Phytoplankton distribution in the upwelling area off NW Africa

Abstract

In spring 1968, the centre of the upwelling region off NW Africa was investigated using R. V. “Meteor“. The hydrographic structure in this area is characterized by irregular upwelling in space and time. The complicated distribution of the biological components (phytoplankton and dissolved plus particulate organic substances) is expressed only within the limits given by the resolving power of the system. Therefore, the basic values have been smoothed statistically to suppress accidental irregularities. The problematics of such manipulation are discussed. The phytoplankton distribution shows a pronounced maximum in the Cap Blanc region with extreme values in the range of > 50µg C · l−1 in the surface layer. A decrease was observed not only in the direction of the open sea, but also along the coast. This distribution pattern is repeated (1) at the depth of the vertical phytoplankton maximum, which is near the surface in the Cap Blanc region; (2) at the large proportion of diatoms characteristic for upwelling water; (3) at the phytoplankton stock in the euphotic zone with maximum values up to > 0.5 g C · m−2. It is calculated that (minimum) daily production is almost equal to the stock. Processes of decomposition demonstrate the upwelling by regionally different relations between O2 consumption and PO4 liberation. An anchor station situated near the centre of the upwelling area indicates another small-scale development of phytoplankton as demonstrated by distribution charts of the total area. Pronounced daily rhythms are revealed after extensive smoothing procedures.