, the net impact on atmospheric carbon levels from a single eruption is not always straightforward. The efficiency of the biological pump can vary, and not all the carbon absorbed by the bloom is successfully exported to the deep ocean. Some geoengineering proposals have suggested artificially fertilizing oceans with iron to combat climate change, but events like the Kasatochi eruption serve as a reminder that the outcomes of such large-scale interventions are complex and not fully predictable. These natural experiments provide invaluable data for refining climate models and understanding the intricate connections between geology, biology, and chemistry in the Earth system.
