Saharan dust effects in different trophic areas off Northwest Africa

The impacts of atmospheric and deposited mineral particles of Saharan dust storms on the light field above and below the water surface were quantified using satellite measurements and model simulations. The investigations were concentrated on the photosynthetic active part of the incident solar radiation as well as the light attenuation and the euphotic depth in waters of the tropical Eastern North Atlantic. In the time period of 2003 to 2012, the Saharan dust storms were characterized by the dust aerosol optical depth derived from satellite data. Statistics of the reduction of photosynthetically active radiation by atmospheric dust were derived in areas of eutrophic, mesotrophic, and oligotrophic waters. The highest reductions of up to 45 % in the photosynthetically active radiation were observed in July and the lowest ones in the winter months between November and January. The reductions became less from on- to offshore areas due to the removal of atmospheric dust. In the 10-year data set, no systematic changes or increasing or decreasing trends could be verified in dust storms and reductions of photosynthetically active radiation. Model simulations in the water column showed that the deposited dust increased the light attenuation up to 28 % and decreased the euphotic depth up to 22 % particularly in the blue spectral range. The strongest impacts on optical water properties were found in low chlorophyll-a oligotrophic areas during strong and long-lasting Saharan dust storms with high deposition rates of small dust particles as well as long residence times and low mixed layer depths.