Dust carried by wind from drought-stricken southern Africa caused a bloom of marine phytoplankton off the southeast Madagascar coast from November 2019 through February 2020, found a new study.
Phytoplankton significantly influence Earth's climate and carbon cycles. Similar to terrestrial plants, they possess chlorophyll and harness sunlight for energy via photosynthesis. During this process, they generate oxygen and absorb a substantial amount of carbon dioxide.
Drylands comprise 41 per cent of the global land area, are vulnerable to extreme drought and are currently at risk of expanding desertification. Vegetation loss in dry regions promotes the wind-driven mobilisation of soil particles, enhancing atmospheric dust emissions.
Dust particles often contain essential nutrients like iron, nitrogen, and phosphorus. These can travel long distances through the atmosphere and settle on the ocean's surface, helping to relieve nutrient shortages and boost primary productivity in the ocean.
The relationship among desertification, dust emissions and ocean fertilisation is not understood well. The study shed light on the role of climate warming in creating complex interactions among land, atmosphere and ocean systems.
The study published in the journal PNAS Nexus on October 1, 2024 showed that dust emitted from southern Africa was transported and deposited into the nutrient-limited surface waters southeast of Madagascar, which stimulated the strongest phytoplankton bloom of the last two decades during a period of the year when blooms are not expected.
Key sources of dust sources are Etosha and Makgadikgadi Pans in Namibia and Botswana, respectively, pans and ephemeral rivers in the coastal Namibian desert, as well as the southwestern Kalahari Pan belt.
Southern Africa is a climate change hotspot, with projections indicating rising temperatures and increasing aridity.
Prolonged and extreme multi-year droughts have occurred in southern Africa over the last decade. This trend culminated in the austral spring (the period from September to November) of 2019, which was among the driest in the last 40 years for parts of Zimbabwe, Namibia, Botswana and South Africa. Around 90,000 livestock were lost in Namibia and over 11 million people encountered remarkable levels of food insecurity during this period.
From November to December of 2019, strong, positive dust aerosol optical depth anomalies were present over parts of Namibia, Botswana and western South Africa.
The study results demonstrate that multiple potential sources of iron-rich dust aerosols over southern Africa were active during the bloom period.
The 2019-20 South–East Madagascar Bloom was remarkable with regards to both its timing and magnitude.
According to the study, the bloom initiated 2.5 months earlier and lasted three weeks longer than previous Madagascar blooms in the austral summer (when a majority of the year’s precipitation falls).
In this study, the researchers used standardised anomalies of dust aerosol optical depth from the Copernicus Atmosphere Monitoring Service (CAMS) and in-situ coarse mode aerosol optical depth retrieved by a nearby Aerosol Robotic Network station to quantify the density of atmospheric dust aerosols over the Madagascar area through time.
Such dust-driven ocean fertilisation events can become more frequest across the world with intensifying droughts due to climate change, according to the authors of the study.
These events can also significantly alter carbon dioxide uptake by the oceans, they added.