Satellite observations of the impact of dust in a low-nutrient, low-chlorophyll region: Fertilization or artifact?

The coupling between dust aerosols and phytoplankton concentrations in the Mediterranean Sea, a low‐nutrient, low‐chlorophyll region, is examined at different timescales using Sea‐viewing Wide Field‐of‐view Sensor observations (1998–2002). Aerosol optical thickness (AOT) and chlorophyll (MCHL) were used as proxies for dust aerosol and phytoplankton biomass, respectively. The AOT data was qualified using quasi‐true color images in order to ascertain the presence of dust. Strong positive correlations were found between AOT and MCHL on weekly but not on seasonal timescales. However, weekly analyses cannot distinguish between real phytoplankton response and artifacts due to residual dust in the atmosphere or water. Daily time series of AOT and MCHL, for single‐dust events, and their temporal cross‐correlation function were analyzed. Apparent AOT‐driven MCHL increases principally occurred within 0–2 days and most cross correlations were significant at zero lag. In contrast, significant negative or positive correlations at lag greater than 2 days were very few, indicating no compelling evidence that dust enhancement of phytoplankton growth is significant and that the response at near‐zero lags is an artifact of the satellite data processing. Our analysis demonstrates that the dust fertilization does not play a significant role in the sustainment of the phytoplankton dynamics in the Mediterranean Sea.