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Phytoplankon: blooming marvellous. Norman Kuring, CC BY

Saharan dust feeds the ocean and locks away carbon too

The Saharan dust that clogged air and dirtied cars recently may seem like a nuisance, but in fact contains some essential nutrients – if, that is, you’re phytoplankton.

The dust and sand blown from Africa into the ocean provides nutrients for these microscopic ocean plants, upon which the entire marine food chain depends. The most important is iron. And although these plants only need very small quantities, iron is essential – and only supplied by dust or by other contact with land masses.

As part of a wide ranging endeavour to study changes in the open ocean and to improve our predictions of how the oceanic system may change over the coming decades, the National Oceanography Centre maintains three multidisciplinary observatories. These are observation and recording stations anchored to the ocean floor around 4,800m deep with but with sensors extending up to the surface.

Attached to these moorings are a variety of sensors and samplers some of which send data every few hours by satellite to the National Oceanography Centre in Southampton. Each is set in a very different environment, in terms of how productive the ocean around it is, how much desert dust reaches it, and the extent of carbon dioxide exchange between the air and sea.

Vital dust cycle

For example, the observatories at the Northern Oligotrophic Gyre (NOG, in the middle of the North Atlantic), and the Southern Oligotrophic Gyre (SOG, between Brazil and West Africa,) are fixed in locations right in the middle of the most unproductive, desert-like parts of the world’s oceans. But one of the crucial differences seen from comparing recordings from them is that although phytoplankton concentrations from satellite observations at each seem very similar, the amount of carbon being sequestered at NOG is about double that at SOG.

a. Richard Lampitt/NOC

The reason for this difference is the greater supply of Saharan dust reaching the NOG, although the exact mechanism for this is still under discussion.

It could be that the iron from the dust enables some specialist phytoplankton called diazotrophs to take up nitrogen from the seawater as dissolved nitrogen gas (a process which requires a lot of iron), providing a growth spurt just as fertiliser would do on crops on land. As the growing phytoplankton use this nitrogen and other nutrients they also take up carbon dioxide, as do plants on land. Another explanation could be that the dust supply itself acts as ballast, making sticky carbon-rich organic particles heavier and dragging them to the bottom where the carbon can be considered sequestered from the atmosphere for hundreds to thousands of years.

The third observatory, the Porcupine Abyssal Plain observatory (PAP, well beyond the edge of the continental shelf southwest of Ireland) is one of the most sophisticated and longest-running observatories in the open ocean. For the past 25 years an increasing number of measurements have been recorded at the site, sending back data on carbon dioxide, oxygen, plant pigment, particle and nutrient concentration by satellite every few hours. Other recordings such as time lapse photographs of the seabed and passing creatures and the downward flux of particulate matter in the deep ocean are recovered once per year when we visit the site on one of the NERC research vessels.

Creatures come for their close up at the PAP cameras. Brian Bett/NOC

Although this recent visit of Saharan dust to Britain’s south and east has caused some consternation, the supply reaching the PAP observatory has not been very great as its trajectory from the Sahara swept across the continent and not out to sea. In any case, at this time of year iron would probably not be in short supply at PAP.

Seeding the oceans

This regular supply of dust to the ocean has changed massively over timescales of 10,000 to 100,000 years, and will do so again in the future – possibly much more quickly as a result of climate change brought about by human activity. It’s of key importance to understand how the system functions and what the consequences of these changes will be on the oceans’ ability to continue to sequester carbon from the atmosphere. At present it draws up about a third of the planet’s carbon emissions, but there is considerable debate about whether this will increase or decrease in the future.

Our ambition to reduce atmospheric CO2 further has driven a debate over whether we could encourage the oceans to take up more CO2 than at present. After all, much of the ocean is short of iron nutrients, and it was suggested around 25 years ago that fertilising the ocean with iron to boost phytoplankton growth and so drawn out more carbon from the atmosphere would be inexpensive as such small quantities are required.

The jury is still out about whether this is a viable proposal, and such research has been strictly regulated due to possible unintended consequences. This is in my opinion the right way forward, although funds for carrying out this research have so far been hard to acquire. It is essential for decisions about geoengineering to be taken with a good knowledge of the benefits and costs; hopefully such research can be conducted before it is too late.

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