A few days ago, after an intense Saharan dust event over the Canary region, I have concluded that I would like to open a series of subtopics related to atmospheric deposition. Dust deposition has been a huge topic in science in the last couple of decades, and it has been growing bigger due to its complexity in understanding how it affects the marine organisms.

The Sahara Desert is exposed to heavy wind erosion and the fine particles created upon abrasion are lifted and transported long distances, mainly by the trade winds. While in the air, these particles undergo several changes, either by absorbing metals and other elements coming from industrial emissions or bush fires, or organic compounds such as pollen, plant fibers, bacteria, and viruses to name the least. In addition, atmospheric processes like rain can furtherly alter dust composition, due to a slightly acidic character.

In fact, deposition is classified as dry when particles fall into the seawater without being hydro-processed beforehand, and as wet when these have been exposed to rain during transportation and have fallen to the sea surface perhaps chemically changed.

These changes occur differently based on the transportation route the particles are subjected to and the travelling distance. Bigger particles will fall nearer the dust source and will not be atmospherically processed to the same extent as the small particles subjected to long-distance transportation. Consequently, their physical and chemical properties will have a different impact in the marine environment once deposited into the water column.

The question mark that has been the source of much curiosity and research in the science world nowadays is how much of the material absorbed on the dust surface adsorbs and biologically alters, positively or negatively, the marine life. This concept, known as bioavailability, has been the cause of many discussions to whether the adsorbed compounds could trigger higher primary productivity rates or not, and thus sequester CO₂ from the atmosphere.

It is believed that the Saharan dust falling over the Canary region positively influences marine phytoplankton communities, especially the filamentous cyanobacteria spp. Trichodesmium, when the seawater temperature and wind conditions are adequate. The reason for this might be the slightest amount of available iron (Fe), that is an essential micronutrient among the photosynthetic organisms, being present within the photosystem apparatus.

Another possibility is that organic compounds like pollen, plant fibers, bacteria, and viruses, on the dust particles are used by marine bacteria upon arrival to the surface water, and that these bacteria, once finalized their life cycle, release nutrients that are used by the phytoplankton. Thus, the fertilization of the sea and its consequent increase in primary productivity is an indirect effect of the dust.

Whether there is a community succession depending on the biotic or abiotic characteristics of the compounds found on the dust particles and released into the water column, keeps the scientists well entertained. What is believed, though, is that the phytoplankton community structure and its efficiency in CO₂ sequestration is thought to change due to the inputs of dust on a regional scale. But why, really?

Aja Trebec