Dust, pollution, and phytoplankton..

Our recent study shows that acid processing of mineral dust increases bioavailable P flux to the ocean by 120%

Give me a quick run down…

Phosphorus (P) is an essential requirement for life. Natural sources of P on land are from rock weathering and fertilizers. By contrast over the open ocean, the major source of P is from falling dust. However, less than 10% of the P in dust is automatically available to phytoplankton for growth, a percentage we call bioavailable. Therefore, changes to the supply of bioavailable P to oceans can have considerable impacts on marine ecosystems and the global carbon cycle. Previous work shows acid processes in the atmosphere can convert nonbioavailable minerals to bioavailable P. In our previous study a simple relationship between acid in the atmosphere and bioavailable P production was found. In this study the new relationship, together with global soil data maps on the amount and type of P in dust, was incorporated into a global aerosol transport model, which predicts where dust and acid interact. Our results show atmospheric acid processing of dust is particularly important in the Mediterranean Sea, North Atlantic Ocean, northwest Pacific Ocean, and the Indian Ocean. As a result, atmospheric acid pollution increases the amount of oceanic plant growth and reduces the quantity of atmospheric anthropogenic carbon dioxide.

Read on for more detail…

Phytoplankton are tiny photosynthesising organisms that live in the ocean and form the base of the aquatic food web. In order to grow they require several key elemental nutrients: nitrogen, iron, and phosphorus. Additionally, these nutrients need to be in a form that they can be readily absorbed and used – we refer to this as a bioavailable form.

Nitrogen, making up 78% of air, gets dissolved into the surface water, but in the vast majority of ocean waters iron (Fe) and phosphorus (P) are harder to get hold of. Previous studies have found that an important source of Fe and P comes from mineral dust that has been transported high in the atmosphere from desert regions, such as the Sahara and Kalahari deserts in Africa, and the Gobi desert in Asia. However, there is still a problem; once the Fe and P are deposited into the ocean they are not automatically bioavailable and may simply fall through the water column. Studies focusing on Fe in mineral dust have found that as the dust is transported through the atmosphere the acids  that are collected (from pollution and fires) are able to dissolve the Fe into a form that is bioavailable.

So, more acid in the atmosphere = more bioavailable Fe = more phytoplankton. What about phosphorus? Does the same process occur? This is what our research at Leeds University aimed to find out.

Dr Anthony Stockdale made the first steps by taking various samples of dust and mixing them with known quantities of acid in a series of laboratory experiments. The study, published in PNAS in 2016, can be found here. The experiments showed two important things: the first was that the majority of P within dust is present in the form of insoluble apatite (which is similar to what teeth are made of) and a labile form that is already bioavailable. The second finding was that when acid was introduced the apatite dissolved to produce a bioavailable form. To reproduce this process we need to know three things: the amount of apatite, the amount of calcium carbonate (which also reacts with acid), and the amount of acid available throughout the dust particle’s journey.

So, dust already contains some bioavailable P (labile P) but this can be significantly increased through interaction with acid. The next question is: do dust particles actually collect enough acid to dissolve the apatite?

To answer this question we used a global aerosol model called GLOMAP to simulate a year’s worth of emission, transport, and deposition of dust from across the globe. The acid gases in the atmosphere, also simulated by GLOMAP, included sulphuric acid and nitric acid. These acids arise from natural processes but are significantly increased via air pollution and biomass burning.

P_paper_fig1
Figure taken from Herbert et al., 2018, GBC

To get the right amount of apatite at each location a global dataset of soil properties was used, and a second one for the calcium carbonate. The resulting maps used, shown to the left, clearly show that different regions of the earth have very different properties. This is important because it tells us that some areas may be more sensitive to acid than others.

The main results of the study are shown below; this includes the total amount of bioavailable P deposited to the surface, the amount of dust, and the amount of acid collected by the dust. The Sahara desert and Gobi desert emit vast quantities of dust and this is transported thousands of miles before it finely falls, or is rained out of, the atmosphere. The central Atlantic Ocean is a dominant sink for the dust, as is the northwest Pacific Ocean. Deserts in South America, south Africa, and Australia supply dust across the Southern Ocean and southern reaches of the Atlantic and Pacific Oceans.

P_paper_fig2
Figure taken from Herbert et al., 2018, GBC

Acid associated with the dust occurs primarily over regions with high rates of air pollution such as Europe, N America, and China, and also regions with high rates of biomass burning, such as central Africa.

The amount of P that is deposited in a bioavailable form shows a strong resemblance to the dust deposition. So, areas with the highest rates of dust deposition coincide with the regions of highest bioavailable P deposition.

Globally, the labile P and acid-dissolved P result in 31 Gg-P per year being deposited to the ocean; the acid-dissolved P made up 55% of this value which demonstrates the importance of this process in the atmosphere. The Mediterranean Sea, North Atlantic Ocean, and North Pacific Ocean were identified as particularly important regions for dust-borne bioavailable P deposition. The bioavailable P input to the ocean is estimated to potentially sequester an additional 1.3 Tg of carbon per year, with 0.7 Tg from the acid dissolution.

P_paper_fig3
Figure taken from Herbert et al., 2018, GBC

The contribution from each source of bioavailable P in dust is shown here (top and middle).

There is quite a distinct pattern: close to the desert sources the labile fraction almost exclusively provides the bioavailable P to the surface, whereas outside of the main plume outflow the dissolution of apatite almost exclusively provides the bioavailable P.

The bottom figure shows the percentage of deposited dust P that is in a bioavailable form, which ranges from ~10 to ~50%. In similar modelling studies it is generally assumed that this percentage is globally constant, with values between 10 and 15% commonly used. This study shows that although the labile fraction of dust P is globally constant at ~10%, the acid-dissolved pool increases the mean bioavailability over oceans to 22% with considerable regional variation: Pacific Ocean (42%), Atlantic Ocean (18%), Indian Ocean (20%), and Mediterranean (15%).

Our study confirmed the importance of acid processes in the atmosphere in increasing the flux of bioavailable P to the global ocean. The effect is spatially and temporally variable, and it is suggested that increased bioavailable P can result in regionally important changes in biogeochemical processes such as nutrient limitation, nitrogen fixation rates, and carbon uptake.

More acid in the atmosphere = more bioavailable Fe and P = more phytoplankton

You can read the full article by following this link.

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