Atmospheric dust deposited on the sea surface appears to play an important role in feeding ocean ecosystems. Using satellite data and modelling, a study shows that the dust promotes the growth of phytoplankton, the food base for all marine organisms. The atmospheric dust thus makes a supplementary contribution to the nutrients that are transported from the depths to the surface by the mixing of the oceans. In view of climate change, however, the researchers assume that this influence will change in the future.
The oceans play an important role in the global carbon cycle. Carbon dioxide from the atmosphere dissolves in water, where it is converted into organic matter by phytoplankton during photosynthesis. Some of these sink to the sea floor, where they form one of the most important carbon sinks in the world. The mixing also brings nutrients from the depths of the ocean back to the surface, where they in turn contribute to the growth of phytoplankton. In addition to nutrients from the depths, phytoplankton also use nutrients from land, blown up by winds or thrown into the air by volcanic eruptions. However, the contribution of this atmospheric dust has so far been difficult to estimate.
Ocean color provides information about phytoplankton
A team led by Toby Westberry from Oregon State University has now quantified these influences globally for the first time. “Soil particles that are blown up into the atmosphere over land can be blown thousands of kilometers across the sea before finally being deposited on the surface where they can stimulate photosynthesis and plankton growth,” explains Westberry. “Using modern observational data, we have shown on a global scale that the nutrients that are transported elicit a response in the biology of the surface ocean.”
Using satellite images, the researchers analyzed the color of the oceans and drew conclusions about the density and health of the phytoplankton. A greenish tint indicates abundant, healthy phytoplankton populations with high chlorophyll levels. Blue water, on the other hand, is found in regions where there is little phytoplankton, which is often under-supplied with nutrients and therefore low in chlorophyll. They combined this data with modeling of dust transport and deposits. “It’s difficult to determine how much dust is deposited in the ocean because much of the deposition occurs during rainfall when satellites can’t see the dust,” explains co-author Lorraine Remer of the University of Maryland in Baltimore. “That’s why we resorted to a model.” They validated this with available observation data.
Influence dependent on latitude
It turned out that depending on the region of origin of the dust, it brings with it different proportions of the relevant nutrients iron, phosphorus and nitrogen. It therefore has different influences on the growth of photoplankton. But in addition to the composition of the dust itself, the environmental conditions at the point of arrival also play an important role. The research team found that the dust in regions close to the equator primarily improves the health of the phytoplankton, but not the quantity. “This result is consistent with the current understanding of plankton ecosystems at lower latitudes,” the team explains. Since the ecosystems in these regions are mostly quite stable, there is a tight balance between phytoplankton and its consumers. So if more phytoplankton is produced due to increased nutrient availability, the new production is quickly consumed.
“By contrast, growth conditions at higher latitudes are constantly changing throughout the seasons, and this variability provides for a stronger decoupling between phytoplankton proliferation and loss rates,” the authors said. In fact, they found that dust deposits at higher latitudes led to an increase in phytoplankton biomass.
Interaction between ocean and climate
The research team estimates that the deposition of dust on the ocean surface ensures that around 255 million tonnes of additional carbon are converted as part of the primary production of phytoplankton. This corresponds to a 4.5 percent share of the global carbon sink. “Regional variations in this contribution can be much larger, as high as 20 to 40 percent of the annual particulate carbon exported from the surface ocean,” Westberry and his team report. Thus, the atmospheric dust in the ocean helps regulate the level of CO2 in the atmosphere, which in turn is an important factor in global warming. “We expect that this connection between the atmosphere and the oceans will change as the planet continues to warm,” Westberry said. The methods established in the study can help to understand these changes.
Source: Toby Westberry (Oregon State University, Covallis, USA) et al., Science, doi: 10.1126/science.abq5252