We know that outflows from rivers and dust clouds from deserts fertilize the oceans, feeding the plankton that everything else feeds on. (Dust clouds also fertilize the land.) I hadn't really groked how important this is to the Carbon cycle. Plankton are a significant sink for atmospheric Carbon, which means that dust storms promote Carbon sequestration and cool the planet.
It is easy enough to see major dust storms and the giant plankton blooms that follow. But it has been difficult to estimate how much dust contributes to plankton overall. There is always dust falling from the air, and it can't drift a long, long way over the ocean. So the effects are potentially everywhere, even when there isn't a massive storm.
NASA has been studying how to measure this process from satellites. For one thing, this work suggests that iron is a key nutrient that comes from dustfalls. Iron is relatively rare in the oceans, so a sprinkle of iron-y dust from the sky can be an important boost for plankton. This hypothesis has been supported by experiments.
In satellite images, phytoplankton is green, so careful analysis of the color of ocean water is used to assess the amount of plankton present [2]. (I gather that there is an entire discipline studying "ocean color". : - ) )
This spring, researchers at Oregon State and UMBC report a study that correlates estimates of dust falling with ocean color over the whole planet [1].
Estimating how much dust falls where is pretty complicated. Obviously, dust is carried by winds. But where it falls depends on many factors, including the composition of the dust, precipitation, and so on. Furthermore, dust from different places carries different chemicals. Which all means that the slow, constant, but tiny rain of nutrients is quite variable from place to place and day to day.
The effects of nutrients are estimated from satellite imagery processed to reflect the level of chlorophyll at the ocean surface. This reflects the presence and health of plankton. The study shows that likely deposition from large dust clouds is followed by increased chlorophyll a few days later.
The picture is a bit complicated. Near the equator, plankton are fairly stable year round. Here, the tasty dust tends to make the plankton healthier, but not increase the amount of plankton. Near the poles, however, plankton is highly seasonal. Here, dust falls tend to produce blooms of growth.
The researchers took these estimates of the surface plankton and modelled the effects on Earth's Carbon. They find that the dust deposition contributes about 4.5% of the global "export" of Carbon (i.e., from the atmosphere into the ocean). In some places dust contributes much higher percentage, up to 40%. A modest contribution to cooling the planet, but a contribution nevertheless.
It is interesting to see that wind erosion and aerosols that are generally not good for people or the land actually have good effects for the ocean and the planet.
The researchers note that climate change will likely cause changes in all these variables in the near future. As land areas get drier or wetter, there will be changes in dust, and changes in wind patterns and precipitation will alter the delivery of dust over the oceans. Which means that the supply of nutrients from dust may increase or decrease in different places and times.
- T. K. Westberry, M. J. Behrenfeld, Y. R. Shi, H. Yu, L. A. Remer, and H. Bian, Atmospheric nourishment of global ocean ecosystems. Science, 380 (6644):515-519, 2023/05/05 2023. https://doi.org/10.1126/science.abq5252
- Sally Younger and Michael Carlowicz, How Desert Dust Nourishes the Growth of Phytoplankton at Sea, in NASA Earth Science News, May 10, 2023. https://www.nasa.gov/feature/esnt/2023/how-desert-dust-nourishes-the-growth-of-phytoplankton-at-sea
- Sally Younger and Michael Carlowicz, Researchers have found that even modest amounts of desert dust can improve the health of the ocean's microscopic, plant-like organisms, in NASA Earth Ovservatory - Image of the Day, May 13, 2023. https://earthobservatory.nasa.gov/images/151330/how-desert-dust-nourishes-phytoplankton
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