During the period of study (1 to 22 November, 1976) primary production in the near-shore part of the Zaire (Congo) river plume was lower as compared with both river and adjacent ocean. Although the Zaire enriches the ocean with nutrients, suspended matter and coloured substances diminish light transmittance, resulting in low primary production. Moreover, riverine phytoplankton dies rapidly in the ocean and it takes some days for marine phytoplankton to develop. Above 20‰ S phytoplankton biomass increases again, but above 30‰ S the nitrate-phosphate ratio drops below the normal assimilation ratio of 16:1. As a result primary production in the outer plume did not reach values exceeding those outside the plume. The Zaire river plume therefore differs markedly from the Amazon river plume, where primary production reaches values one order of magnitude higher than in the adjacent ocean. This difference can be explained by a difference in estuarine mixing processes between the deep Zaire estuary and the Amazon estuary with its extensive shoal. As a result marine phytoplankton in the Amazon river plume can develop at much lower salinities than in the Zaire river plume, and reaches its maximum already below 10‰ S when the discharged nutrients are still less diluted by nutrient-poor ocean water.
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