Scientists have found new evidence that desertification, potentially linked to global warming, leads to large amounts of nutrient-rich dust landing in the sea, causing ocean algae to grow rapidly. Biological oceanographer John A. Gittings and an international group of researchers have found an example of this phenomenon in the Indian Ocean south-east of Madagascar.
They analysed satellite images that showed how the colour of the sea in that area had changed over the years. Phytoplankton (microscopic algae found in the oceans) affect the colour of the water when they grow rapidly in response to higher levels of nutrients - including iron that's found in dust. The researchers found that drought in southern Africa's drylands had caused the strongest phytoplankton bloom in about 27 years, south-east of Madagascar.
What is a phytoplankton bloom?
Millions of tiny organisms called phytoplankton live in every aquatic environment. They are critical components of the Earth system - phytoplankton are estimated to produce about 50% of Earth's oxygen.
They have a crucial role in the global carbon cycle.
Phytoplankton also form the foundation of marine food webs. They provide an essential food source for organisms like zooplankton (tiny marine animals), which in turn sustain larger species such as fish and whales. This directly benefits fisheries and human communities that rely on them.
Just like land plants, they grow more in certain seasons. When light, nutrients and other conditions, such as temperature, are at the best level for phytoplankton, they can rapidly multiply and flourish. This leads to the development of a phytoplankton bloom.
Phytoplankton cells contain chlorophyll, a green pigment that affects the colour of oceanic surface waters. This can be detected from space using specialised satellite ocean colour sensors. It is difficult to say exactly how many phytoplankton cells made up the bloom. However, this bloom off the Madagascar coast covered an estimated 2,000km².
During the spring/summer of 2019/2020 in the southern hemisphere, an exceptional phytoplankton bloom was discovered in the Indian Ocean south-east of Madagascar. This was during a period of the year when blooms are not expected. The bloom began in November 2019 before diffusing into the Mozambique channel and broader Madagascar basin in December 2019 and January 2020.
What were the causes of this?
To determine the cause, we conducted an in-depth analysis of what's known as Lagrangian trajectories in this part of the ocean. This is like following the path of an object as it moves through space. Imagine you're watching a leaf floating on a river: instead of just looking at the river's flow, you focus on the leaf's journey, tracking where it goes and how it moves over time.
We analysed the movement of water parcels (a mass of water of similar properties that can be tracked). This allowed us to investigate whether nutrients important for phytoplankton growth had originated from the east coast of Madagascar and the south-east Africa continental shelf. We then explored whether the settling of dust from the air or atmosphere could have been what had fertilised the ocean.
We found that in the 60 days prior to the bloom beginning, about 75% of water parcels we tracked to the bloom area did not originate from nearby land masses.
The bloom near Madagascar was caused by nutrient-rich dust that blew from drought-stricken drylands in the western parts of southern Africa. The Etosha and Makgadikgadi salt pans in Namibia and Botswana, pans and ephemeral rivers in the coastal Namibian desert, as well as the south-western Kalahari pan belt, are major suppliers of dust to the Southern Ocean and its outer edges.
Carried over long distances by wind, the dust was deposited into the nutrient-limited surface waters south-east of Madagascar through intense rainfall events. Blooms of this magnitude are rare. But rising air temperatures, increasing dryness, and higher dust emissions in southern Africa suggest that such events could become more common in the future.
How was ocean and marine life affected?
The effects of the 2019/2020 bloom on the broader marine food web in waters south-east of Madagascar still need to be fully investigated. But this abundant food supply could have potentially boosted populations of zooplankton and fish species in the region.
The oceans play a crucial role in absorbing carbon dioxide from the atmosphere, making them essential for climate regulation. During the 2019/2020 bloom, the region acted as a significant carbon sink because of the high rates of photosynthesis occurring. (Photosynthesis is the process by which plants and algae use sunlight to produce food; in doing so, they absorb carbon dioxide from the air.) As the phytoplankton thrived during the bloom, they took in large amounts of carbon dioxide.
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Read more: Inside the world of tiny phytoplankton - microscopic algae that provide most of our oxygen
Phytoplankton blooms like this one are uncommon. This one was the first of its kind since the beginning of the satellite ocean colour data record - in other words, the first in about 27 years.
Current trends in air temperatures, aridity and dust emissions in southern Africa suggest that such events could become more probable in the future. Together with recent findings on ocean fertilisation by drought-induced megafires in Australia, our results point towards a potential link between global warming, drought, aerosol emissions and ocean blooms.
Dust that fertilises the ocean and leads to an increased number of phytoplankton blooms could help remove carbon from the atmosphere. This will only be confirmed by further research.
John A Gittings, Post-doctoral Researcher: Department of Biology, University of Athens
