Chapter 6
Impacts of Saharan Dust on the Marine
Environment in the Area off Northwest Africa
Thomas Ohde and Herbert Siegel
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.
6.1 Introduction
This chapter summarises satellite based investigations in the upwelling area off
Northwest (NW) Africa focussed on the biological response to upwelling and
dust deposition, as well as on the quantification of atmospheric dust impact on
T. Ohde () · H. Siegel
Leibniz Institut für Ostseeforschung Warnemünde,
Rostock, Germany
e-mail: thomas.ohde@io-warnemuende.de
V. Barale, M. Gade (eds.), Remote Sensing of the African Seas,
119
DOI 10.1007/978-94-017-8008-7_6,
© Springer Science+Business Media Dordrecht 2014
Impacts of Saharan Dust on the Marine
Environment in the Area off Northwest Africa
Thomas Ohde and Herbert Siegel
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.
6.1 Introduction
This chapter summarises satellite based investigations in the upwelling area off
Northwest (NW) Africa focussed on the biological response to upwelling and
dust deposition, as well as on the quantification of atmospheric dust impact on
T. Ohde () · H. Siegel
Leibniz Institut für Ostseeforschung Warnemünde,
Rostock, Germany
e-mail: thomas.ohde@io-warnemuende.de
V. Barale, M. Gade (eds.), Remote Sensing of the African Seas,
119
DOI 10.1007/978-94-017-8008-7_6,
© Springer Science+Business Media Dordrecht 2014