Impacts of Saharan Dust on the Marine Environment in the Area off Northwest Africa

Abstract

The main sources of nutrient supply in the area off Northwest Africa are upwelling and Saharan dust deposition. Their influence on phytoplankton development was evaluated by different methods. Statistical analyses showed that alongshore wind stress and induced upwelling were most significantly responsible for surface Chl-a variability mainly in winter and spring with delay of up to 16 days. Only 5 % of the Chl-a variability was related to the dust input. All strong Saharan dust storms between 2000 and 2008 were identified by remote sensed dust aerosol optical depth. Only some of the events caused a biological response expressed by an increase of surface chlorophyll-a. Time lags between dust deposition and enhancement of chlorophyll-a between 8 and 16 days were determined. The chlorophyll-a concentration was increased up to 2.4 mg m⁻³. Atmospheric dust modifies the amount and spectral distribution of the water light field. In the applied optical model the dust effect on incident solar radiation was parameterized by radiation measurements. The photosynthetically active radiation was reduced up to 19.3 % in the upper water column. The impacts of spectral effects on photosynthetically active radiation were different in oceanic and coastal regions. A compensation of the spectral effects at water depths of about 7 m was only observed in ocean regions because of different light attenuation in coastal areas. Saharan dust also impacts satellite derived ocean surface wind speeds. The influence depended mainly on the strength of dust storms, the microwave frequency and the wind speed. The impact was higher for stronger dust storms, higher frequencies and lower wind speeds.