Microbial activity is a fundamental component of oceanic nutrient cycles. Photosynthetic microbes, collectively termed phytoplankton, are responsible for the vast majority of primary production in marine waters. The availability of nutrients in the upper ocean frequently limits the activity and abundance of these organisms. Experimental data have revealed two broad regimes of phytoplankton nutrient limitation in the modern upper ocean. Nitrogen availability tends to limit productivity throughout much of the surface low-latitude ocean, where the supply of nutrients from the subsurface is relatively slow. In contrast, iron often limits productivity where subsurface nutrient supply is enhanced, including within the main oceanic upwelling regions of the Southern Ocean and the eastern equatorial Pacific. Phosphorus, vitamins and micronutrients other than iron may also (co-)limit marine phytoplankton. The spatial patterns and importance of co-limitation, however, remain unclear. Variability in the stoichiometries of nutrient supply and biological demand are key determinants of oceanic nutrient limitation. Deciphering the mechanisms that underpin this variability, and the consequences for marine microbes, will be a challenge. But such knowledge will be crucial for accurately predicting the consequences of ongoing anthropogenic perturbations to oceanic nutrient biogeochemistry.

Air pollution creates high Fe solubility in pyrogenic aerosols, raising the flux of biologically essential Fe to the oceans.

Two types of samples were used to chemically characterize the Saharan end‐member: fine fractions of surface soil samples collected in Northern Africa and particulate phases of typical Saharan rains. Since the concentrations measured in the particulate phase of the Saharan rains were corrected from the dissolution losses in rainwater, these particles were considered to be representative of the transported Saharan dust before being blended into rainwater. Al, Fe, P, and Pb were analyzed: except for lead, the chemical composition of the transported Saharan dust was more homogeneous than the composition of individual soils. As confirmed by the air mass back trajectories, the higher level of homogeneity of the aerosol is partly due to the fact that a dust event affects a large area of the Saharan desert, and the composition of the particles reflects the average composition of the eroded areas. Pb concentration in the transported dust reflected an anthropogenic fraction. By using Pb/Al measurements from the soils it was shown that a typical Saharan rain event with no mixing with air masses from Europe appears to be very rare in the Mediterranean environment. The following values are proposed to characterize the Saharan dust end‐member: Al (%) = 7.09 ± 0.79; Fe (%) = 4.45 ± 0.49; P (%) = 0.082 ± 0.011; Pb (ppm) = 24 ± 9. This study suggests that the [element/Al or Fe)] ratio is also useful to characterize the Saharan end‐member as they are very homogeneous for the two sample types. Saharan dust represents a potential source of nutrients (P, Fe) for the Mediterranean water. Indeed, it accounts for ∼30–40% of the total atmospheric flux of phosphorus in the western Mediterranean, and it governs the biogeochemical cycle of iron being the main source of dissolved iron in the western Mediterranean waters.

A Saharan soil, considered as a proxy for Saharan aerosols, was used to perform a series of dissolution experiments: various amounts of Saharan soil were exposed to ultrapure water and seawater for varying lengths of time. The concentration of phosphate released was proportional to the amount of dust introduced. In the case of Saharan events associated with a significant amount of rain, the main dissolution of phosphorus will occur in the air column; for Saharan events associated with only few drops of rainwater, the main dissolution will occur in the surface seawater. Saharan dust represents a source of phosphate to the surface water and may play a role in biological activity, especially during the oligotrophic period. In the western Mediterranean in oligotrophic conditions, biological production is P‐limited, and the atmosphere becomes the main pathway of nutrients to the surface mixed layer. At the scale of the oligotrophic season, the input of “Saharan dissolved inorganic phosporus (DIP)” are negligible compared with the new production integrated over the productive layer. At the event timescale, the production induced by “Saharan DIP” can represent up to 15% of the integrated new production and up to 14% of the total primary production in the mixed surface layer. These inputs of atmospheric DIP would promote fixation of atmospheric N₂ that, in return, may enhance the new production in the surface layer.