Primary production and distribution of phytopigments in the water column and sediments on the upwelling shelf off the Mauritanian coast (Northwest Africa)

Abstract

In May 1988, water and sediments of the shelf area off the Banc d’Arguin were studied for plant pigments and organic carbon. Phytoplankton primary production was also measured and sedimentation of organic matter was estimated using moored sediment traps. On the basis of simultaneous hydrographic measurements, two water types could be distinguished. Upwelling water covered the southern shelf and the outer part of the northern shelf. The other water type had more coastal characteristics and covered the inshore part of the northern transect. The distribution of chlorophyll a and particulate organic carbon conformed with the distribution of the two water types. A similarity analysis of the pigments from the sediment traps showed a difference in the composition of the algal material depositing from the two water types. The primary production was also found to be distinctly different, i.e. higher in the coastal water type. The pigments in the surface sediments did not reflect the respective overlying water types due to the increasing accumulation of pigments at the offshore stations of the northern transect. This accumulation seems to correspond with the progressive reduction of the benthic respiratory activity in an offshore direction. Along the southern transect such a relation could not be established, i.e. pigment accumulation along the shelf edge was not apparent. Special attention was paid to the presence of specific seagrass pigments on the shelf. No evidence for a major influx of seagrass material was found.