Sahara dust impact on optical properties and phytoplankton development in waters of the Cape Verde region

Dr. Herbert Siegel and Dr. Thomas Ohde

The main topic in SOPRAN II was the detailed investigation of the influence of Saharan dust on magnitude and spectral distribution of solar radiation in the water column. The theoretical basics were developed to include the dust effect in ocean circulation models. The impact depends on the wavelength, the water depth, and the optical water type. Atmospheric conditions, especially the dust/cloud ratio and the cloud type were important. Dust reduced the photosynthetically available radiation (PAR) and the cloud influence on PAR. The influence was smaller in coastal than in ocean waters due to the different optical properties. In ocean waters, the spectral dust modification of PAR is compensated at a distinct water depth. Dust also modified the spectral effect of clouds on PAR. The euphotic depth decreased much more in clear than in turbid coastal waters.

The relationship between dust storms and algae blooms has been studied statistically. Methods of cross-correlation and multiple correlations established that only 5% of the surface chlorophyll variability could be ascribed to the dust aerosol optical depth. Six of 57 strong storms were connected with an increase of chlorophyll-a in the period 2000 to 2008. In the next year the relation between scattering phytoplankton blooms and dust events will be analyzed.

In future studies, results and data sets of other SOPRAN- subprojects will be implemented to improve our findings. The dust aerosol optical depth used as an indicator for dust events will be replaced by mass concentration reaching the water surface, which is measured at CVAO at Sao Vicente (IfT). The statistical analyses of the new defined dust events will be connected to statistics of absorbing and scattering phytoplankton blooms to improve the relationships between Saharan dust and algae blooms. Turbidity measurements on Gliders (IFM-GEOMAR) will be included to extract dust impact and influence on phytoplankton development, and to use the results for tuning of the optical model. The University of Heidelberg plans to study the trajectories of the dust transport and to distinguish between the different sources, which can be important for the development of phytoplankton blooms because of their mineral composition.