Phytoplankton are tiny, plant-like organisms found in the world’s oceans. While they are small, these microorganisms underpin oceanic food webs, are a source of blue carbon storage, and produce oxygen. One of the least studied influences on these microscopic plants is dust from the Saharan Desert, which can travel thousands of miles to fertilize distant oceans.
Saharan dust storms
The Sahara Desert is one of the largest sources of airborne dust in the world, generating enormous dust storms that can last for days and cover thousands of miles. These dust storms result from strong winds that pick up loose soil particles, which are then carried high into the atmosphere. Once airborne, this dust can be transported across continents, with significant quantities reaching the Atlantic Ocean, parts of Europe, and even crossing as far as the Caribbean and South America.
Natural ocean fertilizer
The transport of debris from the world’s deserts as atmospheric dust that eventually deposits into the ocean creates what is known as a “natural ocean fertilization event” that can trigger phytoplankton blooms. A phytoplankton bloom is a short period of time during which the population of these microorganisms increases dramatically in response to an increase in nutrients.
Dust from the Sahara contains essential nutrients, including iron and phosphorus, that are critical for phytoplankton growth. In many regions of the ocean, especially the open ocean, these nutrients are scarce. When Saharan dust is deposited into these nutrient-poor waters, it provides a an infusion of minerals that can stimulate phytoplankton blooms.
The sudden increase in phytoplankton provides a food source for zooplankton, tiny marine animals that feed on these microorganisms. In turn, larger marine animals like fish, whales, and seabirds benefit from the increased availability of food. This chain reaction of productivity supports entire marine food webs.
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In addition to supporting marine life, phytoplankton blooms play an important role in the global carbon cycle. Through the process of photosynthesis, phytoplankton absorb large amounts of carbon dioxide from the atmosphere. Some of this carbon is eventually transported to the deep ocean when phytoplankton die and sink, effectively removing carbon from the atmosphere and storing it in the ocean.
Atmospheric dust supports 4.5% of ocean carbon production
Research published in the journal Science in 2023 analyzed the contribution of atmospheric dust to phytoplankton populations in the world’s oceans. A team of researchers from Oregon State University looked at 14 years of data on dust deposition and satellite measurements of ocean color, which helps indicate the presence of chlorophyll—a marker of phytoplankton activity.
To do this, satellite data from Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite between 2003 and 2018 was analyzed for changes in ocean color. The distinct signatures in ocean color can reveal the timing, location, and the health and abundance of phytoplankton blooms.
The geographic information about phytoplankton blooms was then overlayed with information about Saharan dust deposition events collected from NASA’s Goddard Earth Observing System (GEOS) model in order to see correlations between the two phenomenon.
The findings showed that dust, even in small amounts, has a widespread impact on phytoplankton by improving the health of these microscopic animals and sometimes increasing their biomass, particularly in regions where iron is limited.
The research identifies two primary effects: in low-latitude regions with stable ocean conditions, dust mainly improves the health of phytoplankton, allowing them to photosynthesize more efficiently. In higher-latitude, seasonally varying regions, dust deposition often leads to an increase in phytoplankton biomass.
Overall, the study estimates that dust supports about 4.5% of the global ocean’s carbon production, with much higher regional contributions, especially in areas downwind of large dust sources.
Desertification, drought, and increased phytoplankton blooms
An international team of researchers looked at the connectivity of desertification, dust emissions, and ocean fertilization. The research, published in PNAS Nexus, analyzed how dust from Southern Africa was carried over and deposited into nutrient-poor waters southeast of Madagascar, causing an unusually strong phytoplankton bloom during a time of year when such blooms are rare.
In late 2019, dust from Southern Africa, rich in nutrients like iron, was transported to nutrient-limited waters southeast of Madagascar, triggering the largest phytoplankton bloom in the region in the last two decades. This bloom occurred earlier and lasted longer than usual, reaching unprecedented levels of chlorophyll concentration. The 2019/2020 bloom was driven by the relief in iron stress on phytoplankton, as confirmed by satellite data showing decreased chlorophyll fluorescence quantum yield, a marker of reduced nutrient stress.
Dust emissions from regions like the Kalahari and Namib deserts are increasingly contributing to oceanic nutrient enrichment, especially in iron-limited waters. The authors of this study on aeolian dust from South Africa hypothesize that rising temperatures, increased dryness, and higher dust emissions in Southern Africa are likely to increase such phytoplankton blooms.
Increased blooms fueled by dust emissions from regions like the Kalahari and Namib deserts could enhance the ocean’s ability to absorb CO2, providing a potential feedback mechanism that influences atmospheric carbon levels and the global carbon cycle.
References
Gittings, J. A., Dall’Olmo, G., Tang, W., Llort, J., Jebri, F., Livanou, E., Nencioli, F., Darmaraki, S., Theodorou, I., Brewin, R. J. W., Srokosz, M., Cassar, N., & Raitsos, D. E. (2024). An exceptional phytoplankton bloom in the southeast Madagascar Sea driven by African dust deposition. PNAS Nexus, 3(10), pgae386. DOI: 10.1093/pnasnexus/pgae386
Westberry, T. K., Behrenfeld, M. J., Shi, Y. R., Yu, H., Remer, L. A., & Bian, H. (2023). Atmospheric nourishment of global ocean ecosystems. Science, 380(6644), 515-519. DOI: 10.1126/science.abq5252
